@article{li_atik_zheng_hajibabai_hajbabaie_2024, title={A relaxation-based Voronoi diagram approach for equitable resource distribution}, volume={9}, ISSN={["1467-8667"]}, DOI={10.1111/mice.13339}, abstractNote={Abstract This paper introduces a methodology designed to reduce cost, improve demand coverage, and ensure equitable vaccine distribution during the initial stages of the vaccination campaign when demand significantly exceeds supply. We formulate an enhanced maximum covering problem as a mixed integer linear program, aiming to minimize the total vaccine distribution cost while maximizing the allocation of vaccines to population blocks under equity constraints. Block‐level census data are employed to define demand locations, identifying gender, age, and racial groups within each block using population data. A Lagrangian relaxation technique integrated with a modified Voronoi diagram is proposed to solve the location–allocation problem efficiently. Empirical case studies in Pennsylvania, using real‐world data from the Centers for Disease Control and Prevention and health department websites, were conducted for the first 4 months of the COVID‐19 vaccination campaign. Preliminary results show that the proposed solution algorithm effectively solves the problem, achieving a 5.92% reduction in total transportation cost and a 28.15% increase in demand coverage. Moreover, our model can reduce the deviation from equity to 0.07 (∼50% improvement).}, journal={COMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING}, author={Li, Kuangying and Atik, Asya and Zheng, Dayang and Hajibabai, Leila and Hajbabaie, Ali}, year={2024}, month={Sep} }
 @article{niroumand_hajibabai_hajbabaie_2024, title={Advancing the white phase mobile traffic control paradigm to consider pedestrians}, volume={3}, ISSN={["1467-8667"]}, DOI={10.1111/mice.13178}, abstractNote={AbstractCurrent literature on joint optimization of intersection signal timing and connected automated vehicle (CAV) trajectory mostly focuses on vehicular movements paying no or little attention to pedestrians. This paper presents a methodology to safely incorporate pedestrians into signalized intersections with CAVs and connected human‐driven vehicles (CHVs). The movements of vehicles are controlled using both traffic lights and mobile CAV controllers during our newly introduced “white phase.” CAVs navigate platoons of CHVs through the intersection when the white phases are active. In addition to optimizing CAV trajectories, the model optimally selects the status of the traffic light signal among white and green indications for vehicular and walk and do‐not‐walk intervals for pedestrian movements. A receding horizon‐based methodology is used to capture the stochastic nature of the problem and to reduce computational complexity. The case study results show the successful operation of fleets consisting of pedestrians, CAVs, and CHVs with various demand levels through isolated intersections. The results also show that increasing the CAV market penetration rate (MPR) can decrease average intersection delay by up to 27%. Moreover, the average pedestrian, CHV, and CAV delays decrease as the CAV MPR increases and reach their minimum values with a fully CAV fleet. In addition, the presence of the white phase can decrease the intersection average delay by up to 14.7%.}, journal={COMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING}, author={Niroumand, Ramin and Hajibabai, Leila and Hajbabaie, Ali}, year={2024}, month={Mar} }
 @article{mohebifard_hajbabaie_2024, title={Deep Reinforcement Learning Technique for Traffic Metering in Connected Urban Street Networks}, volume={6}, ISSN={["2169-4052"]}, url={https://doi.org/10.1177/03611981241253581}, DOI={10.1177/03611981241253581}, abstractNote={Proper metering can improve traffic operations in congested urban street networks. The available approaches either (a) use macroscopic fundamental diagrams to model traffic dynamics or (b) use numerical time–space discretization of the hydrodynamic traffic flow model with high computational requirements. Therefore, they either (a) do not represent traffic dynamics accurately or (b) are not suitable for online applications. This study introduces a deep reinforcement learning (DRL) methodology to capture traffic dynamics on a micro-level scale with the capability of capturing detailed traffic dynamics with low computational time. The DRL methodology employs two neural networks that map the location of connected vehicles in a network to traffic metering signal indications and estimate the objective function of the traffic metering problem. The outputs of the neural networks are used to construct a loss function, whose optimization provides the optimal parameters for the neural networks. Because of the complexity of the loss function, the gradient descent optimization technique with Monte Carlo simulation is used to optimize the loss function. The proposed methodology was tested on a simulated case study network in Vissim software with 20 intersections. The numerical results showed that the methodology increased throughput by 41.2% and 21.3% and reduced the total travel time of vehicles by 3.4% and 15.5% compared to a no-metering strategy. Comparing the computational time of the proposed methodology with one of the existing traffic metering approaches also showed the potential of the methodology for online applications.}, journal={TRANSPORTATION RESEARCH RECORD}, author={Mohebifard, Rasool and Hajbabaie, Ali}, year={2024}, month={Jun} }
 @article{al farabi_mohebifard_niroumand_hajbabaie_hadi_elefteriadou_2024, title={Integrated corridor management by cooperative traffic signal and ramp metering control}, volume={4}, ISSN={["1467-8667"]}, DOI={10.1111/mice.13199}, abstractNote={Abstract This paper formulates a cooperative traffic control methodology that integrates traffic signal timing and ramp metering decisions into an optimization model to improve traffic operations in a corridor network. A mixed integer linear model is formulated and is solved in real time within a model predictive controller framework, where the cell transmission model is used as the system state predictor. The methodology is benchmarked in a case study corridor in San Mateo, CA, in VISSIM with two optimization scenarios, namely, optimal metering and optimal signal control, and two simulation scenarios with a preset metering plan and no metering. The numerical results show that integrated traffic signal and ramp metering control reduces delays, stops, and travel times of the corridor by up to 33.1%, 36%, and 16.4%, respectively, compared to existing benchmark conditions. With appropriate weights prioritizing freeway or arterial street operations, the integrated control balances traffic congestion between the arterial street and the freeway.}, journal={COMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING}, author={Al Farabi, Abdullah and Mohebifard, Rasool and Niroumand, Ramin and Hajbabaie, Ali and Hadi, Mohammed and Elefteriadou, Lily}, year={2024}, month={Apr} }
 @article{bin al islam_hajbabaie_2024, title={Joint Signal Timing and Trajectory Control With Uncertainty in Connected Automated Vehicle Dynamics}, volume={5}, ISSN={["1558-0016"]}, url={https://doi.org/10.1109/TITS.2024.3389818}, DOI={10.1109/TITS.2024.3389818}, abstractNote={Optimizing the trajectory of connected automated vehicles (CAVs) through cooperation with signal controllers can smoothen the traffic flow and reduce energy consumption. However, most existing research efforts in this domain do not consider the effect of stochastic disturbances generated by exogenous systems. Ignoring these stochasticities may cause a mismatch between the estimated and real vehicle dynamics, which may result in a deviation among implemented and optimized trajectories, inefficient operational performance, and even collisions in the worst-case condition. This paper introduces a two-stage optimization model for CAVs trajectory and signal timing control that considers and responds to uncertainty in vehicle dynamics. The signal controller receives the speed, acceleration, and position of incoming CAVs within the communication range and identifies incoming platoons based on vehicle headways. At the upper stage, a mixed-integer linear program within the signal controller optimizes the trajectories of the platoon leaders and signal timing parameters. At the lower stage, platoon leaders optimize the trajectories of all other vehicles within the platoon based on a chance-constrained concept to consider uncertainties involved in implementing optimized trajectories. We utilize a sample-based approximation of the collision probabilities to formulate constraints to control vehicle trajectory. The resulting formulation ensures that the probability of satisfying inter-vehicle safety distance is above a certain threshold and reduces the probability of longitudinal crashes between vehicles. The proposed framework shows a 48%-67% reduction in travel delays in comparison with optimized fixed-time signal timing plans in a simulated signalized intersection under different levels of uncertainty in vehicle dynamics.}, journal={IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS}, author={Bin Al Islam, S. M. A. and Hajbabaie, Ali}, year={2024}, month={May} }
 @article{islam_newaz_song_lartey_lin_fan_hajbabaie_khan_partovi_phuapaiboon_et al._2023, title={Connected autonomous vehicles: State of practice}, volume={5}, ISSN={["1526-4025"]}, DOI={10.1002/asmb.2772}, abstractNote={AbstractConnected autonomous vehicles (CAVs) have the potential to deal with the steady increase in road traffic while solving transportation related issues such as traffic congestion, pollution, and road safety. Therefore, CAVs are becoming increasingly popular and viewed as the next generation transportation solution. Although modular advancements have been achieved in the development of CAVs, these efforts are not fully integrated to operationalize CAVs in realistic driving scenarios. This paper surveys a wide range of efforts reported in the literature about the CAV developments, summarizes the CAV impacts from a statistical perspective, explores current state of practice in the field of CAVs in terms of autonomy technologies, communication backbone, and computation needs. Furthermore, this paper provides general guidance on how transportation infrastructures need to be prepared in order to effectively operationalize CAVs. The paper also identifies challenges that need to be addressed in near future for effective and reliable adoption of CAVs.}, journal={APPLIED STOCHASTIC MODELS IN BUSINESS AND INDUSTRY}, author={Islam, Muhammad Mobaidul and Newaz, Abdullah Al Redwan and Song, Li and Lartey, Benjamin and Lin, Shih-Chun and Fan, Wei and Hajbabaie, Ali and Khan, Mubbashar Altaf and Partovi, Alireza and Phuapaiboon, Tienake and et al.}, year={2023}, month={May} }
 @article{hajbabaie_tajalli_bardaka_2023, title={Effects of Connectivity and Automation on Saturation Headway and Capacity at Signalized Intersections}, volume={8}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/03611981231187386}, DOI={10.1177/03611981231187386}, abstractNote={ This paper analyzes the potential effects of connected and automated vehicles on saturation headway and capacity at signalized intersections. A signalized intersection is created in Vissim as a testbed, where four vehicle types are modeled and tested: (I) human-driven vehicles (HVs), (II) connected vehicles (CVs), (III) automated vehicles (AVs), and (IV) connected automated vehicles (CAVs). Various scenarios are defined based on different market-penetration rates of these four vehicle types. AVs are assumed to move more cautiously than HVs. CVs and CAVs are supposed to receive information about the future state of traffic lights and adjust their speeds to avoid stopping at the intersection. As a result, their movements are expected to be smoother with a lower number of stops. The effects of these vehicle types in mixed traffic are investigated in relation to saturation headway, capacity, travel time, delay, and queue length in different lane groups of an intersection. A Python script code developed by Vissim is used to provide the communication between the signal controller and CVs and CAVs to adjust their speeds accordingly. The results show that increasing CV and CAV market-penetration rate reduces saturation headway and consequently increases capacity at signalized intersections. On the other hand, increasing the AV market-penetration rate deteriorates traffic operations. Results also indicate that the highest increase (80%) and decrease (20%) in lane-group capacity are observed respectively in a traffic stream of 100% CAVs and 100% AVs. }, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Hajbabaie, Ali and Tajalli, Mehrdad and Bardaka, Eleni}, year={2023}, month={Aug} }
 @article{niroumand_hajibabai_hajbabaie_2023, title={White Phase Intersection Control Through Distributed Coordination: A Mobile Controller Paradigm in a Mixed Traffic Stream}, volume={24}, ISSN={1524-9050 1558-0016}, url={http://dx.doi.org/10.1109/TITS.2022.3226557}, DOI={10.1109/TITS.2022.3226557}, abstractNote={This study presents a vehicle-level distributed coordination strategy to control a mixed traffic stream of connected automated vehicles (CAVs) and connected human-driven vehicles (CHVs) through signalized intersections. We use CAVs as mobile traffic controllers during a newly introduced “white phase”, during which CAVs will negotiate the right-of-way to lead a group of CHVs while CHVs must follow their immediate front vehicle. The white phase will not be activated under low CAV penetration rates, where vehicles must wait for green signals. We have formulated this problem as a distributed mixed-integer non-linear program and developed a methodology to form an agreement among all vehicles on their trajectories and signal timing parameters. The agreement on trajectories is reached through an iterative process, where CAVs update their trajectory based on shared trajectory of other vehicles to avoid collisions and share their trajectory with other vehicles. Additionally, the agreement on signal timing parameters is formed through a voting process where the most voted feasible signal timing parameters are selected. The numerical experiments indicate that the proposed methodology can efficiently control vehicle movements at signalized intersections under various CAV market shares. The introduced white phase reduces the total delay by 3.2% to 94.06% compared to cooperative trajectory and signal optimization under different CAV market shares in our tests. In addition, our numerical results show that the proposed technique yields reductions in total delay, ranging from 40.2% – 98.9%, compared to those of a fully-actuated signal control obtained from a state-of-practice traffic signal optimization software.}, number={3}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Niroumand, Ramin and Hajibabai, Leila and Hajbabaie, Ali}, year={2023}, month={Mar}, pages={2993–3007} }
 @article{mehrabipour_hajbabaie_2022, title={A Distributed Gradient Approach for System Optimal Dynamic Traffic Assignment}, volume={23}, ISSN={1524-9050 1558-0016}, url={http://dx.doi.org/10.1109/TITS.2022.3163369}, DOI={10.1109/TITS.2022.3163369}, abstractNote={This study presents a distributed gradient-based approach to solve system optimal dynamic traffic assignment (SODTA) formulated based on the cell transmission model. The algorithm distributes SODTA into local sub-problems, who find optimal values for their decision variables within an intersection. Each sub-problem communicates with its immediate neighbors to reach a consensus on the values of common decision variables. A sub-problem receives proposed values for common decision variables from all adjacent sub-problems and incorporates them into its own offered values by weighted averaging and enforcing a gradient step to minimize its objective function. Then, the updated values are projected onto the feasible region of the sub-problems. The algorithm finds high quality solutions in all tested scenarios with a finite number of iterations. The algorithm is tested on a case study network under different demand levels and finds solutions with at most a 5% optimality gap.}, number={10}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Mehrabipour, Mehrzad and Hajbabaie, Ali}, year={2022}, month={Oct}, pages={17410–17424} }
