@article{ahmed_karr_rouphail_chase_2023, title={Modeling Framework for Predicting Lane Change Intensity at Freeway Weaving Segments}, volume={4}, ISSN={["2169-4052"]}, url={https://doi.org/10.1177/03611981231165206}, DOI={10.1177/03611981231165206}, abstractNote={ This study proposes a modeling framework for predicting discretionary lane change (DLC) intensity at weaving segments using traffic flow and site data. The database used to develop the models comprises 294 field observations from 19 sites. Two modeling techniques, using regression trees and linear regression, were employed to predict DLCs per hour and DLCs per vehicle. The proposed models were compared with the lane change model for weaving segments in the Highway Capacity Manual (HCM7). The lane change data were clustered by site, which cautioned the applicability of linear regression for this dataset. Nonetheless, both the regression tree and linear regression models yielded high R-squared values, varying from 0.93 to 0.96. The relative root mean squared error (RMSE)—the ratio of the error to the mean values—varied between 0.18 and 0.30. However, a site-specific validation showed that the linear regression models performed poorly for most sites, although measures were taken to cope with outliers, nonlinearity, and interactions. The tree model improved the prediction of DLCs per hour for more than two-thirds of the sites when compared with the mean value at each site. It also performed well in most cases when applied to a site that was omitted from the model development. The HCM7 model performed well when applied to an omitted site. However, it exhibited the highest overall relative RMSE (0.57), underscoring the necessity of advanced modeling tools with additional predictors. We recommend incorporating observations from more extended periods and varying traffic conditions for each site for future research. }, journal={TRANSPORTATION RESEARCH RECORD}, author={Ahmed, Ishtiak and Karr, Alan and Rouphail, Nagui M. and Chase, R. Thomas}, year={2023}, month={Apr} }
 @article{das_ahmed_williams_rouphail_2023, title={Response time of mixed platoons with traditional and autonomous vehicles in field trials: impact assessment on flow stability and safety}, volume={12}, ISSN={["2324-9943"]}, url={https://doi.org/10.1080/23249935.2023.2298498}, DOI={10.1080/23249935.2023.2298498}, abstractNote={This study investigates the response times of autonomous vehicles (AVs) equipped with adaptive cruise control (ACC) and traditional human-driven vehicles (TVs) in mixed traffic scenarios. The primary objective is to assess how these response times impact the stability and safety of mixed traffic flow, considering the growing prevalence of ACC technology in vehicles worldwide. Utilising a trajectory dataset from OpenACC totalling 3389.70 s, this research introduces a response time estimation framework that combines cross-correlation and partial autocorrelation techniques. The study calibrates Gazis, Herman, and Rothery's (GHR) car-following model to evaluate mixed traffic flow stability and employs a modified time-to-collision (MTTC) surrogate for safety analysis. The study also delves into the influence of vehicle manufacturer diversity on study outcomes. Key findings reveal that the AVs exhibit significantly longer response times, ranging from 1.10–3.20 s, compared to the 0.30–1.90-second range of traditional vehicles (p value < 0.005). These extended response times in AVs contribute to prolonged traffic flow instability and increased traffic conflicts. Moreover, the type of lead vehicle does not significantly affect the response times of either AVs or TVs (p value > 0.005). The study also highlights that vehicle manufacturer diversity does not substantially affect these response times. Additionally, the examination of fitted GHR parameters underscores AVs’ heightened sensitivity to spacing and relative speed, providing insights into AV dynamics in the presence of mixed traffic.}, journal={TRANSPORTMETRICA A-TRANSPORT SCIENCE}, author={Das, Tanmay and Ahmed, Ishtiak and Williams, Billy M. and Rouphail, Nagui M.}, year={2023}, month={Dec} }