 @article{niroumand_bahrami_aashtiani_hajbabaie_2022, title={Battery Electric Vehicles Network Equilibrium With Flow-Dependent Energy Consumption}, volume={2677}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/03611981221131813}, DOI={10.1177/03611981221131813}, abstractNote={ Recent studies show that energy consumption of battery electric vehicles (BEVs) increases in traffic congestion. Therefore, it is important to consider the effect of link flow on BEV energy consumption. The flow-dependent energy consumption changes the route choice and user equilibrium conditions. In this paper, some shortcomings of available BEV flow-dependent energy consumption user equilibrium models are shown first. Then, “sufficient” as well as “sufficient and necessary” user equilibrium based on the generalized travel time of each path and sub-path penalties are defined and modeled for flow-dependent energy consumption. While it is difficult to solve the sufficient and necessary model, the sufficient model can be solved directly with commercial solvers for small to medium-sized networks by generating all paths. An iterative algorithm is also presented to generate paths as required to solve the problem for larger networks. Numerical examples demonstrate the model and proposed algorithm, and analyze the impact of flow-dependent energy consumption on equilibrium conditions. }, number={5}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Niroumand, Ramin and Bahrami, Sina and Aashtiani, Hedayat Z. and Hajbabaie, Ali}, year={2022}, month={Nov}, pages={444–462} }
 @article{tajalli_niroumand_hajbabaie_2022, title={Distributed cooperative trajectory and lane changing optimization of connected automated vehicles: Freeway segments with lane drop}, volume={143}, ISSN={0968-090X}, url={http://dx.doi.org/10.1016/j.trc.2022.103761}, DOI={10.1016/j.trc.2022.103761}, abstractNote={• Developing a formulation to couple the discrete lane changing decisions with the polynomial longitudinal and lateral equations of motion without assuming a predefined function for the lateral movement of vehicles. • Establishing cooperation among all vehicles on the road to promote system-level operational optimality while maintaining safety. • Introducing a vehicle-level distributed algorithm to reduce the cooperative problem’s complexity so that the algorithm can work with traffic demand flow rates as high as 2400 vehicles per hour per lane. • Improving mobility on freeway facilities by reducing the average travel time by up to 86.4% and increasing the throughput by at most 134.3% depending on traffic demand and lane configuration. This study presents a methodology for optimal control of connected automated vehicles (CAVs) in freeway segments with a lane drop. Lane drops can create bottlenecks with a considerable number of mandatory and discretionary lane-changing maneuvers when traffic volume is high, which can eventually lead to stop-and-go conditions. Proper motion planning aligned with optimal lane changing upstream of a lane drop can increase capacity and reduce the number of stops and the risk of collision. This paper introduces a vehicle-level mixed-integer program to control longitudinal and lateral movement of CAVs, provide a smooth flow of traffic, and avoid congestion in freeway segments with lane drops. To ensure the feasibility of vehicle-level decisions and promote system-level optimality, a cooperative distributed algorithm is established, where CAVs coordinate their decisions to find the optimal longitudinal and lateral maneuvers that avoid collisions among all vehicles. The proposed coordination scheme lets CAVs find their optimal trajectories based on predictive information from surrounding vehicles (i.e., future locations and speeds) and coordinate their lane-changing decisions to avoid collisions. The results show that optimal lane changing of CAVs smoothens the traffic flow and increases freeway capacity in congested traffic conditions. Compared with all-knowing CAVs simulated in Vissim, the proposed methodology reduced the average travel time by up to 86.4%. It increased the number of completed trips by up to 134.3% based on various traffic demands and lane drop layout combinations.}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier BV}, author={Tajalli, Mehrdad and Niroumand, Ramin and Hajbabaie, Ali}, year={2022}, month={Oct}, pages={103761} }
 @article{niroumand_hajibabai_hajbabaie_tajalli_2022, title={Effects of Autonomous Driving Behavior on Intersection Performance and Safety in the Presence of White Phase for Mixed-Autonomy Traffic Stream}, volume={2676}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/03611981221082580}, DOI={10.1177/03611981221082580}, abstractNote={ This paper studies the effects of different autonomous driving behaviors on an isolated intersection’s safety and mobility performance measures in a mixed-autonomy environment. The movement of vehicles through the intersection is controlled by green, red, and “white” signal indications. Traffic operations during green and red signals are identical to a typical intersection. However, in the presence of the white phase, connected human-driven vehicles (CHVs) should follow connected and autonomous vehicles (CAVs) to pass the intersection safely. Three levels of driving aggressiveness for CAVs are considered: (1) cautious behavior, (2) normal behavior, and (3) aggressive behavior. The mobility and safety impacts of these CAV behaviors are studied based on different CAV market penetration rates and demand levels. The results indicate that a more aggressive CAV driving behavior leads to a lower average delay while increasing the average number of stops for CAVs. Additionally, a more aggressive CAV driving behavior leads to more frequent activation of the white phase that contributes to significant reduction in the speed variance. Moreover, the total number of rear-end near-collision observations with a time to collision of less than 10 s decreases as the CAV penetration rate and aggressiveness level increase. The main reason for this observation is that aggressive CAVs have higher acceleration and lower deceleration values and, therefore, have more flexibility to avoid a crash. }, number={8}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Niroumand, Ramin and Hajibabai, Leila and Hajbabaie, Ali and Tajalli, Mehrdad}, year={2022}, month={Apr}, pages={112–130} }
 @article{tajalli_al islam_list_hajbabaie_2022, title={Testing Connected Vehicle-Based Accident Mitigation for Red-Light Violation Using Simulation Strategies}, volume={2676}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/03611981221075630}, DOI={10.1177/03611981221075630}, abstractNote={ Simulation is often suggested as a way to analyze the safety improvements of geometric changes and operational strategies. But the results from simulations are mixed. This paper presents new ideas about how to do such assessments, especially in the context of testing the value of vehicle to vehicle (V2V), vehicle to infrastructure (V2I), and vehicle to pedestrian (V2P) communications in preventing crashes because of red-light violation at signalized intersections. Algorithms are created that watch for impending collisions through sensing and then issue speed and trajectory changes to avoid accidents. Red-light violation is a primary focus because it increases the likelihood that collisions will occur. VISSIM is used to test these ideas, including new communication and control algorithms that link to vehicles, pedestrians, and signal controllers through the communication interface. The algorithms predict unsafe conditions, determine an appropriate crash remedial decision, and communicate those controls with the appropriate vehicles and pedestrians. The impacts of these algorithms are explored under various demand patterns, connected vehicle market penetration rates, and red-light violation rates in a hypothetical simulated environment. The simulation analysis suggests that the number of near-crash events can be reduced significantly if V2V and V2P communications are implemented. Moreover, adding V2I communication on top of these may further reduce the number of near-crash events. These results suggest that not only could such control strategies have significant impacts, but also those impacts can be assessed through simulation. }, number={6}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Tajalli, Mehrdad and Al Islam, S. M. A. Bin and List, George F. and Hajbabaie, Ali}, year={2022}, month={Mar}, pages={583–600} }
 @article{hajibabai_hajbabaie_swann_vergano_2022, title={Using COVID-19 Data on Vaccine Shipments and Wastage to Inform Modeling and Decision-Making}, volume={56}, ISSN={0041-1655 1526-5447}, url={http://dx.doi.org/10.1287/trsc.2022.1134}, DOI={10.1287/trsc.2022.1134}, abstractNote={ Since the start of the COVID-19 pandemic, disruptions have been experienced in many supply chains, particularly in personal protective equipment, testing kits, and even essential household goods. Effective vaccines to protect against COVID-19 were approved for emergency use in the United States in late 2020, which led to one of the most extensive vaccination campaigns in history. We continuously collect data on vaccine allocation, shipment and distribution, administration, and inventory in the United States, covering the entire vaccination campaign. In this article, we describe some data sets that we collaborated to obtain. We are publishing the data and making them freely available to researchers, media organizations, and other stakeholders so that others may use the data to develop insights about the distribution and wastage of vaccines during the current pandemic or to provide an informed future pandemic response. This article gives an overview of vaccine distribution logistics in the United States, describes the data we obtain, outlines how they may be accessed and used by others, and describes some high-level analyses demonstrating some aspects of the data (for data collected during January 1, 2021–March 31, 2021). This article also provides directions for future research using the collected data. Our goal is two-fold: (i) We would like the data to be used in many creative ways to inform the current and future pandemic response. (ii) We also want to inspire other researchers to make their data publicly available in a timely manner. }, number={5}, journal={Transportation Science}, publisher={Institute for Operations Research and the Management Sciences (INFORMS)}, author={Hajibabai, Leila and Hajbabaie, Ali and Swann, Julie and Vergano, Dan}, year={2022}, month={Sep}, pages={1135–1147} }
 @article{islam_hajbabaie_2022, title={An Enhanced Cell Transmission Model for Multi-Class Signal Control}, volume={23}, ISSN={1524-9050 1558-0016}, url={http://dx.doi.org/10.1109/TITS.2021.3101838}, DOI={10.1109/TITS.2021.3101838}, abstractNote={Existing multi-class cell transmission model (CTM) based methodologies for signal timing or traffic assignment may transfer prioritized transit vehicles from one cell to the next one before processing their preceding passenger cars. In addition, existing CTM-based methodologies process a proportion of a slow-moving transit vehicle in each time step. As such a portion of each transit vehicle remains in each cell and it never clears them. This paper presents constraints to project the position of transit vehicles based on the speed and cell occupancy variations between different classes of vehicles and incorporates them into the CTM. The resulting optimization program is a mixed-integer nonlinear problem. We used a distributed receding horizon control framework to solve it in real-time. The proposed formulation is executed in a simulated arterial street with four signalized intersections in Springfield, IL with different traffic volume levels and transit vehicle frequencies. The results showed that the proposed algorithm addressed the mentioned issues of the existing multi-class CTM, and yielded more efficient network performance than the conventional transit signal priority-based (CTSP) systems. The proposed formulation reduced average bus delay by 1% to 70% and car delay by 52% to 76% compared to CTSP.}, number={8}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Islam, S M A Bin Al and Hajbabaie, Ali}, year={2022}, month={Aug}, pages={11215–11226} }
 @article{mohebifard_hajbabaie_2021, title={Connected automated vehicle control in single lane roundabouts}, volume={131}, ISSN={0968-090X}, url={http://dx.doi.org/10.1016/j.trc.2021.103308}, DOI={10.1016/j.trc.2021.103308}, abstractNote={This paper introduces a methodology to optimize the trajectory of connected automated vehicles (CAVs) in roundabouts using a two-dimensional point-mass model. We formulate an optimization problem that includes vehicle dynamics and collision-avoidance constraints with explicit representation of vehicle paths. The objective function of the problem minimizes the distance of CAVs to their destinations and their acceleration magnitudes. The methodology also involves a customized solution technique that convexifies the collision-avoidance constraints and employs the alternating direction method of multipliers to decompose the convexified problem into two sub-problems. The first sub-problem only includes vehicle dynamics constraints while the second sub-problem projects the solutions of the first sub-problem onto a collision-free region. The first sub-problem is then transformed into a quadratic problem by redefining its decision variables along vehicle paths. The transformation allows solving this sub-problem with several vehicle-level problems in a distributed architecture. Moreover, we show that iterating between the two sub-problems leads to convergence to the optimal solutions of the convexified problem. The methodology is applied to a case study roundabout with different demand levels. The results show that the trajectory optimization reduced the total travel times and average delays respectively by 9.1% to 36.8% and 95.8% to 98.5% compared to a scenario with human-driven vehicles.}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier BV}, author={Mohebifard, Rasool and Hajbabaie, Ali}, year={2021}, month={Oct}, pages={103308} }
 @article{islam_tajalli_mohebifard_hajbabaie_2021, title={Effects of Connectivity and Traffic Observability on an Adaptive Traffic Signal Control System}, volume={2675}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/03611981211013036}, DOI={10.1177/03611981211013036}, abstractNote={ The effectiveness of adaptive signal control strategies depends on the level of traffic observability, which is defined as the ability of a signal controller to estimate traffic state from connected vehicle (CV), loop detector data, or both. This paper aims to quantify the effects of traffic observability on network-level performance, traffic progression, and travel time reliability, and to quantify those effects for vehicle classes and major and minor directions in an arterial corridor. Specifically, we incorporated loop detector and CV data into an adaptive signal controller and measured several mobility- and event-based performance metrics under different degrees of traffic observability (i.e., detector-only, CV-only, and CV and loop detector data) with various CV market penetration rates. A real-world arterial street of 10 intersections in Seattle, Washington was simulated in Vissim under peak hour traffic demand level with transit vehicles. The results showed that a 40% CV market share was required for the adaptive signal controller using only CV data to outperform signal control with only loop detector data. At the same market penetration rate, signal control with CV-only data resulted in the same traffic performance, progression quality, and travel time reliability as the signal control with CV and loop detector data. Therefore, the inclusion of loop detector data did not further improve traffic operations when the CV market share reached 40%. Integrating 10% of CV data with loop detector data in the adaptive signal control improved traffic performance and travel time reliability. }, number={10}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Islam, S M A Bin Al and Tajalli, Mehrdad and Mohebifard, Rasool and Hajbabaie, Ali}, year={2021}, month={May}, pages={800–814} }
 @misc{niroumand_hajibabai_hajbabaie_2021, title={The Effects of Connectivity on Intersection Operations with “White Phase”}, url={http://dx.doi.org/10.1109/itsc48978.2021.9564622}, DOI={10.1109/itsc48978.2021.9564622}, abstractNote={This paper analyzes the impact of infrastructure to vehicle communication on mobility and safety performance measures at isolated signalized intersections with white phase. During white phases, the intersection controller optimizes the trajectory of connected automated vehicles (CAV) to lead a group of connected human-driven vehicles (CHV) and navigate them through the intersection. White phase can be assigned to conflicting movements simultaneously to mimic the signal-free intersection logic within a mixed autonomy environment where the safety of the shared fleet is ensured by optimally controlling the movements of CAVs. Red and green signal indications can be used to ensure safety when there are not enough CAVs in the vicinity of the intersection. The problem is formulated as an optimization model and solved using a receding horizon framework. One demand level and five CAV market shares are used to assess the effects of connectivity on traffic operations and safety. Case studies are tested with three connectivity levels: 1) no-connectivity, 2) connected without white phase (no-white-phase), and 3) connected with white phase (optimized-white-phase). The results indicate that average delay decreases as the CAV market share and level of connectivity increase.}, journal={2021 IEEE International Intelligent Transportation Systems Conference (ITSC)}, publisher={IEEE}, author={Niroumand, Ramin and Hajibabai, Leila and Hajbabaie, Ali}, year={2021}, month={Sep} }
 @article{tajalli_hajbabaie_2022, title={Traffic Signal Timing and Trajectory Optimization in a Mixed Autonomy Traffic Stream}, volume={23}, ISSN={1524-9050 1558-0016}, url={http://dx.doi.org/10.1109/TITS.2021.3058193}, DOI={10.1109/TITS.2021.3058193}, abstractNote={This study introduces a methodology for cooperative signal timing and trajectory optimization at intersections with a mix of connected automated vehicles (CAVs) and human-driven vehicles (HVs). We represent joint signal timing and trajectory control as a mixed-integer non-linear program, which is computationally complex. The developed methodology provides a balance between computational efficiency and solution quality by (a) linearizing the nonlinear constraints and reformulating the problem with a tight convex hull of the mixed-integer solutions and (b) decomposing the intersection-level program into several lane-level programs. Hence, a unique controller jointly optimizes the trajectories of CAVs on a lane and the signal timing parameters associated with that lane. This setting will allow finding near-optimal solutions with small duality gaps for complex intersections with different demand levels. Case study results show that the proposed methodology finds solutions efficiently with at most 0.1% duality gap. We compared the developed methodology with an existing signal timing and trajectory control approach and found 13% to 41% reduction in average travel time and 1% to 31% reduction in fuel consumption under different scenarios.}, number={7}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Tajalli, Mehrdad and Hajbabaie, Ali}, year={2022}, month={Jul}, pages={6525–6538} }
 @article{mohebifard_hajbabaie_2021, title={Trajectory control in roundabouts with a mixed fleet of automated and human‐driven vehicles}, volume={37}, ISSN={1093-9687 1467-8667}, url={http://dx.doi.org/10.1111/mice.12711}, DOI={10.1111/mice.12711}, abstractNote={AbstractThis paper presents a methodology to control the trajectory of cooperative connected automated vehicles (CAVs) at roundabouts with a mixed fleet of CAVs and human‐driven vehicles (HVs). We formulate an optimization program in a two‐dimensional space for this purpose. A model predictive control‐based solution technique is developed to optimize the trajectories of CAVs at discretized time steps based on the estimated driving behavior of HVs, while the actual behavior of HVs is controlled by a microscopic traffic simulator. At each time step, the location and speed of vehicles are collected, and a decomposition‐based methodology optimizes CAV trajectories for a few time steps ahead of the system time. The optimization methodology has convexification, alternating direction method of multipliers, and cutting plane decomposition components to tackle the complexities of the problem. We tested the solution technique in a case study roundabout with different traffic demand flow rates and CAV market penetration rates. The results showed that increasing the CAV market penetration rate from 20% to 100% reduced total travel times by 2.8% to 35.8%. The analyses indicate that the presence of cooperative CAVs in roundabouts can lead to considerable improvements.}, number={15}, journal={Computer-Aided Civil and Infrastructure Engineering}, publisher={Wiley}, author={Mohebifard, Rasool and Hajbabaie, Ali}, year={2021}, month={Jun}, pages={1959–1977} }
 @article{tajalli_mirheli_hajbabaie_hajibabai_2021, title={Utilization and Cost Estimation Models for Highway Fleet Equipment}, volume={2675}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/03611981211032215}, DOI={10.1177/03611981211032215}, abstractNote={ Highway agencies need to manage the utilization of their highway equipment assets to reduce fleet management costs, balance equipment use, and provide the required services. Predictive equipment utilization and operational cost models are required for optimal management; however, there are no widely accepted models for this purpose. Although the utilization data is collected by state DOTs, the literature does not show any specific statistical model to predict equipment utilization as a function of contributing factors such as asset age, fleet size, costs, and demand for service. This study will bridge this gap and develop a predictive model to estimate the utilization of fleet equipment. The main objective of this paper is to develop a set of predictive models to estimate the annual utilization of seven non-stationary highway equipment types based on several explanatory variables including their annual fuel cost, downtime hours, age, and weight. Furthermore, another set of models are fit to predict the annual operational cost for these equipment types based on the most important contributing factors. The prediction models are developed after a nationwide data collection. Several years of collected data from seven states are processed and used for model development. This research has identified annual mileage as an appropriate and widely used utilization metric. Various model structures to predict annual mileage are considered. The logarithmic function of annual mileage has provided the most appropriate structure. The final annual mileage predictive models have R-squared values that are between 0.65 and 0.89, which indicates a good fit for all models. The models are validated by performing several statistical tests and they have satisfied all required assumptions of regression analysis. The result of modeling and statistical analysis showed that the proposed models accurately estimated the utilization and operational cost for highway equipment assets. }, number={12}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Tajalli, Mehrdad and Mirheli, Amir and Hajbabaie, Ali and Hajibabai, Leila}, year={2021}, month={Aug}, pages={1172–1186} }
 @article{islam_hajbabaie_aziz_2020, title={A real-time network-level traffic signal control methodology with partial connected vehicle information}, volume={121}, url={http://dx.doi.org/10.1016/j.trc.2020.102830}, DOI={10.1016/j.trc.2020.102830}, abstractNote={This paper presents two algorithms to estimate traffic state in urban street networks with a mixed traffic stream of connected and unconnected vehicles and incorporates them in a real-time and distributed traffic signal control methodology. The first algorithm integrates connected vehicles (CV) and loop detector data to estimate the trajectory of unconnected vehicles based on car-following concepts. The second algorithm converts the temporal point vehicle detections to a spatial vehicle distribution on a link. The signal control methodology utilizes either algorithm to estimate traffic state on all network links at a time, optimizes the signal timing parameters over a prediction period constituting several time steps, implements the optimal decisions in the next time step, and continues this process until the end of the study period. We applied the methodology to a real-world case study network simulated in Vissim. The results show that both algorithms are effective under a wide range of CV market penetration rates in all tested demand patterns: at 0% market penetration rate, the proposed methodology reduced travel time by 2% to 10% and average delay by 7% to 20% compared to the existing signal timing parameters and traffic demand. At a 40% penetration rate, the proposed algorithms reduced travel time by 27% to 33% and average delay by 50% to 61% compared to the existing signal and demand pattern in the case study network. Similar trends were found for all other demand patterns tested in this study.}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier BV}, author={Islam, S.M.A. Bin Al and Hajbabaie, Ali and Aziz, H.M. Abdul}, year={2020}, month={Dec}, pages={102830} }
 @misc{mohebifard_hajbabaie_2020, title={Effects of Automated Vehicles on Traffic Operations at Roundabouts}, url={http://dx.doi.org/10.1109/itsc45102.2020.9294563}, DOI={10.1109/itsc45102.2020.9294563}, abstractNote={This study evaluates the effects of various market penetration rates of connected autonomous vehicles (CAV) on traffic operations at roundabouts. We have utilized a simulation-and an optimization-based approach for this purpose. The simulation-based approach included calibrated car following models with different driving behavior parameters for a mixed fleet of conventional vehicles and CAVs with various penetration rates ranging from 0% to 100%. We also used an optimization-based approach for the 100% CAV market penetration rate case to evaluate operations while trajectories of CAVs were optimized by a central controller. The simulation results showed that CAVs improved traffic operations in under-and semi-saturated flow conditions. Nevertheless, the optimization of CAV trajectories resulted in significant delay reductions and improvements in the roundabout performance. These results indicate that CAVs have great potentials for improving traffic operations once an effective control algorithm is available.}, journal={2020 IEEE 23rd International Conference on Intelligent Transportation Systems (ITSC)}, publisher={IEEE}, author={Mohebifard, Rasool and Hajbabaie, Ali}, year={2020}, month={Sep} }
 @article{niroumand_tajalli_hajibabai_hajbabaie_2020, title={Joint optimization of vehicle-group trajectory and signal timing: Introducing the white phase for mixed-autonomy traffic stream}, volume={116}, ISSN={0968-090X}, url={http://dx.doi.org/10.1016/j.trc.2020.102659}, DOI={10.1016/j.trc.2020.102659}, abstractNote={This study develops a novel mixed-integer non-linear program to control the trajectory of mixed connected-automated vehicles (CAVs) and connected human-driven vehicles (CHVs) through signalized intersections. The trajectory of CAVs is continuously optimized via a central methodology, while a new “white” phase is introduced to enforce CHVs to follow their immediate front vehicle. The movement of CHVs is incorporated in the optimization framework utilizing a customized linear car-following model. During the white phase, CAVs lead groups of CHVs through an intersection. The proposed formulation determines the optimal signal indication for each lane-group in each time step. We have developed a receding horizon control framework to solve the problem. The case study results indicate that the proposed methodology successfully controls the mixed CAV-CHV traffic under various CAV market penetration rates and different demand levels. The results reveal that a higher CAV market penetration rate induces more frequent white phase indication compared to green-red signals. The proposed program reduces the total delay by 19.6%–96.2% compared to a fully-actuated signal control optimized by a state-of-practice traffic signal timing optimization software.}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier BV}, author={Niroumand, Ramin and Tajalli, Mehrdad and Hajibabai, Leila and Hajbabaie, Ali}, year={2020}, month={Jul}, pages={102659} }
 @article{tajalli_mehrabipour_hajbabaie_2021, title={Network-Level Coordinated Speed Optimization and Traffic Light Control for Connected and Automated Vehicles}, volume={22}, ISSN={1524-9050 1558-0016}, url={http://dx.doi.org/10.1109/TITS.2020.2994468}, DOI={10.1109/TITS.2020.2994468}, abstractNote={This study develops a methodology for coordinated speed optimization and traffic light control in urban street networks. We assume that all vehicles are connected and automated. The signal controllers collect vehicle data through vehicle to infrastructure communications and find optimal signal timing parameters and vehicle speeds to maximize network throughput while harmonizing speeds. Connected and automated vehicles receive these dynamically assigned speeds, accept them, and implement them. The problem is formulated as a mixed-integer non-linear program and accounts for the trade-offs between maximizing the network throughput and minimizing speed variations in the network to improve the network operational performance and at the same time smoothen the traffic flow by harmonizing the speed and reducing the number of stops at signalized intersections. A distributed optimization scheme is developed to reduce the computational complexity of the proposed program, and effective coordination ensures near-optimality of the solutions. The case study results show that the proposed algorithm works in real-time and provides near-optimal solutions with a maximum optimality gap of 5.4%. The proposed algorithm is implemented in Vissim. The results show that coordinated signal timing and speed optimization improved network performance in comparison with cases that either signal timing parameters or average speed of vehicles are optimized. The coordinated approach reduced the travel time, average delay, average number of stops, and average delay at stops by 1.9%, 5.3%, 28.5%, and 5.4%, respectively compared to the case that only signal timing parameters are optimized.}, number={11}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Tajalli, Mehrdad and Mehrabipour, Mehrzad and Hajbabaie, Ali}, year={2021}, month={Nov}, pages={6748–6759} }
 @article{bin al islam_abdul aziz_hajbabaie_2021, title={Stochastic Gradient-Based Optimal Signal Control With Energy Consumption Bounds}, volume={22}, ISSN={1524-9050 1558-0016}, url={http://dx.doi.org/10.1109/TITS.2020.2979384}, DOI={10.1109/TITS.2020.2979384}, abstractNote={This paper develops a stochastic gradient-based optimization model for traffic signal control with bounds on network-level vehicular energy consumption. The signal control problem is formulated as a mixed-integer linear mathematical program, which incorporates inequality constraints to limit the total energy consumption in the network. The developed stochastic gradient approximation algorithm provides a near-optimal solution to the non-convex optimization problem. To account for the energy consumption constraints, a penalty function method leveraging the pseudo gradient estimation technique is developed. Empirical results from a signalized arterial street show that it is possible to achieve optimized signal settings at the desired energy consumption bound without compromising delay. Further, we report the sensitivity of the energy bounds to the mobility metrics—system delay. Our novel gradient-approximation-based solution technique offers a functional and feasible way to accommodate non-convex energy consumption bounds within a signal control optimization model to achieve maximal mobility with minimal energy consumption.}, number={5}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Bin Al Islam, S. M. A. and Abdul Aziz, H. M. and Hajbabaie, Ali}, year={2021}, month={May}, pages={3054–3067} }