 @article{ahmed_karr_rouphail_chase_tanvir_2022, title={Characterizing lane changing behavior and identifying extreme lane changing traits}, volume={4}, ISSN={["1942-7875"]}, url={https://doi.org/10.1080/19427867.2022.2066856}, DOI={10.1080/19427867.2022.2066856}, abstractNote={ABSTRACT This study characterizes lane changing behavior of drivers under differing congestion levels and identifies extreme lane changing traits using high-resolution trajectory data. Total lane change frequency exhibited a reciprocal relationship with congestion level, but the distribution of lane change per vehicle remained unchanged as congestion increased. On average, the speed of trajectories increased by 5.4 ft/s after changing a lane. However, this gain significantly diminished as congestion worsened. Further, the average speed of lane changing vehicles was 3.9 ft/s higher than those that executed no lane changes. Two metrics were employed to identify extreme lane changing behavior: critical time-to-line-crossing (TLCc) and lane changes per unit distance. The lowest 1% TLCc varied between 0.71–1.57 seconds. The highest 1% of lane change rates for all lane changing vehicles was 2.5 lane changes per 1,000 ft traveled. Interestingly, no drivers in thisdataset had both excessive lane changes and lane changes with low TLCc.}, journal={TRANSPORTATION LETTERS-THE INTERNATIONAL JOURNAL OF TRANSPORTATION RESEARCH}, publisher={Informa UK Limited}, author={Ahmed, Ishtiak and Karr, Alan F. and Rouphail, Nagui M. and Chase, R. Thomas and Tanvir, Shams}, year={2022}, month={Apr} }
 @article{ahmed_williams_samandar_chun_2022, title={Investigating the relationship between freeway rear-end crash rates and macroscopically modeled reaction time}, volume={18}, ISSN={["2324-9943"]}, url={https://doi.org/10.1080/23249935.2021.1914769}, DOI={10.1080/23249935.2021.1914769}, abstractNote={This study tests the hypothesis that an analytically estimated driver reaction time required for asymptotic stability, based on the macroscopic Gazis-Herman-Rothery (GHR) model, serves as an indicator of the impact of traffic oscillations on rear-end crashes. If separate GHR models are fit discontinuously for different traffic regimes, the local drop in required reaction time between these regimes can also be estimated. This study evaluates the relationship between rear-end crash rates and that drop in required reaction time. Traffic data from 28 sensors were used to fit the GHR model. Rear-end crash rates, estimated from four years of crash data, exhibited a positive correlation with the drop in required reaction time at the congested regime’s density-breakpoint. A linear relationship provided the best fit. These results motivate follow-on research to incorporate macroscopically derived reaction time in road-safety planning. More generally, the study demonstrates a useful application of a discontinuous macroscopic traffic model.}, number={3}, journal={TRANSPORTMETRICA A-TRANSPORT SCIENCE}, publisher={Informa UK Limited}, author={Ahmed, Ishtiak and Williams, Billy M. and Samandar, M. Shoaib and Chun, Gyounghoon}, year={2022}, month={Dec}, pages={1001–1024} }
 @article{avr_tanvir_rouphail_ahmed_2021, title={Dynamically Collected Local Density using Low-Cost Lidar and its Application to Traffic Models}, volume={5}, ISSN={["2169-4052"]}, DOI={10.1177/03611981211010184}, abstractNote={ This article demonstrates the use of traffic density observations collected dynamically in the vicinity of probe vehicles. Fixed position sensors cannot capture the longitudinal evolution of local traffic density in the corridor. In this research, dynamic traffic density observations were collected in a naturalistic driving setting that was free of any controlled experiment biases. Speed from global positioning system and space headway from a light detection and ranging module was collected on one arterial and one freeway segment, 2 and 4 mi long, respectively. The combined data frequency was approximately 3 Hz. Space headway was used to estimate the local density and consequently to identify the density of a specific location in a corridor. Besides, driver behavior was characterized using the relationship between instantaneous speed and local density under different regimes of the Wiedemann car-following model. Macroscopic traffic stream models were used to investigate the relationship between dynamically collected instantaneous speed and local density. Using the longitudinal evolution of density, precise local density across the corridor can be obtained along with the leader and follower trajectories. A method to identify driver behavior across density ranges was developed for different facility types using a microscopic relationship between instantaneous speed and local density. Overall driving behavior on the freeway segment can be represented by translating the instantaneous speed and local density relationship to macroscopic stream models. }, journal={TRANSPORTATION RESEARCH RECORD}, author={Avr, Azhagan and Tanvir, Shams and Rouphail, Nagui M. and Ahmed, Ishtiak}, year={2021}, month={May} }