 @misc{niroumand_tajalli_hajibabai_hajbabaie_2020, title={The Effects of the “White Phase” on Intersection Performance with Mixed-Autonomy Traffic Stream}, url={http://dx.doi.org/10.1109/itsc45102.2020.9294741}, DOI={10.1109/itsc45102.2020.9294741}, abstractNote={This study investigates the effects of the “white phase” on the performance of isolated signalized intersections. During the white phase, connected automated vehicles (CAV) control traffic flow through an intersection, and connected human-driven vehicles (CHV) follow their front vehicle (either CAV or CHV). Traffic controller ensures collision-free movement of vehicles through the intersection by determining 1) the sequence and duration of phases (green and white) and 2) trajectory of CAVs during white phases. White phases can be assigned to conflicting movements simultaneously. We have formulated this problem as a mixed-integer non-linear program (MINLP) and solved it using a receding horizon algorithm. Two demand patterns with five different CAV market penetration rates are used to evaluate the effects of the white phase on mobility and safety in an isolated intersection. Each case study is tested with three different control scenarios: 1) No-white-phase, 2) white-phase-only, and 3) optimal-white-phase activation (combination of white, green, and red phases). The results indicate that the white phase yields significant improvement in intersection performance while maintaining the same safety level.}, journal={2020 IEEE 23rd International Conference on Intelligent Transportation Systems (ITSC)}, publisher={IEEE}, author={Niroumand, Ramin and Tajalli, Mehrdad and Hajibabai, Leila and Hajbabaie, Ali}, year={2020}, month={Sep} }
 @article{mirheli_tajalli_mohebifard_hajibabai_hajbabaie_2020, title={Utilization Management of Highway Operations Equipment}, volume={2674}, ISSN={["2169-4052"]}, url={http://dx.doi.org/10.1177/0361198120927400}, DOI={10.1177/0361198120927400}, abstractNote={ This paper presents fleet utilization management processes for highway operations equipment based on actual tracked and reported usage data obtained from transportation agencies. The objective is to minimize total fleet utilization costs, including operational, purchase, and relocation expenses that yield the optimal utilization values and fleet composition of specific equipment types within each region in a year. The framework includes utilization prediction and optimization models, rather than relying on pre-determined utilization thresholds in existing strategies, to avoid under-utilization, over-utilization, or both. The prediction models are structured using equipment explanatory variables with their significant contributing factors, for example, annual equipment usage, annual fuel cost, downtime hours, age, and class code, to predict operational costs. The optimization model is formulated as a set of mathematical formulations, with embedded predictive models, that minimizes the total costs of (i) keeping an asset in-service using predictive annual operational cost functions, (ii) purchasing new assets in a region in the following year, and (iii) relocating assets by capturing the distance between regions. The costs include equipment purchase, operation, maintenance, and transportation expenses. The proposed framework captures the remedial actions to balance under-/over-utilized assets in the fleet in a cost-efficient manner. The proposed methodology is applied to utilization management of a set of operations equipment, and the findings of the dump trucks are presented. Several scenarios are designed to analyze the sensitivity of the costs to various decisions and parameters. The numerical experiments reveal that the proposed framework can facilitate the utilization prediction and management of highway operations equipment and save up to 16.6% in operational costs considering different demand scenarios. }, number={9}, journal={TRANSPORTATION RESEARCH RECORD}, author={Mirheli, Amir and Tajalli, Mehrdad and Mohebifard, Rasool and Hajibabai, Leila and Hajbabaie, Ali}, year={2020}, month={Sep}, pages={202–215} }
 @article{mirheli_tajalli_hajibabai_hajbabaie_2019, title={A consensus-based distributed trajectory control in a signal-free intersection}, volume={100}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85060455583&partnerID=MN8TOARS}, DOI={10.1016/j.trc.2019.01.004}, abstractNote={This paper develops a distributed cooperative control logic to determine conflict-free trajectories for connected and automated vehicles (CAVs) in signal-free intersections. The cooperative trajectory planning problem is formulated as vehicle-level mixed-integer non-linear programs (MINLPs) that aim to minimize travel time of each vehicle and their speed variations, while avoiding near-crash conditions. To push vehicle-level solutions towards global optimality, we develop a coordination scheme between CAVs on conflicting movements. The coordination scheme shares vehicle states (i.e., location) over a prediction horizon and incorporates such information in CAVs’ respective MINLPs. Therefore, the CAVs will reach consensus through an iterative process and select conflict-free trajectories that minimize their travel time. The numerical experiments quantify the effects of the proposed methodology on traffic safety and performance measures in an intersection. The results show that the proposed distributed coordinated framework converges to near-optimal CAV trajectories with no conflicts in the intersection neighborhood. While the solutions are found in real-time, the comparison to a central intersection control logic for CAVs indicates a maximum marginal objective value of 2.30%. Furthermore, the maximum marginal travel time, throughput, and average speed do not exceed 0.5%, 0.1%, and 0.5%, respectively. The proposed control logic reduced travel time by 43.0–70.5%, and increased throughput and average speed respectively by 0.8–115.6% and 59.1–400.0% compared to an optimized actuated signal control, while eliminating all near-crash conditions.}, journal={Transportation research part C: emerging technologies}, publisher={Elsevier}, author={Mirheli, Amir and Tajalli, Mehrdad and Hajibabai, Leila and Hajbabaie, Ali}, year={2019}, pages={161–176} }
 @article{mehrabipour_hajibabai_hajbabaie_2019, title={A decomposition scheme for parallelization of system optimal dynamic traffic assignment on urban networks with multiple origins and destinations}, volume={34}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85066156002&partnerID=MN8TOARS}, DOI={10.1111/mice.12455}, abstractNote={AbstractThis paper presents a decomposition scheme to find near‐optimal solutions to a cell transmission model‐based system optimal dynamic traffic assignment problem with multiple origin‐destination pairs. A linear and convex formulation is used to define the problem characteristics. The decomposition is designed based on the Dantzig–Wolfe technique that splits the set of decision variables into subsets through the construction of a master problem and subproblems. Each subproblem includes only a single origin‐destination pair with significantly less computational burden compared to the original problem. The master problem represents the coordination between subproblems through the design of interactive flows between the pairs. The proposed methodology is implemented in two case study networks of 20 and 40 intersections with up to 25 origin‐destination pairs. The numerical results show that the decomposition scheme converges to the optimal solution, within 2.0% gap, in substantially less time compared to a benchmark solution, which confirms the computational efficiency of the proposed algorithm. Various network performance measures have been assessed based on different traffic state scenarios to draw managerial insights.}, number={10}, journal={Computer-Aided Civil and Infrastructure Engineering}, publisher={Wiley Online Library}, author={Mehrabipour, Mehrzad and Hajibabai, Leila and Hajbabaie, Ali}, year={2019}, pages={915–931} }
 @article{mohebifard_islam_hajbabaie_2019, title={Cooperative traffic signal and perimeter control in semi-connected urban-street networks}, volume={104}, url={https://doi.org/10.1016/j.trc.2019.05.023}, DOI={10.1016/j.trc.2019.05.023}, abstractNote={This paper presents an integrated formulation and a distributed solution technique for cooperative signal control and perimeter traffic metering in urban street networks with various market penetration rates of connected vehicles. The problem is formulated as a mixed integer nonlinear program thus, does not scale well with the size of the network in a centralized optimization framework due to the presence of many mixed integer decision variables and nonlinear constraints. To address this limitation, we will develop a distributed model predictive control that distributes the network-level cooperative problem into several intersection-level sub-problems and coordinates their decisions. Our numerical analyses show that the proposed distributed methodology finds solutions to the problem in real-time with the optimality gap of at most 3.6% in our case studies. We have implemented the distributed methodology in Vissim and observed that cooperative signal timing and perimeter control yielded significant improvements in traffic operations. Our case study results show that the cooperative approach increases the number of completed trips by 6.0–12.8% and 10.9–11.0% and reduces the total travel times by 8.1–9.0% and 23.6–24.2% compared to independent signal control and independent perimeter control, respectively.}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier BV}, author={Mohebifard, Rasool and Islam, S.M.A. Bin Al and Hajbabaie, Ali}, year={2019}, month={Jul}, pages={408–427} }
 @article{mohebifard_hajbabaie_2019, title={Optimal network-level traffic signal control: A benders decomposition-based solution algorithm}, volume={121}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85061027902&partnerID=MN8TOARS}, DOI={10.1016/j.trb.2019.01.012}, abstractNote={This paper formulates the network-level traffic signal timing optimization problem as a Mixed-Integer Non-Linear Program (MINLP) and presents a customized methodology to solve it with a tight optimality gap. The MINLP is based on the Cell Transmission Model (CTM) network loading concept and captures the fundamental flow-density diagram of the CTM explicitly by considering closed-form constraints in the model and thus, eliminates the flow holding-back problem. The proposed solution algorithm is based on the Benders decomposition technique and decomposes the original MINLP to an equivalent Integer Program (IP) (Master problem), and a new MINLP (Primal problem). We will show that the new MINLP has only one optimal non-holding-back solution that can be found by a CTM simulation run. We will prove that the proposed solution technique guarantees convergence to optimal solutions with a finite number of iterations. Furthermore, we propose a dual estimation algorithm for the new MINLP (the Primal problem), which utilizes a simulation-based approach to generate Benders cuts instead of solving a complex optimization program. We applied the proposed solution technique to a simulated network of 20 intersections under various demand patterns and observed an optimality gap of at most 2% under all tested conditions. We compared the solutions of the proposed algorithm with two benchmark algorithms and found reductions in total travel time ranging from 7.0% to 35.7%.}, journal={Transportation Research Part B: Methodological}, publisher={Elsevier}, author={Mohebifard, Rasool and Hajbabaie, Ali}, year={2019}, pages={252–274} }
 @article{mirheli_hajibabai_hajbabaie_2018, title={Development of a signal-head-free intersection control logic in a fully connected and autonomous vehicle environment}, volume={92}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85047272483&partnerID=MN8TOARS}, DOI={10.1016/j.trc.2018.04.026}, abstractNote={Establishment of effective cooperation between vehicles and transportation infrastructure improves travel reliability in urban transportation networks. Lack of collaboration, however, exacerbates congestion due mainly to frequent stops at signalized intersections. It is beneficial to develop a control logic that collects basic safety message from approaching connected and autonomous vehicles and guarantees efficient intersection operations with safe and incident free vehicle maneuvers. In this paper, a signal-head-free intersection control logic is formulated into a dynamic programming model that aims to maximize the intersection throughput. A stochastic look-ahead technique is proposed based on Monte Carlo tree search algorithm to determine the near-optimal actions (i.e., acceleration rates) over time to prevent movement conflicts. Our numerical results confirm that the proposed technique can solve the problem efficiently and addresses the consequences of existing traffic signals. The proposed approach, while completely avoids incidents at intersections, significantly reduces travel time (ranging between 59.4% and 83.7% when compared to fixed-time and fully-actuated control strategies) at intersections under various demand patterns.}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier}, author={Mirheli, Amir and Hajibabai, Leila and Hajbabaie, Ali}, year={2018}, pages={412–425} }
 @article{mohebifard_hajbabaie_2019, title={Distributed Optimization and Coordination Algorithms for Dynamic Traffic Metering in Urban Street Networks}, volume={20}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050987048&partnerID=MN8TOARS}, DOI={10.1109/TITS.2018.2848246}, abstractNote={Previous research has shown that proper metering of entry traffic to urban street networks, similar to metering traffic on on-ramps in freeway facilities, reduces traffic congestion, especially in oversaturated flow conditions. Building on the previous research, this paper presents a real-time and scalable methodology for finding near-optimal metering rates dynamically in urban street networks. The problem is formulated into a mixed-integer linear program (MILP) based on the cell transmission model. We propose a distributed optimization scheme that decomposes the network level MILP into several link-level MILPs to reduce the complexity of the problem. We convert the link-level MILPs to linear programs to reduce the computational complexity further. Moreover, we create distributed coordination between the link-level linear programs to push the solutions toward optimality. The distributed optimization and coordination solution algorithm is incorporated into a rolling horizon technique to account for the time-varying demand and capacity and to reduce the computational complexity further. We applied the proposed solution technique to a number of case studies and observed that it was scalable and real time and found solutions that were at most 2.2% different from the optimal solution of the problem. Like the previous studies, we found significant improvements in network operations as a result of traffic metering.}, number={5}, journal={IEEE Transactions on Intelligent Transportation Systems}, author={Mohebifard, R. and Hajbabaie, A.}, year={2019}, pages={1930–1941} }
 @article{tajalli_hajbabaie_2018, title={Distributed optimization and coordination algorithms for dynamic speed optimization of connected and autonomous vehicles in urban street networks}, volume={95}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85051259651&partnerID=MN8TOARS}, DOI={10.1016/j.trc.2018.07.012}, abstractNote={Dynamic speed harmonization has shown great potential to smoothen the flow of traffic and reduce travel time in urban street networks. The existing methods, while providing great insights, are neither scalable nor real-time. This paper develops Distributed Optimization and Coordination Algorithms (DOCA) for dynamic speed optimization of connected and autonomous vehicles in urban street networks to address this gap. DOCA decomposes the nonlinear network-level speed optimization problem into several sub-network-level nonlinear problems thus, it significantly reduces the problem complexity and ensures scalability and real-time runtime constraints. DOCA creates effective coordination in decision making between each two sub-network-level nonlinear problems to push solutions towards optimality and guarantee attaining near-optimal solutions. DOCA is incorporated into a model predictive control approach to allow for additional consensus between sub-network-level problems and reduce the computational complexity further. We applied the proposed solution technique to a real-world network in downtown Springfield, Illinois and observed that it was scalable and real-time while finding solutions that were at most 2.7% different from the optimal solution of the problem. We found significant improvements in network operations and considerable reductions in speed variance as a result of dynamic speed harmonization.}, journal={Transportation research part C: emerging technologies}, publisher={Elsevier}, author={Tajalli, Mehrdad and Hajbabaie, Ali}, year={2018}, pages={497–515} }
 @article{tajalli_hajbabaie_2018, title={Dynamic speed harmonization in connected urban street networks}, volume={33}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85043677737&partnerID=MN8TOARS}, DOI={10.1111/mice.12360}, abstractNote={AbstractConnected vehicle technology, the Internet of Things, and other advanced communication technologies create possibilities to facilitate the movement of vehicles through transportation networks and reduce their travel time. Harmonizing the speed of vehicles in different network links not only yields a more efficient network capacity utilization, but also regulates the movement of vehicles to achieve a “smoother” flow of traffic. This study develops a mathematical nonlinear formulation for dynamic speed harmonization in urban street networks aiming at improving traffic operations. We have converted the nonlinear problem into a linear program utilizing the fundamental flow–density relationship and developed a model predictive control approach to account for stochastic changes in traffic demand and further improve the efficiency of the developed solution algorithm. Results showed that the algorithm efficiently found dynamic optimal advisory speeds on various network links, and speed harmonization significantly reduced the travel time (up to 5.4%), speed variance (19.8%–29.4%), and the number of stops (8.3%–18.5%), while increasing the average speed (up to 5.9%) and the number of completed trips (up to 4%) in our case study network under all tested demand patterns.}, number={6}, journal={Computer-Aided Civil and Infrastructure Engineering}, publisher={Wiley Online Library}, author={Tajalli, Mehrdad and Hajbabaie, Ali}, year={2018}, pages={510–523} }
 @article{mohebifard_hajbabaie_2018, title={Dynamic traffic metering in urban street networks: Formulation and solution algorithm}, volume={93}, url={https://doi.org/10.1016/j.trc.2018.04.027}, DOI={10.1016/j.trc.2018.04.027}, abstractNote={Traffic metering offers great potential to reduce congestion and enhance network performance in oversaturated urban street networks. This paper presents an optimization program for dynamic traffic metering in urban street networks based on the Cell Transmission Model (CTM). We have formulated the problem as a Mixed-Integer Linear Program (MILP) capable of metering traffic at network gates with given signal timing parameters at signalized intersections. Due to the complexities of the MILP model, we have developed a novel and efficient solution approach that solves the problem by converting the MILP to a linear program and several CTM simulation runs. The solution algorithm is applied to two case studies under different conditions. The proposed solution technique finds solutions that have a maximum gap of 1% of the true optimal solution and guarantee the maximum throughput by keeping some vehicles at network gates and only allowing enough vehicles to enter the network to prevent gridlocks. This is confirmed by comparing the case studies with and without traffic metering. The results in an adapted real-world case study network show that traffic metering can increase network throughput by 4.9–38.9% and enhance network performance.}, journal={Transportation research part C: emerging technologies}, publisher={Elsevier}, author={Mohebifard, Rasool and Hajbabaie, Ali}, year={2018}, pages={161–178} }
 @article{mehrabipour_hajbabaie_2017, title={A cell-based distributed-coordinated approach for network-level signal timing optimization}, volume={32}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021053257&partnerID=MN8TOARS}, DOI={10.1111/mice.12272}, abstractNote={AbstractThis article develops an efficient methodology to optimize the timing of signalized intersections in urban street networks. Our approach distributes a network‐level mixed‐integer linear program (MILP) to intersection level. This distribution significantly reduces the complexity of the MILP and makes it real‐time and scalable. We create coordination between MILPs to reduce the probability of finding locally optimal solutions. The formulation accounts for oversaturated conditions by using an appropriate objective function and explicit constraints on queue length. We develop a rolling‐horizon solution algorithm and apply it to several case‐study networks under various demand patterns. The objective function of the optimization program is to maximize intersection throughput. The comparison of the obtained solutions to an optimal solution found by a central optimization approach (whenever possible) shows a maximum of 1% gap on a number of performance measures over different conditions.}, number={7}, journal={Computer-Aided Civil and Infrastructure Engineering}, publisher={Wiley Online Library}, author={Mehrabipour, Mehrzad and Hajbabaie, Ali}, year={2017}, pages={599–616} }
 @article{al islam_hajbabaie_2017, title={Distributed coordinated signal timing optimization in connected transportation networks}, volume={80}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85019418628&partnerID=MN8TOARS}, DOI={10.1016/j.trc.2017.04.017}, abstractNote={This paper presents a Distributed-Coordinated methodology for signal timing optimization in connected urban street networks. The underlying assumption is that all vehicles and intersections are connected and intersections can share information with each other. The novelty of the work arises from reformulating the signal timing optimization problem from a central architecture, where all signal timing parameters are optimized in one mathematical program, to a decentralized approach, where a mathematical program controls the timing of only a single intersection. As a result of this distribution, the complexity of the problem is significantly reduced thus, the proposed approach is real-time and scalable. Furthermore, distributed mathematical programs continuously coordinate with each other to avoid finding locally optimal solutions and to move towards global optimality. We proposed a real-time and scalable solution technique to solve the problem and applied it to several case study networks under various demand patterns. The algorithm controlled queue length and maximized intersection throughput (between 1% and 5% increase compared to the actuated coordinated signals optimized in VISTRO) and reduced travel time (between 17% and 48% decrease compared to actuated coordinated signals) in all cases.}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier}, author={Al Islam, SMA Bin and Hajbabaie, Ali}, year={2017}, pages={272–285} }
 @article{hajbabaie_kim_schroeder_aghdashi_rouphail_tabrizi_2017, title={Estimation of saturation headway in work zones on urban streets}, volume={2615}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85015163407&partnerID=MN8TOARS}, DOI={10.3141/2615-04}, abstractNote={ Work zones and lane closures on urban arterials can cause significant disruptions to the traveling public, and methods are increasingly needed to estimate the reductions to saturation flow rates that result from work zones at signalized intersections. A set of statistical models that estimate saturation headways as a function of the presence and configuration of the work zone on signalized arterial streets is presented. More than 10,000 individual vehicular headway observations were collected from video observations in and after work zones at six study sites in North Carolina. Conventional multiple-regression and path-based-regression models (structural equation model) were used to develop the saturation headway models. Three models are provided at different aggregation levels of the collected data with identical work zone configurations. The models developed at cycle-length, 15-min, and full aggregation produced adjusted R-squared values of .3259, .7209, and .895, respectively. The proposed model incorporates the effects of lane configuration, pavement condition, turning percentage from shared lanes, work intensity, and number of closed exclusive turning lanes. Based on path analysis, the structural equation model satisfies all the rule-of-thumb criteria for goodness-of-fit indices. The model uses Highway Capacity Manual default values for turning-vehicle headway effect as its intercept coefficient value. }, number={1}, journal={Transportation research record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Hajbabaie, Ali and Kim, SangKey and Schroeder, Bastian J and Aghdashi, Seyedbehzad and Rouphail, Nagui M and Tabrizi, Kambiz}, year={2017}, pages={26–34} }
 @article{yeom_hajbabaie_rouphail_rasdorf_schroeder_2017, title={Freeway work zone free-flow speed model development}, volume={87}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85044119212&partnerID=MN8TOARS}, number={11}, journal={ITE Journal (Institute of Transportation Engineers)}, author={Yeom, C. and Hajbabaie, A. and Rouphail, N.M. and Rasdorf, W. and Schroeder, B.J.}, year={2017}, pages={38–44} }
 @article{tajalli_hajbabaie_2017, title={On the relationships between commuting mode choice and public health}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85008457031&partnerID=MN8TOARS}, DOI={10.1016/j.jth.2016.12.007}, abstractNote={This paper studies the associations that may exist between commuting mode choice and public health. For this purpose, we used Community Health Survey data collected in New York City in 2010. Obesity, blood pressure, and diabetes are used as indicators of respondents' physical health, and Non-Specific Psychological Distress as an indicator of respondents' mental health. After rigorous statistical analyses, a binary probit model was fitted for each physical and mental health indicator to quantify the associations between different commuting modes and physical/mental health. Results show that walking, as expected, is associated with a lower probability of obesity, hypertension, diabetes, and mental disorders (all statistically significant) when compared to using private transportation. Using subway is related to a lower probability of obesity and diabetes while using the city bus was linked with a higher probability of obesity (all statistically significant) compared to using personal vehicles. Finally, in comparison with using personal vehicles, working at home is associated with a higher probability of having mental disorders (statistically significant).}, journal={Journal of Transport & Health}, publisher={Elsevier}, author={Tajalli, Mehrdad and Hajbabaie, Ali}, year={2017}, pages={267–277} }
 @article{hajbabaie_aghdashi_rouphail_2016, title={Enhanced Decision-Making Framework Using Reliability Concepts for Freeway Facilities}, volume={142}, ISSN={0733-947X 1943-5436}, url={http://dx.doi.org/10.1061/(ASCE)TE.1943-5436.0000797}, DOI={10.1061/(ASCE)TE.1943-5436.0000797}, abstractNote={This paper presents a decision-making framework based on a travel time reliability methodology developed under the U.S. Strategic Highway Research Program. Existing methods consider a set of predefined prevailing conditions for the analysis of freeway facilities as the base case. However, a reliability analysis accounts for multiple recurring and nonrecurring congestion sources to estimate the travel time distribution over a long time horizon. This approach considers variations in traffic demand levels, inclement weather conditions, and incidents that occur stochastically on a freeway facility. Several performance measures are defined based on the travel time distribution, which comprehensively cover the full range of operational conditions on the system. Based on the proposed decision-making framework, mobility strategies can be identified, evaluated, and improved.}, number={4}, journal={Journal of Transportation Engineering}, publisher={American Society of Civil Engineers (ASCE)}, author={Hajbabaie, Ali and Aghdashi, Seyedbehzad and Rouphail, Nagui M.}, year={2016}, month={Apr}, pages={04016008} }
 @inproceedings{hale_hajbabaie_ma_hu_park_bared_2016, title={Proposed Data-Driven Performance Measures for Comparing and Ranking Traffic Bottlenecks}, volume={15}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84999816139&partnerID=MN8TOARS}, DOI={10.1016/j.trpro.2016.06.041}, abstractNote={To justify investments towards improved traffic operations, engineers and policy-makers need scientific and accurate methods of congestion measurement. However, status-quo methods are limited and/or outdated. Peak-hour analyses are becoming outdated as a sole source of traffic assessment, because they fail to account for changing conditions throughout the year. There has been a movement towards "reliability" modeling, which attempts to capture these annual effects. But due to significant input data and calibration requirements, the reliability models suffer from practicality issues. Next, there have been recent improvements in data-driven ITS technologies, which identify congestion in real time. However, there is room for improvement in the robustness of performance measures derived from these technologies. Finally, some engineers have compared and ranked congested locations (i.e., bottlenecks) on the basis of experience and judgment. Despite their cost-effectiveness, judgment-based qualitative assessments will lack credibility unless backed by quantitative results. In a recent Federal Highway Administration study, congestion measurement was a primary area of emphasis. This paper discusses project-specific software development, which produced new and innovative performance measures for congestion measurement. It will present concepts and evidence to imply superiority of the proposed new measures. This paper is intended to serve as a preview of a future full journal paper; which will rank ten or more real-world bottlenecks according to new and old performance measures, to demonstrate impacts of the new measures. It is hoped that the new performance measures will be adopted by states and/or commercial products, for a new level of robustness in congestion measurement.}, booktitle={Transportation Research Procedia}, author={Hale, D. and Hajbabaie, A. and Ma, J. and Hu, J. and Park, H. and Bared, J.}, year={2016}, pages={483–494} }
 @inproceedings{hajbabaie_rouphail_schroeder_dowling_2015, title={A Planning-Level Methodology for Freeway Facilities}, booktitle={The 94th Annual Meeting of the Transportation Research Board}, author={Hajbabaie, A. and Rouphail, N.M. and Schroeder, B.J. and Dowling, R.}, year={2015} }
 @article{hajbabaie_benekohal_2015, title={A program for simultaneous network signal timing optimization and traffic assignment}, volume={16}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84959530041&partnerID=MN8TOARS}, DOI={10.1109/TITS.2015.2413360}, abstractNote={This study formulates a program for simultaneous traffic signal optimization and system optimal traffic assignment for urban transportation networks with added degree of realism. The formulation presents a new objective function, i.e., weighted trip maximization, and explicit constraints that are specifically designed to address oversaturated conditions. This formulation improves system-wise performance while locally prevents queue spillovers, de-facto reds, and gridlocks. A meta-heuristic algorithm is developed that incorporates microscopic traffic flow models and system optimal traffic assignment in genetic algorithms. This solution technique efficiently optimizes signal timing parameters, at the same time solves system optimal traffic assignment, and accounts for oversaturated conditions and different driver's behaviors. This study also proposes a framework to calculate an upper bound on the value of the objective function by solving the problem while several constraints (i.e., network loading and traffic assignment) are relaxed. An empirical case study for a portion of downtown Springfield, Illinois has been conducted under four demand patterns. Findings indicate that our solution approach can solve the problem effectively. Several managerial insights have also been drawn.}, number={5}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={IEEE}, author={Hajbabaie, Ali and Benekohal, Rahim F}, year={2015}, pages={2573–2586} }