 @article{ahmed_warchol_cunningham_rouphail_2021, title={Mobility Assessment of Pedestrian and Bicycle Treatments at Complex Continuous Flow Intersections}, volume={147}, ISSN={["2473-2893"]}, url={https://doi.org/10.1061/JTEPBS.0000512}, DOI={10.1061/JTEPBS.0000512}, abstractNote={AbstractThis study evaluated the mobility performance of pedestrian-bicycle crossing alternatives at continuous flow intersections (CFIs). CFI crossing types were compared with a standard intersect...}, number={5}, journal={JOURNAL OF TRANSPORTATION ENGINEERING PART A-SYSTEMS}, publisher={American Society of Civil Engineers (ASCE)}, author={Ahmed, Ishtiak and Warchol, Shannon and Cunningham, Chris and Rouphail, Nagui}, year={2021}, month={May} }
 @article{xu_rouphail_aghdashi_ahmed_elefteriadou_2020, title={Modeling Framework for Capacity Analysis of Freeway Segments: Application to Ramp Weaves}, volume={2674}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85085987562&partnerID=MN8TOARS}, DOI={10.1177/0361198119900157}, abstractNote={ This research proposes a new modeling framework for the analysis of freeway segments. The framework provides a continuum from the operation of ramp weave segments to an equivalent basic segment serving the same traffic with the same number of lanes and free-flow speed. This approach distinguishes between congestion effects caused by high v/c ratios from turbulence caused by merging, diverging, and weaving traffic, thus greatly simplifying the model form, and its extensibility to other freeway segment types. The paper presents an application of this new framework to the analysis of ramp weaves, which were not sufficiently sampled in the development of the HCM6 methodology. The proposed model is shown to be superior to the HCM6 method both in relation to explaining field observations of speeds and in its simplicity in application. The results include a new formula for capacity estimation that is highly sensitive to segment length, and a speed estimation model that converges for low weaving volumes or at very high weaving segment lengths to that observed at a basic segment. Because the proposed model is calibrated with data mostly from North Carolina, it is recommended that data at additional sites be included in a larger calibration effort to ensure its applicability to a broader set of weaving segment configurations. }, number={1}, journal={TRANSPORTATION RESEARCH RECORD}, author={Xu, Dezhong and Rouphail, Nagui M. and Aghdashi, Behzad and Ahmed, Ishtiak and Elefteriadou, Lily}, year={2020}, month={Jan}, pages={148–159} }
 @article{berglund_monroe_ahmed_noghabaei_do_pesantez_khaksar fasaee_bardaka_han_proestos_et al._2020, title={Smart Infrastructure: A Vision for the Role of the Civil Engineering Profession in Smart Cities}, volume={26}, ISSN={1076-0342 1943-555X}, url={http://dx.doi.org/10.1061/(ASCE)IS.1943-555X.0000549}, DOI={10.1061/(ASCE)IS.1943-555X.0000549}, abstractNote={AbstractSmart city programs provide a range of technologies that can be applied to solve infrastructure problems associated with ageing infrastructure and increasing demands. The potential for infr...}, number={2}, journal={Journal of Infrastructure Systems}, publisher={American Society of Civil Engineers (ASCE)}, author={Berglund, Emily Zechman and Monroe, Jacob G. and Ahmed, Ishtiak and Noghabaei, Mojtaba and Do, Jinung and Pesantez, Jorge E. and Khaksar Fasaee, Mohammad Ali and Bardaka, Eleni and Han, Kevin and Proestos, Giorgio T. and et al.}, year={2020}, month={Jun}, pages={03120001} }
 @article{ahmed_karr_rouphail_chun_tanvir_2019, title={Characterizing Lane Changes via Digitized Infrastructure and Low-Cost GPS}, volume={2673}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85064917506&partnerID=MN8TOARS}, DOI={10.1177/0361198119841277}, abstractNote={ With the expected increase in the availability of trajectory-level information from connected and autonomous vehicles, issues of lane changing behavior that were difficult to assess with traditional freeway detection systems can now begin to be addressed. This study presents the development and application of a lane change detection algorithm that uses trajectory data from a low-cost GPS-equipped fleet, supplemented with digitized lane markings. The proposed algorithm minimizes the effect of GPS errors by constraining the temporal duration and lateral displacement of a lane change detected using preliminary lane positioning. The algorithm was applied to 637 naturalistic trajectories traversing a long weaving segment and validated through a series of controlled lane change