 @article{kim_hajbabaie_williams_rouphail_2016, title={Dynamic Bandwidth Analysis for Coordinated Arterial Streets}, volume={20}, ISSN={["1547-2442"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84941243905&partnerID=MN8TOARS}, DOI={10.1080/15472450.2015.1074575}, abstractNote={A commonly used strategy for improving mobility along signalized arterials is to coordinate neighboring intersections to minimize vehicle stops by maximizing the duration of green bands, otherwise known as arterial bandwidth. Signal coordination has been researched, developed, and refined for five decades. In lieu of traditional methods that are based on the analysis of programmed green times (which assume all phases operate at their maximum settings), a dynamic bandwidth analysis method is presented that reproduces actual dynamic bandwidth durations using closed loop signal data. The analysis is intended to help assess the performance of semi-actuated coordinated signal systems on arterial streets. In addition, the study highlights the arterial progression benefits that result from changing coordinated intersection offsets based on optimizing the dynamic, rather than the programmed, bandwidths. Detailed analysis at three arterial sites revealed that coordinated green phase time distributions are complex and multimodal and cannot be represented by a single-valued statistic. Dynamic bandwidth analysis confirmed that programmed green bandwidth consistently underestimates the size of the actual dynamic bandwidth, and exhaustive search results highlighted the potential for further improvements in coordination. Future research will include field and simulation comparative studies and the development of efficient methods for dynamic bandwidth optimization.}, number={3}, journal={JOURNAL OF INTELLIGENT TRANSPORTATION SYSTEMS}, author={Kim, Sangkey and Hajbabaie, Ali and Williams, Billy M. and Rouphail, Nagui M.}, year={2016}, pages={294–310} }
 @inproceedings{hajbabaie_kim_schroeder_aghdashi_rouphail_song_tabrizi_2015, title={Estimating Saturation Headways at Work Zones on Urban Arterials}, booktitle={The 94th Annual Meeting of the Transportation Research Board}, author={Hajbabaie, A. and Kim, S. and Schroeder, B.J. and Aghdashi, S. and Rouphail, N.M. and Song, T. and Tabrizi, K.}, year={2015} }
 @inproceedings{aghdashi_hajbabaie_rouphail_schroeder_2015, title={Freeway Reliability Scenario Generation: A Hybrid Approach}, booktitle={The 94th Annual Meeting of the Transportation Research Board}, author={Aghdashi, S. and Hajbabaie, A. and Rouphail, N.M. and Schroeder, B.J.}, year={2015} }
 @inproceedings{hajbabaie_yoem_rouphail_rasdorf_schroeder_2015, title={Freeway Work Zone Speed Prediction from Multi-State Sensor Data}, booktitle={The 94th Annual Meeting of the Transportation Research Board}, author={Hajbabaie, A. and Yoem, C. and Rouphail, N.M. and Rasdorf, W.J. and Schroeder, B.J.}, year={2015} }
 @article{aghdashi_hajbabaie_rouphail_schroeder_trask_2015, title={Generating Scenarios of Freeway Reliability Analysis: Hybrid Approach}, volume={2483}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84975722795&partnerID=MN8TOARS}, DOI={10.3141/2483-17}, abstractNote={ The freeway reliability methodology proposed for the Highway Capacity Manual, which is based on SHRP2 L08 methodology, produces an approach to scenario generation that can result in several thousand scenarios to be evaluated to estimate travel time reliability. This large number of scenarios can result in cumbersome user input, a demanding computational burden, and more important, extensive challenges posed when trying to error-check and interpret individual scenarios or to calibrate the model on the basis of real-world observations. This paper presents a novel scenario-generating methodology that accounts for multiple operating conditions. The objective of the proposed approach is to increase the quality of each scenario to make it more representative of the expected congestion patterns on the freeway. This paper shows that the new approach estimates reliability performance measures more accurately than current methods, while reducing the number of scenarios significantly. Thus, the new approach results in a more direct interpretation of results, while simultaneously relaxing many assumptions in the present approach to scenario generation and decreasing biases and errors. The proposed approach uses three core mathematical schemes: (a) a deterministic mathematical model for demand generation and scheduled work zones, (b) a Monte Carlo simulation for incident and weather events, and (c) an optimization algorithm to maximize similarities between the generated set of scenarios and the population of all scenarios. A comparison of results between the proposed method and the SHRP 2 Project L08 approach confirms that the proposed approach yields a higher level of accuracy in matching observed freeway reliability performance measures. }, number={1}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Aghdashi, S. and Hajbabaie, A. and Rouphail, N.M. and Schroeder, B.J. and Trask, J.L.}, year={2015}, pages={148–159} }
 @inproceedings{aghdashi_rouphail_hajbabaie_schroeder_2015, title={Generic Speed Flow Models for Basic Freeway Segments on General Purpose and Managed Lanes}, booktitle={The 94th Annual Meeting of the Transportation Research Board}, author={Aghdashi, S. and Rouphail, N.M. and Hajbabaie, A. and Schroeder, B.J.}, year={2015} }
 @article{aghdashi_rouphail_hajbabaie_schroeder_2015, title={Generic Speed--Flow Models for Basic Freeway Segments on General-Purpose and Managed Lanes in Undersaturated Flow Conditions}, volume={2483}, number={1}, journal={Transportation Research Record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Aghdashi, Seyedbehzad and Rouphail, Nagui M and Hajbabaie, Ali and Schroeder, Bastian J}, year={2015}, pages={102–110} }
 @book{aghdashi_rouphail_hajbabaie_schroeder_2015, title={Generic speed-flow models for basic freeway segments on general-purpose and managed lanes in undersaturated flow conditions}, volume={2483}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84975744162&partnerID=MN8TOARS}, DOI={10.3141/2483-12}, abstractNote={ This paper presents a generic set of undersaturated speed–flow models for basic freeway segments on general purpose and managed lanes (MLs) consistent with the Highway Capacity Manual 2010 (HCM 2010). The proposed models predict segment space mean speed under a wide set of freeway operational conditions that can affect its free-flow speed (FFS) and capacity. Furthermore, the proposed models allow quantifying the impacts of nonrecurring events, such as severe weather conditions, incidents, and work zones on the speed–flow relationship. In addition, the model allows calibration of real-world facilities through adjustments to FFS and capacity. The incorporation of analyses of reliability and active traffic and demand management in the HCM context requires a set of speed–flow models capable of accounting for the effect of nonrecurring sources of congestion. Currently, the HCM 2010 provides a set of speed–flow models to predict space mean speed and consequently other freeway performance measures. This family of equations provides a limited adjustment to FFS. With guidance from NCHRP Project 3-96, separate speed–flow models are proposed for MLs through use of a different form from that in the HCM. The proposed generic equations describing the speed–flow relationship provide consistency between speed–flow relationships of managed and general purpose lanes and can incorporate any capacity or FFS adjustments to predict segment speed under different circumstances. The proposed generic equations are wholly consistent with the speed–flow models in the HCM 2010 and predict the same speed under any flow rate. }, journal={Transportation Research Record}, author={Aghdashi, S. and Rouphail, N.M. and Hajbabaie, A. and Schroeder, B.J.}, year={2015}, pages={102–110} }
 @inproceedings{yoem_hajbabaie_schroeder_vaughan_xuan_rouphail_2015, title={Innovative Work Zone Capacity Models from Nationwide Field and Archival Sources}, booktitle={The 94th Annual Meeting of the Transportation Research Board}, author={Yoem, C. and Hajbabaie, A. and Schroeder, B.J. and Vaughan, C. and Xuan, X. and Rouphail, N.M.}, year={2015} }
 @article{yeom_hajbabaie_schroeder_vaughan_xuan_rouphail_2015, title={Innovative work zone capacity models from nationwide field and archival sources}, volume={2485}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84975853844&partnerID=MN8TOARS}, DOI={10.3141/2485-07}, abstractNote={ Freeway work zone capacity has been the focus of significant research, but most studies were limited to specific geographic regions or work zone configurations or both. To date, no work zone predictive capacity model exists for U.S. freeways that is based on a geographically representative data set or sensitive to key geometric and operational attributes of the work zone. This paper proposes a new capacity model for freeway work zones, developed from nationwide field data fused with data obtained from an in-depth review of archived literature of prior work zone capacity studies in the United States. The proposed model estimates work zone capacity as a function of the lane closure severity index, barrier type, area type, lateral clearances, and daytime or nighttime work conditions. The model was successfully validated with 25% of the combined data that were not used for the model development. In addition, findings from literature archives suggested a 13.4% difference between prebreakdown, uninterrupted capacity and the ensuing queue discharge rate. The developed capacity model is intended to be incorporated into the next release of the Highway Capacity Manual and provides important insights on the relative effects of work zone configuration and other variables on the expected capacity of freeway work zones. }, number={1}, journal={Transportation Research Record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Yeom, Chunho and Hajbabaie, Ali and Schroeder, Bastian J and Vaughan, Christopher and Xuan, Xingyu and Rouphail, Nagui M}, year={2015}, pages={51–60} }
 @article{hajbabaie_rouphail_schroeder_dowling_2015, title={Planning-Level Methodology for Freeway Facilities}, volume={2483}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84975682966&partnerID=MN8TOARS}, DOI={10.3141/2483-06}, abstractNote={ This paper presents a planning-level methodology for the analysis of freeway facilities. The proposed approach is based on and compatible with the operational method of the Highway Capacity Manual 2010 (HCM 2010). The approach is specifically constructed with the intent to minimize input data requirements. The method covers both under- and oversaturated flow conditions and produces estimates of travel time, speed, density, and level of service. The underlying methodology relies on developing a relationship between a basic freeway segment's delay rate per unit distance and its demand-to-capacity ratio. For weaving segments, the study develops capacity adjustment factors on the basis of volume ratio and segment length. With these factors, demand-to-capacity ratios on weave segments were adjusted and the segment was treated similarly to a basic freeway segment. For merge and diverge segments, a novel methodology is proposed to estimate their capacity on the basis of demand level, free-flow speed, and space mean speed. Subsequently, capacity adjustment factors are calculated on those segments and their demand-to-capacity ratios are adjusted accordingly. The proposed approach is applied to two examples in the HCM 2010 and produced very promising results. For undersaturated flow conditions, facility travel time is at most 3.4% and density was at most 1.1% at variance from the results found by applying the HCM 2010 operational methodology. The corresponding differences for oversaturated conditions are 6.7% and 13.0%, respectively. }, number={1}, journal={Transportation Research Record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Hajbabaie, Ali and Rouphail, Nagui M and Schroeder, Bastian J and Dowling, Richard}, year={2015}, pages={47–56} }
 @article{medina_hajbabaie_benekohal_2013, title={Effects of metered entry volume on an oversaturated network with dynamic signal timing}, volume={2356}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84886571705&partnerID=MN8TOARS}, DOI={10.3141/2356-07}, abstractNote={ This paper analyzes the effects of different traffic metering levels at the entry points of a simulated signalized network to maintain efficient vehicle processing. Metering signals were placed along the network perimeter in advance of the bordering intersections to reduce the vehicle arrival rate and prevent oversaturation. In the simulation environment, traffic signals were externally controlled by independent agents using a learning algorithm based on approximate dynamic programming. Agents operated the signals in a cycle-free mode, reacting in real time to current demands and occupancy estimated from detectors placed at the entry and exit points of all links. The metering strategies were analyzed for delay, throughput, network congestion, and queue management. Results indicate that metering have a significant effect on network performance. Metering to levels just below the maximum throughput capacity of an intersection resulted in increased network throughput (up to 5%); reduced delay (up to 10.9%), including vehicles inside and those metered outside of the network; and queue lengths inside the network that allowed efficient use of green time. However, metering to points well below or above the capacity of an intersection did not always provide network improvements. This finding suggests that an optimal congestion level exists inside the network that can be achieved by a metering strategy. An analysis of the metering effects is presented in a case study, and field implementations and scenarios in which metering can be applied are discussed. }, number={1}, journal={Transportation Research Record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Medina, J.C. and Hajbabaie, A. and Benekohal, R.F.}, year={2013}, pages={53–60} }
 @article{medina_hajbabaie_benekohal_2013, title={Effects of metered entry volume on an oversaturated network with dynamic signal timing}, number={2356}, journal={Transportation Research Record}, author={Medina, J. C. and Hajbabaie, A. and Benekohal, R. F.}, year={2013}, pages={53–60} }
 @inproceedings{aghdashi_rouphail_hajbabaie_2013, title={Estimating Incident Propensity Analysis in the Highway Capacity Manual}, booktitle={The 92nd Annual Meeting of the Transportation Research Board}, author={Aghdashi, S. and Rouphail, N.M. and Hajbabaie, A.}, year={2013} }
 @article{aghdashi_rouphail_hajbabaie_2013, title={Estimation of incident propensity for reliability analysis in the Highway Capacity Manual}, volume={2395}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897037320&partnerID=MN8TOARS}, DOI={10.3141/2395-14}, abstractNote={ This paper presents the method used to generate the incident probabilities required by the freeway scenario generator for travel time reliability analysis in the Highway Capacity Manual. The freeway scenario generator requires the estimation of monthly probabilities of different levels of incident severity during specified study periods. Incident probability in this context is the fraction of time that an incident of a specific level of severity is active somewhere on the freeway facility during the study period for the month considered. The proposed method is designed to recognize and deal with the varying levels of incident and facility data availability at the implementing agencies. A queuing model is proposed for the conversion of incident frequencies into incident probabilities when agencies have access only to frequencies instead of probabilities. }, number={1}, journal={Transportation Research Record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Aghdashi, Seyedbehzad and Rouphail, Nagui M and Hajbabaie, Ali}, year={2013}, pages={123–131} }
 @inproceedings{chase_hajbabaie_schroeder_2013, title={Incorporating Weather Effects in HCM Reliability Analysis}, booktitle={The 92nd Annual Meeting of the Transportation Research Board}, author={Chase, R.T. and Hajbabaie, A. and Schroeder, B.J.}, year={2013} }
 @article{hajbabaie_benekohal_2013, title={Traffic Signal Timing Optimization Choosing the Objective Function}, volume={2355}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84886558107&partnerID=MN8TOARS}, DOI={10.3141/2355-02}, abstractNote={ Choosing an appropriate objective function in optimizing traffic signals in urban transportation networks is not a simple and straightforward task because the choice likely will affect the set of constraints, modeling variables, obtained outputs, and necessary computer and human resources. A methodology for selection of an appropriate objective function for the problem of signal timing optimization was developed. The methodology was applied to a realistic case study network under four demand patterns (symmetric, asymmetric, undersaturated, oversaturated). Selection is made from a pool of five candidates: minimizing the delay, minimizing the travel time, maximizing the throughput-minus-queue, maximizing the number of completed trips (or trip maximization), and maximizing the weighted number of completed trips (or weighted trip maximization). Findings indicate that for all demand patterns, weighted trip maximization improved network performance compared with the other objective functions. Weighted trip maximization reduced system total delay by 0.1% to 5.2% in symmetric undersaturated demand, by 1.0% to 2.4% in asymmetric undersaturated demand, by 1.2% to 16.6% in symmetric oversaturated demand, and by 11.7% to 27.4% in asymmetric partially oversaturated demand. These figures indicate that the weighted trip maximization objective function is the most suitable of the candidates in oversaturated conditions, especially when demand is not symmetric. Throughput-minus-queue and trip maximization were the second most suitable objective functions for oversaturated conditions, and trip maximization was slightly more suitable when demand was asymmetric. }, number={2355}, journal={TRANSPORTATION RESEARCH RECORD}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Hajbabaie, Ali and Benekohal, Rahim F.}, year={2013}, pages={10–19} }
 @inproceedings{medina_hajbabaie_benekohal_2011, title={A comparison of approximate dynamic programming and simple genetic algorithm for traffic control in oversaturated conditions - Case study of a simple symmetric network}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-83755196408&partnerID=MN8TOARS}, DOI={10.1109/ITSC.2011.6082999}, abstractNote={The performance of two algorithms for finding traffic signal timings in a small symmetric network with oversaturated conditions was analyzed. The two algorithms include an approximate dynamic programming approach using a “post-decision” state variable (ADP) and a simple genetic algorithm (GA). Results were found by using microscopic simulation and compared based on typical measures of performance (delay, throughput, number of stops) and also on measures that considered the efficiency of green time utilization and queue occupancy of the links. The symmetric characteristics of the small network allowed a straightforward analysis of the operation of the signals, providing some insights on the quality of the solutions. Results showed that even though the solutions from ADP were very different from those in GA, the network performance for both methods was similar, used green time efficiently preventing queue backups, and served all approaches according to current demands. The potential of ADP using the “post-decision” state variable is currently under further analysis using more challenging conditions, additional constraints, and domain knowledge as part of the algorithm formulation.}, booktitle={IEEE Conference on Intelligent Transportation Systems, Proceedings, ITSC}, author={Medina, J.C. and Hajbabaie, A. and Benekohal, R.F.}, year={2011}, pages={1815–1820} }
 @inproceedings{hajbabaie_benekohal_2011, title={Common or Variable Cycle Length Policy for a More Efficient Network Performance?}, url={http://dx.doi.org/10.1061/41167(398)109}, DOI={10.1061/41167(398)109}, abstractNote={To coordinate signals along a corridor or in a network, the conventional wisdom of using a common cycle length for all coordinated intersections has been widely used and has led to very desirable results. On the other hand, using variable cycle length for this purpose may result in good signal coordination as well, however, the effectiveness and appropriateness of this method remains unknown. In this study, we compare the effects of using a common cycle length, to the effects of using a variable cycle length on signal coordination in a network of oversaturated intersections. For this comparison, we have used our microscopic-simulation-based Intelligent Dynamic Signal Timing Optimization Procedure (IDSTOP) that uses Genetic Algorithms to find near-optimal signal timing parameter in a network of oversaturated intersections. IDSTOP determines near-optimal signal timing parameters on a network of oversaturated intersections with a) fixed cycle length policy, and b) variable cycle policy. In both cases, we have made sure that the network was working at its near-optimal condition. In the first policy, IDSTOP determined a common cycle of 144 seconds and offsets and green splits so that in each corridor the through movements were coordinated. For the common cycle policy, the network processed 4854 vehicles that traveled 482 vehicle-miles, and on average each vehicle experienced 1.27 minutes of delay. The analysis of offsets showed that through traffic movements were coordinated in each corridor of the network. In the second policy, IDSTOP determined cycle length that ranged from 136 to 156 seconds. This shows that the cycle lengths were not similar but the range of that was narrow. The network processed 5298 vehicles, that travelled 501 miles, and the delay was 1.23 minutes. With the variable cycle policy the network processed 8.4% vehicles more than the first policy, vehicle miles travelled were significantly increased, and the average delay per vehicle did not change significantly. The analysis of the offsets showed that through traffic movements were coordinated when needed. These figures show a significant improvement over the traditional common cycle policy. The finding, as an example of other conditions that were analyzed, indicated that using variable signal timing policy results in a considerably higher number of vehicles processed by the network as well as a significantly lower delay in the network compared to a common cycle length.}, booktitle={Transportation and Development Institute Congress 2011}, publisher={American Society of Civil Engineers}, author={Hajbabaie, Ali and Benekohal, Rahim F.}, year={2011}, month={Mar}, pages={1138–1146} }
 @article{medina_hajbabaie_benekohal_2011, title={Detection Performance of Wireless Magnetometers at Signalized Intersection and Railroad Grade Crossing under various Weather Conditions}, volume={2259}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.3141/2259-22}, DOI={10.3141/2259-22}, abstractNote={ The performance of wireless magnetometers for vehicle detection was evaluated under favorable and adverse weather conditions at two sites: (a) a signalized intersection with stop bar and advance detection zones and (b) a railroad grade crossing, as a potential backup system to control the operation of gates. Loop detectors and magnetometers were installed at the same locations, providing similar detection areas. Discrepancies between activations from loops and magnetometers provided pointers to potential errors in the systems, and video images were used to verify them visually. At the signalized intersection, the most common type of detection error by the magnetometers in the stop bar zones was a false call caused by vehicles in the adjacent lane, with 5.6% to 7.6% error per lane in favorable weather and 7.7% to 15.4% in winter conditions. At the advance zones the most frequent error in all weather conditions was a missed call caused by vehicles traveling between two lanes, ranging from 2.9% to 9.7% in the left-turn lanes. At the railroad grade crossing, however, most errors were false and stuck-on calls. On average, there was one stuck-on call per every 150 trains, and one per every 2,800 vehicles. False calls varied from 13.4% to 14.1% in the through lane and from 55.9% to 59.9% in the left-turn lane, caused mostly by vehicles traveling in the opposing direction (this was an atypical location with no median separating the two directions of traffic). }, number={1}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Medina, Juan C. and Hajbabaie, Ali and Benekohal, Rahim (Ray) F.}, year={2011}, month={Jan}, pages={233–241} }
 @inproceedings{medina_hajbabaie_benekohal_2011, title={Detection Performance of Wireless Magnetometers at a Signalized and a Railroad Grade Crossing under Different Weather Conditions}, booktitle={The 90th Annual Meeting of the Transportation Research Board}, author={Medina, J.C. and Hajbabaie, A. and Benekohal, R.F.}, year={2011} }
 @inproceedings{hajbabaie_benekohal_2011, title={Does traffic metering improve network performance efficiency?}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-83755196369&partnerID=MN8TOARS}, DOI={10.1109/ITSC.2011.6083011}, abstractNote={Traffic metering at on-ramps in interstate highways has been widely used and led to desirable results. In urban transportation networks when demand reaches network capacity level, traffic metering may also increase network performance efficiency. In this paper, we apply different metering strategies to a case study network to see if they result in a different network operation and potentially a more efficient performance. To make sure if any observed differences in network performance efficiency is due to metering strategies and not due to an inappropriate signal timing, we determine near optimal signal timing of the network by using our Intelligent Dynamic Signal Timing Optimization Program (IDSTOP). IDSTOP incorporates Genetic Algorithms (GAs) with microscopic traffic simulation to find near-optimal signal timing parameters of the network. Our results showed that letting all traffic enter the network or metering a large portion of the traffic are not the best options. Instead metering around 20% of the traffic resulted in the best network performance in terms of average delay (16% reduction compared to no metering and 17% reduction compared to extremely heavy metering strategies), network throughput (18% increase compared to heavy metering), and average travel time (14% reduction compared to no metering and 10% reduction compared to heavy metering). Our findings suggested that in an urban network, there is an optimal point that sending more vehicles into the network than that deteriorates network performance efficiency.}, booktitle={IEEE Conference on Intelligent Transportation Systems, Proceedings, ITSC}, author={Hajbabaie, A. and Benekohal, R.F.}, year={2011}, pages={1114–1119} }
 @inproceedings{medina_hajbabaie_benekohal_2011, title={Evaluation of wireless magnetometers for vehicle detection at signalized intersections}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79955447171&partnerID=MN8TOARS}, DOI={10.1061/41167(398)110}, abstractNote={Wireless magnetometers for vehicle detection were evaluated at a signalized intersection at both stop bar and advance locations. The studied approach had three lanes, and for each lane one sensor was installed at the stop bar and one more at the advance zones. Loop detectors were installed at the same location of the magnetometers and were used as a pointer to identify potential detection errors. Initially, data was collected after the system was installed by the manufacturer. Then, the system setup was adjusted by the manufacturer based on the analysis of the initial data, and data was collected again. Results show that the most frequent error at the stop bar was false calls (5.6%–7.6% due to vehicles in adjacent lanes, and additional 7.8%–9.6% due to single vehicles placing multiple calls), and at the advanced zones it was missed calls (0.9% to 10% per zone). Stuck-on calls and dropped calls were very rare.}, booktitle={T and DI Congress 2011: Integrated Transportation and Development for a Better Tomorrow - Proceedings of the 1st Congress of the Transportation and Development Institute of ASCE}, author={Medina, J.C. and Hajbabaie, A. and Benekohal, R.F.}, year={2011}, pages={1147–1156} }
 @article{wang_benekohal_ramezani_nassiri_medina_hajbabaie_2011, title={Safety and headway characteristics in highway work zones with automated speed enforcement}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79952977853&partnerID=MN8TOARS}, DOI={10.4399/97888548388646}, abstractNote={This paper investigated the headway distribution of platooning vehicles in work zones with and without automated Speed Photo Enforcement (SPE). Data from two work zones were collected and analyzed. The portion of vehicles traveling with a very short headway (less than 0.7 sec) was also examined. In addition, the behavior of vehicles near the law enforcement vehicles was analyzed in terms of applying brakes and changing lane in one of the work zones. The results indicated that the mean headway of cars in the median lane significantly increased in both work zones when the SPE van was present compared to the base condition. For trucks, the mean headway increase was significant only in the shoulder lane when the SPE was present. This is a beneficial effect and indicates that drivers had a longer time to react to the lead vehicles. It was also found that the proportion of cars traveling with a very short headway in the median lane significantly decreased when a SPE van was present at one work zone (I-64). In the shoulder lane, no very short headway was observed when SPE was implemented. The difference in braking behavior was not significant when SPE was compared to police car presence. However, lane changing behavior was significantly different for cars traveling in the median lane and also for trucks traveling in the shoulder lane. The proportion of cars changing lanes was 5 percent with a police car present and increased to 8 percent when the SPE van was present. A significant number of trucks changed lanes from the shoulder to the median when the police car was present to comply with state law.}, number={23}, journal={Advances in Transportation Studies}, author={Wang, M.-H. and Benekohal, R.F. and Ramezani, H. and Nassiri, H. and Medina, J.C. and Hajbabaie, A.}, year={2011}, pages={67–76} }
 @inproceedings{hajbabaie_ramezani_benekohal_2011, title={Speed photo enforcement effects on headways in work zones}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79955383453&partnerID=MN8TOARS}, DOI={10.1061/41167(398)117}, abstractNote={Determining the effects of Speed Photo-radar Enforcement (SPE) on headways of vehicles traveling in a work zone is important since maintaining a larger headway provides drivers with a longer time to avoid a potential rear-end collision. In addition, when the headway between two vehicles is less than the follower’s reaction time, a rear end collision is likely to happen if the leader suddenly decreases its speed. In this paper, the effects of the SPE on the headway of vehicles traveling along a work zone are studied. For this purpose, we collected data from two locations that were around 1.5 miles apart in a work zone on interstate highway 55 at. The SPE van was present at the upstream location. The headway of all vehicles within a period of 45 minutes was measured at the upstream location as well as the downstream location. The results of this study showed that the presence of the SPE did not reduce the headway of the traffic stream at the location that it was present, as well as at the downstream location. This was confirmed by comparing the headway of followers (headway less than four seconds) when the SPE was deployed in the work zone to the headway of them when the SPE was not present, for both lanes combined, and for the shoulder and median lanes separately. The same result was observed for different pairs of leaders and followers. In addition, our analysis showed that the SPE prevented a reduction in the headway of car-car pairs traveling on the median lane at the downstream location.}, booktitle={T and DI Congress 2011: Integrated Transportation and Development for a Better Tomorrow - Proceedings of the 1st Congress of the Transportation and Development Institute of ASCE}, author={Hajbabaie, A. and Ramezani, H. and Benekohal, R.F.}, year={2011}, pages={1226–1234} }