experiments. Analysis of naturalistic trajectory data revealed that ramp-to-freeway trips had the highest number of discretionary lane changes in excess of 1 lane change/vehicle. Overall, excessive lane change rates were highest between the two middle freeway lanes at 0.86 lane changes/vehicle. These results indicate that extreme lane changing behavior may significantly contribute to the peak-hour congestion at the site. The average lateral speed during lane change was 2.7 fps, consistent with the literature, with several freeway–freeway and ramp–ramp trajectories showing speeds up to 7.7 fps. All ramp-to-freeway vehicles executed their first mandatory lane change within 62.5% of the total weaving length, although other weaving lane changes were spread over the entire segment. These findings can be useful for implementing strategies to lessen abrupt and excessive lane changes through better lane pre-positioning. }, number={8}, journal={TRANSPORTATION RESEARCH RECORD}, author={Ahmed, Ishtiak and Karr, Alan and Rouphail, Nagui M. and Chun, Gyounghoon and Tanvir, Shams}, year={2019}, month={Aug}, pages={298–309} }
 @article{ahmed_xu_rouphail_karr_2019, title={Lane Change Rates at Freeway Weaving Sites: Trends in HCM6 and from NGSIM Trajectories}, volume={2673}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85064555642&partnerID=MN8TOARS}, DOI={10.1177/0361198119841281}, abstractNote={ Concerns have been raised about the HCM6 weaving method’s lack of sensitivity to weaving segment length. This study explores the trends in HCM6 as they relate to lane change estimates and their impact on the segment speed and level of service (LOS). The study also compares HCM6 estimates of lane changes against empirical data from an NGSIM weaving site. Thus, the objectives of this study are twofold: ( a) critically investigate the effect of weaving length on lane change and associated speed model estimates in HCM6, and ( b) analyze trends in lane changes against congestion levels using detailed NGSIM trajectory data, comparing against HCM6 estimates. For ( a) it was found that the lack of sensitivity to weave length is because of the absence of this parameter in the nonweaving lane change and speed models. For ( b), a comparison of HCM6 lane change rates with NGSIM, US-101 data confirmed that the HCM6 estimates for weaving vehicles are fully consistent with those at the NGSIM site, controlling for density. In contrast, nonweaving lane change estimates in HCM6 did not deliver the expected trends, with more discretionary lane changes predicted as congestion increased. Finally, analysis of lane change patterns at the NGSIM site revealed a tendency for early merging for freeway to ramp traffic and uniform merging for ramp to freeway traffic over the length of the weave. Interestingly, a speed analysis showed that in most cases, a higher frequency of discretionary lane changes yielded lower travel times for drivers executing them. }, number={5}, journal={TRANSPORTATION RESEARCH RECORD}, author={Ahmed, Ishtiak and Xu, Dezhong and Rouphail, Nagui and Karr, Alan}, year={2019}, month={May}, pages={627–636} }
 @article{ahmed_williams_samandar_2018, title={Application of a Discontinuous Form of Macroscopic Gazis-Herman-Rothery Model to Steady-State Freeway Traffic Stream Observations}, volume={2672}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85060947455&partnerID=MN8TOARS}, DOI={10.1177/0361198118799166}, abstractNote={In this study, a two-regime, steady-state, traffic stream model is developed by applying the macroscopic Gazis–Herman–Rothery model to fixed sensor data on freeways. The uncongested and congested regimes are modeled discontinuously with an overlap range defined in terms of density. The overlap is important as various phenomena related to the change in traffic state can be modeled by introducing this overlap. Two empirical tools for removing non-stationary, mixed-state, and erroneous observations are applied at different stages of the model development process. Three constraints justified by the Highway Capacity Manual (HCM) were applied to fit the model so that the fitted parameters have reasonable and physically interpretable values. The proposed model is applied to one year of data (2013) obtained from fixed sensors located at five freeway basic segments near Raleigh, North Carolina. The resulting fundamental diagrams show that the fitted models reasonably represent the steady-state observations. Two forms of the freeway flow model described in the HCM were applied to the same observations to provide a continuous model comparison. Two statistical performance measures, mean squared error of flow rate and Bayesian Information Criterion, verify that the proposed model is preferable to the HCM models both in terms of fit alone and when considering the tradeoff between fit and model complexity. It is expected that the proposed discontinuous steady-state model will be useful to researchers and practitioners to study various site-specific freeway traffic stream characteristics.}, number={20}, journal={TRANSPORTATION RESEARCH RECORD}, author={Ahmed, Ishtiak and Williams, Billy M. and Samandar, M. Shoaib}, year={2018}, month={Dec}, pages={51–62} }