 @article{hajbabaie_medina_wang_benekohal_chitturi_2011, title={Sustained and Halo Effects of Various Speed Reduction Treatments in Highway Work Zones}, volume={2265}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.3141/2265-13}, DOI={10.3141/2265-13}, abstractNote={ This paper analyzes the speed reductions achieved with the use of an automated speed photo–radar enforcement (SPE) system in highway work zones. A comparison with the following three traditional treatments used to reduce speeds is also presented: (a) a speed feedback trailer (trailer treatment), (b) a police patrol car (police treatment), and (c) the combination of a police patrol car and a speed feedback trailer (police–trailer treatment). The results indicated that the SPE system and police–trailer treatments reduced the mean speed of both the general traffic stream and free-flowing vehicles by about 5 to 7 mph. The magnitudes of the speed reductions while the treatments were deployed were sustained over time. Police presence alone also reduced the speed significantly but to a lesser degree, and the effects of the trailer treatment alone were limited. The frequency and the degree of speeding were also influenced by the treatments to various degrees. The proportion of drivers speeding by more than 10 mph was reduced by 8.7% by the SPE system treatment and by 8.9% by the police–trailer and police treatments, which brought the rate of speeding down to 0.2% for the SPE system treatment and 0% for the police–trailer and police treatments. These treatments also reduced the frequency of speeding by 10 mph or less by 36% to 46%. The halo effect for the SPE system after the treatment was removed was limited to a reduction in the mean speed of 2 mph or less, and that for the police treatments was not significant. }, number={1}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Hajbabaie, Ali and Medina, Juan C. and Wang, Ming-Heng and Benekohal, Rahim (Ray) F. and Chitturi, Madhav}, year={2011}, month={Jan}, pages={118–128} }
 @inproceedings{hajbabaie_wang_medina_chitturi_benekohal_2011, title={Sustained and Halo Effects of Various Speed Reduction Treatments in Highway Work Zones}, booktitle={The 90th Annual Meeting of the Transportation Research Board}, author={Hajbabaie, A. and Wang, M. and Medina, J.C. and Chitturi, M.V. and Benekohal, R.F.}, year={2011} }
 @inproceedings{medina_hajbabaie_benekohal_2010, title={Arterial traffic control using reinforcement learning agents and information from adjacent intersections in the state and reward structure}, url={http://dx.doi.org/10.1109/ITSC.2010.5624977}, DOI={10.1109/ITSC.2010.5624977}, abstractNote={An application that uses reinforcement learning (RL) agents for traffic control along an arterial under high traffic volumes is presented. RL agents were trained using Q learning and a modified version of the state representation that included information on the occupancy of the links from neighboring intersections. The proposed structure also includes a reward that considers potential blockage from downstream intersections (due to saturated conditions), as well as pressure to coordinate the signal response with the future arrival of traffic from upstream intersections. Experiments using microscopic simulation software were conducted for an arterial with 5 intersections under high conflicting volumes, and results were compared with the best settings of coordinated pre-timed phasing. Data showed lower delays and less number of stops with RL agents, as well as a more balanced distribution of the delay among all vehicles in the system. Evidence of coordinated-like behavior was found as the number of stops to traverse the 5 intersections was on average lower than 1.5, and also since the distribution of green times from all intersections was very similar. As traffic approached to capacity, however, delays with the pre-timed phasing were lower than with RL agents, but the agents produced lower maximum delay times and lower maximum number of stops per vehicle. Future research will analyze variable coefficients in the state and reward structures for the system to better cope with a wide variety of traffic volumes, including transitions from oversaturation to undersaturation and vice versa.}, booktitle={13th International IEEE Conference on Intelligent Transportation Systems}, publisher={IEEE}, author={Medina, Juan C. and Hajbabaie, Ali and Benekohal, Rahim F.}, year={2010}, month={Sep}, pages={525–530} }
 @article{chitturi_hajbabaie_wang_medina_others_2010, title={Effectiveness of Automated Speed Enforcement in Work Zones}, volume={80}, number={6}, journal={Institute of Transportation Engineers. ITE Journal}, publisher={Institute of Transportation Engineers}, author={Chitturi, Madhav and Hajbabaie, Ali and Wang, Ming-Heng and Medina, Juan C and others}, year={2010}, pages={26} }
 @article{chitturi_benekohal_hajbabaie_wang_medina_2010, title={Effectiveness of automated speed enforcement in work Zones}, volume={80}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77954149350&partnerID=MN8TOARS}, number={6}, journal={ITE Journal (Institute of Transportation Engineers)}, author={Chitturi, M. and Benekohal, R.F. and Hajbabaie, A. and Wang, M.-H. and Medina, J.C.}, year={2010}, pages={26–35} }
 @inproceedings{hajbabaie_medina_benekohal_2010, title={Effects of ITS-based left turn policies on network performance}, url={http://dx.doi.org/10.1109/ITSC.2010.5625269}, DOI={10.1109/ITSC.2010.5625269}, abstractNote={Increasing transportation network capacity is very important, particularly in congested traffic condition. In certain conditions, providing left turn green time could reduce intersection capacity because of: a) larger lost time due to added phase, and b) shorter green time for the through traffic movements. In this study, we will compare the effects of three ITS-based left turn policies on the performance of a congested transportation network. The base policy is allowing left turns in all intersections along a corridor of the network with high traffic demand. The second policy is prohibiting them in some intersections of that corridor, while the third policy is removing these turning movements from all intersections along that corridor. When a left turn is removed, the left turners are rerouted in the network. To make sure that in each policy the network is working at its optimal condition, we used Genetic Algorithms (GAs) to determine optimal signal timing parameters for each policy. The results showed that prohibiting left turns in every other intersection along an arterial of the case study network increased the total number of vehicles processed by the network by 6.6% and resulted in a total of 2550 processed vehicles and reduced average delay per vehicle by 10.7% resulting in delay of 196.3 seconds per vehicle. Removing left turns from all intersections along an arterial of the network increased the total number of vehicles processed by the network by 9.1% and resulted in 2607 processed vehicles. This policy resulted in a decrease in average delay per vehicle by 3.1% and resulted in 213.0 seconds of delay per vehicle. Thus, in periods of heavy traffic demand for through movement and low traffic demand for left turns, prohibiting left turns in all or some intersections of a network could result in a significant increase in the performance of the network as well as a significant decrease in delay per vehicle.}, booktitle={13th International IEEE Conference on Intelligent Transportation Systems}, publisher={IEEE}, author={Hajbabaie, Ali and Medina, Juan C. and Benekohal, Rahim F.}, year={2010}, month={Sep}, pages={80–84} }
 @inproceedings{hajbabaie_benekohal_2010, title={The Effects of Heavy Vehicle Percentage on Passenger Car Equivalency Factor}, booktitle={Proceedings of the TFT 2010 Conference}, author={Hajbabaie, A. and Benekohal, R.F.}, year={2010}, month={Jul} }
 @inproceedings{medina_hajbabaie_benekohal_2010, title={Wireless Magnetometers for Traffic Data Collection at Signalized Intersections}, booktitle={Proceedings of the TFT 2010 Conference}, author={Medina, J.C. and Hajbabaie, A. and Benekohal, R.F.}, year={2010}, month={Jul} }
 @inproceedings{hajbabaie_benekohal_chitturi_wang_medina_2009, title={Comparison of Effects of Automated Speed Enforcement and Police Presence on Speeding in Work Zones}, booktitle={The 88th Annual Meeting of the Transportation Research Board}, author={Hajbabaie, A. and Benekohal, R.F. and Chitturi, M.V. and Wang, M. and Medina, J.C.}, year={2009} }
 @article{medina_benekohal_hajbabaie_wang_chitturi_2009, title={Downstream Effects of Speed Photo--Radar Enforcement and Other Speed Reduction Treatments on Work Zones}, volume={2107}, number={1}, journal={Transportation Research Record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Medina, Juan C and Benekohal, Rahim F and Hajbabaie, Ali and Wang, Ming-Heng and Chitturi, Madhav V}, year={2009}, pages={24–33} }
 @inproceedings{medina_benekohal_hajbabaie_wang_chitturi_2009, title={Downstream Effects of Speed Photo–Radar Enforcement and Other Speed Reduction Treatments on Work Zones}, booktitle={The 88th Annual Meeting of the Transportation Research Board}, author={Medina, J.C. and Benekohal, R.F. and Hajbabaie, A. and Wang, M. and Chitturi, M.V.}, year={2009} }
 @book{medina_benekohal_hajbabaie_wang_chitturi_2009, title={Downstream effects of speed photo-radar enforcement and other speed reduction treatments on work zones}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76249112826&partnerID=MN8TOARS}, DOI={10.3141/2107-03}, abstractNote={ The effects of automated speed photo–radar enforcement (SPE) and traditional speed reduction treatments (speed feedback trailer, presence of police vehicles with emergency lights on and off, and combinations of the speed feedback trailer and police presence) on speed were studied at a location 1.5 mi downstream of the actual treatment (spatial effects). Three data sets from two Interstate highway work zones were used. Field data consistently showed significant spatial (downstream) effects for SPE. The combination of speed feedback trailer and police vehicle with emergency lights off had downstream effects in some cases but to a lesser degree than SPE. Other treatments showed no significant downstream effects. For free-flowing traffic, SPE reduced the average downstream speed by 2 to 3.8 mph for cars and by 0.8 to 5.3 mph for trucks. Also, SPE reduced speeding cars by 7.1% to 23.4% (except for cars in median in Data Set 1), and speeding trucks by 4.2% to 48.3% (except for trucks in shoulder in Data Set 3). For the general traffic stream, SPE reduced the average downstream speed by 1.1 to 2.9 mph on cars and by 0.9 to 3.3 mph on trucks. When SPE was used, the percentage of speeding cars and trucks in the general traffic stream was reduced by 2.9% to 28.6%, and by 7.5% to 36.1%, respectively. SPE also reduced the percentage of cars in the general traffic stream exceeding the speed limit by more than 10 mph in virtually all cases, and eliminated such trucks in all but one case. }, number={2107}, journal={Transportation Research Record}, author={Medina, J.C. and Benekohal, R.F. and Hajbabaie, A. and Wang, M.-H. and Chitturi, M.V.}, year={2009}, pages={24–33} }
 @inproceedings{hajbabaie_benekohal_2009, title={Evolution strategies compared to genetic algorithms in finding optimal signal timing for oversaturated transportation network}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77955445784&partnerID=MN8TOARS}, booktitle={IJCCI 2009 - International Joint Conference on Computational Intelligence, Proceedings}, author={Hajbabaie, A. and Benekohal, R.F.}, year={2009}, pages={298–301} }
 @article{benekohal_wang_chitturi_hajbabaie_medina_2009, title={Speed Photo--Radar Enforcement and Its Effects on Speed in Work Zones}, volume={2096}, number={1}, journal={Transportation research record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Benekohal, Rahim F and Wang, Ming-Heng and Chitturi, Madhav V and Hajbabaie, Ali and Medina, Juan C}, year={2009}, pages={89–97} }
 @inproceedings{benekohal_wang_chitturi_hajbabaie_medina_2009, title={Speed Photo–Radar Enforcement and Its Effects on Speed in Work Zones}, booktitle={The 88th Annual Meeting of the Transportation Research Board}, author={Benekohal, R.F. and Wang, M. and Chitturi, M.V. and Hajbabaie, A. and Medina, J.C.}, year={2009} }
 @book{benekohal_wang_chitturi_hajbabaie_medina_2009, title={Speed photo-radar enforcement and Its effects on speed in work zones}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76649143367&partnerID=MN8TOARS}, DOI={10.3141/2096-12}, abstractNote={ Automated speed photo–radar enforcement (SPE) in work zones was implemented for the first time in the United States in Illinois. This paper presents the results of the effectiveness of SPE on the basis of three data sets collected in two work zones. SPE was effective in reducing the average speed and increasing compliance with the work zone speed limit in all three data sets. In almost all cases in which SPE was implemented, the average speeds were significantly lower than the work zone speed limit. The average free-flowing speed of cars was reduced by 4.2 to 7.9 mph, and that of trucks by 3.4 to 6.9 mph. SPE reduced the percentage of cars and heavy vehicles exceeding the speed limit significantly. The percentages of free-flowing cars exceeding the speed limit were reduced from 39.8% to 8.3% in Data Set 1, from 30.4% to 4.2% in Data Set 2, and from 93.2% to 45.5% in Data Set 3. The percentages of free-flowing heavy vehicles exceeding the speed limit were reduced from 17.3% to 4.2% in Data Set 1; from 6.1% to 1.2% in Data Set 2; and from 69.2% to 13.9% in Data Set 3. Trucks did not exceed the speed limit by more than 10 mph in any of the data sets when SPE was implemented. In two data sets no cars exceeded the speed limit by more than 10 mph, while in the third data set only 2.5% did. Field data were also collected after the SPE van left the work zone to examine the halo (temporal) effects of SPE. SPE had a halo effect of 1.8∼2.7 mph on free-flowing trucks in one work zone but none in the other work zone. The halo effect of SPE on free-flowing cars was a limited 1.2 mph on the shoulder lane in only one data set. }, number={2096}, journal={Transportation Research Record}, author={Benekohal, R.F. and Wang, M.-H. and Chitturi, M.V. and Hajbabaie, A. and Medina, J.C.}, year={2009}, pages={89–97} }
 @inproceedings{benekohal_hajbabaie_chitturi_wang_medina_2008, title={Automated Speed Enforcement Effects on Speeding and Platooning in Work Zones}, booktitle={The 10th International Conference on Applications of Advanced Technologies in Transportation}, author={Benekohal, R.F. and Hajbabaie, A. and Chitturi, M.V. and Wang, M.-H. and Medina, J.C.}, year={2008} }
 @inproceedings{benekohal_chitturi_hajbabaie_wang_medina_2008, title={Automated Speed Photo Enforcement Effects on Speeds in Work Zones}, booktitle={The 87th Annual Meeting of the Transportation Research Board}, author={Benekohal, R.F. and Chitturi, M.V. and Hajbabaie, A. and Wang, M.-H. and Medina, J.C.}, year={2008} }
 @article{benekohal_chitturi_hajbabaie_wang_medina_2008, title={Automated speed photo enforcement effects on speeds in work zones}, volume={2055}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-56749177408&partnerID=MN8TOARS}, DOI={10.3141/2055-02}, abstractNote={ Automated speed enforcement in construction zones has the potential to increase compliance with the speed limit and improve safety. The effectiveness of speed photo enforcement (SPE) (by radar) in reducing speeds and increasing speed limit compliance in work zones was evaluated for the first time in the United States, at Illinois work zones. Details are presented on SPE implementation and its effectiveness at the point it was stationed and at a downstream location in a work zone. Speed data were collected at the location of SPE and at a location 1.5 mi downstream in the work zone to determine the point and spatial effects of SPE. Speeds were measured for free-flowing and platooned cars and heavy vehicles in shoulder and median lanes. Results showed that SPE is effective in reducing the average speed and increasing compliance with work zone speed limit. The SPE reduced speed in the median lane more than in the shoulder lane, as expected. In addition, the speed of free-flowing vehicles was reduced more than for platooned vehicles. The reduction of the mean speed varied from 3.2 to 7.3 mph. The percentage of vehicles exceeding the speed limit near SPE was reduced from about 40% to 8% for free-flowing cars and from 17% to 4% for free-flowing heavy vehicles. Near the SPE van, none of the cars exceeded the speed limit by more than 10 mph, and none of the heavy vehicles exceeded it by more than 5 mph. The data also showed a mixed spatial effect for SPE. At the downstream location, the speed reduction for cars was not significant, while it varied from 0.9 to 2.5 mph for heavy vehicles. }, number={1}, journal={Transportation Research Record}, publisher={SAGE Publications Sage CA: Los Angeles, CA}, author={Benekohal, Rahim F and Chitturi, Madhav V and Hajbabaie, Ali and Wang, Ming-Heng and Medina, Juan C}, year={2008}, pages={11–20} }