 @article{ahmed_rouphail_tanvir_2018, title={Characteristics and Temporal Stability of Recurring Bottlenecks}, volume={2672}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85060989618&partnerID=MN8TOARS}, DOI={10.1177/0361198118798991}, abstractNote={ This study applies and updates a method which identifies and quantifies the extent of traffic congestion from recurring freeway bottlenecks. Additionally, the spatiotemporal stability of bottlenecks over an extended period was tested. Over time congestion at bottlenecks may increase, may decrease, or may migrate to other nearby locations. Analysis of stability is important since prioritizing and applying treatments at bottlenecks is a multiyear process. In addition, a robust method for selecting sensitivity based parameters to identify and quantify bottleneck effects is presented. Subsequently, a systematic framework is developed for tracking and archiving the spatiotemporal changes in the recurring bottlenecks. The proposed method is demonstrated on a case study on Interstate 40 in North Carolina using three years of probe data. A congestion speed ratio detection threshold of 0.7 and a probability of activation threshold of 33% for the study area were determined from a sensitivity test to ascertain their recurrence. The method identified 13 bottlenecks with their impacts ranging from 35 to 3,278 mi-hours of congestion per year. Eight bottlenecks either newly emerged or had their queues merged or shifted between successive years. Even spatially stable bottlenecks had significant variation in their activation frequency and queue length. Exploration of the changes in bottleneck severity and locations revealed the influence of a long-term work zone in the area and the effect of the rapid growth in traffic demand. Local agencies can use this method to shortlist recurring bottlenecks and track changes to plan mitigation strategies. }, number={42}, journal={TRANSPORTATION RESEARCH RECORD}, author={Ahmed, Ishtiak and Rouphail, Nagui M. and Tanvir, Shams}, year={2018}, month={Dec}, pages={235–246} }
 @article{samandar_williams_ahmed_2018, title={Weigh Station Impact on Truck Travel Time Reliability: Results and Findings from a Field Study and a Simulation Experiment}, volume={2672}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85052711188&partnerID=MN8TOARS}, DOI={10.1177/0361198118791667}, abstractNote={ Weigh stations are necessary for safeguarding highway infrastructure by enforcing truck weight limits. However, mandating all trucks to stop at all weigh stations decreases travel time reliability. This decrease in travel time reliability adversely impacts the productivity of the trucking industry and to a lesser degree impacts personal travel reliability as well. This study, conducted at the Lumberton weigh station on Interstate 95 in North Carolina, quantifies the impact of weigh stations on truck travel time reliability. Truck travel times were observed over periods of weigh station operation and weigh station closure. Comparison of these two states sheds light on the variability in travel time caused by weigh station operation. Results show that when the weigh station is operational, truck travel time reliability degrades significantly. VISSIM microsimulation software was used to quantify the expected impact of weigh in motion (WIM) on truck travel time reliability assuming different scenarios of WIM truck bypass. The model results indicate that WIM technology does increase travel time reliability and provides benefits to both trucking companies and enforcement agencies. It was also found that increases in the proportion of WIM bypass result in increases in route travel time reliability. However, this simulation model improvement was not uniform, with the highest marginal improvement occurring in the 30%–40% WIM bypass range. }, number={9}, journal={TRANSPORTATION RESEARCH RECORD}, author={Samandar, M. Shoaib and Williams, Billy M. and Ahmed, Ishtiak}, year={2018}, month={Dec}, pages={120–129} }