@article{das_samandar_rouphail_williams_2023, title={A Limited, Real-World Assessment of Key Autonomous Vehicle Car Following Models}, ISSN={["2153-0009"]}, DOI={10.1109/ITSC57777.2023.10421835}, journal={2023 IEEE 26TH INTERNATIONAL CONFERENCE ON INTELLIGENT TRANSPORTATION SYSTEMS, ITSC}, author={Das, Tanmay and Samandar, Shoaib and Rouphail, Nagui and Williams, Billy}, year={2023}, pages={966–972} }
 @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_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{hasnat_bardaka_samandar_rouphail_list_williams_2021, title={Impacts of Private Autonomous and Connected Vehicles on Transportation Network Demand in the Triangle Region, North Carolina}, volume={147}, ISSN={["1943-5444"]}, url={https://doi.org/10.1061/(ASCE)UP.1943-5444.0000649}, DOI={10.1061/(ASCE)UP.1943-5444.0000649}, abstractNote={Abstract Autonomous and connected vehicle technologies have the potential to bring profound changes in travel behavior and transportation network performance with moderate to significant market pen...}, number={1}, journal={JOURNAL OF URBAN PLANNING AND DEVELOPMENT}, publisher={American Society of Civil Engineers (ASCE)}, author={Hasnat, Md. Mehedi and Bardaka, Eleni and Samandar, M. Shoaib and Rouphail, Nagui and List, George and Williams, Billy}, year={2021}, month={Mar} }
 @article{song_kim_williams_rouphail_list_2020, title={Crash Classification by Congestion Type for Highways}, volume={10}, ISSN={["2076-3417"]}, url={https://doi.org/10.3390/app10072583}, DOI={10.3390/app10072583}, abstractNote={Effective management of highway networks requires a thorough understanding of the conditions under which vehicular crashes occur. Such an understanding can and should inform related operational and resource allocation decisions. This paper presents an easily implementable methodology that can classify all reported crashes in terms of the operational conditions under which each crash occurred. The classification methodology uses link-based speed data. Unlike previous secondary collision identification schemes, it neither requires an a priori identification of the precipitating incident nor definition of the precipitating incident’s impact area. To accomplish this objective, the methodology makes use of a novel scheme for distinguishing between recurrent and non-recurrent congestion. A 500-crash case study was performed using a 274 km section of the I-40 in North Carolina. Twelve percent of the case study crashes were classified as occurring in non-recurrent congestion. Thirty-seven percent of the crashes in non-recurrent congestion classified were identified within unreported primary incidents or crashes influence area. The remainder was classified as primary crashes occurring in either uncongested conditions (84%) or recurrent congestion (4%). The methodology can be implemented in any advanced traffic management system for which crash time and link location are available along with corresponding archived link speed data are available.}, number={7}, journal={APPLIED SCIENCES-BASEL}, publisher={MDPI AG}, author={Song, Tai-Jin and Kim, Sangkey and Williams, Billy M. and Rouphail, Nagui M. and List, George F.}, year={2020}, month={Apr} }
 @article{rosamond_johnson_bogle_arnold_cunningham_picinich_williams_zègre-hemsey_2020, title={Drone Delivery of an Automated External Defibrillator}, volume={383}, url={https://doi.org/10.1056/NEJMc1915956}, DOI={10.1056/NEJMc1915956}, abstractNote={Drone Delivery of an Automated External Defibrillator Delivery of an AED by drone was compared with on-the-ground retrieval of a fixed-location AED for simulated out-of-hospital cardiac arrest in s...}, number={12}, journal={New England Journal of Medicine}, publisher={Massachusetts Medical Society}, author={Rosamond, Wayne D. and Johnson, Anna M. and Bogle, Brittany M. and Arnold, Evan and Cunningham, Christopher J. and Picinich, Michael and Williams, Billy M. and Zègre-Hemsey, Jessica K.}, year={2020}, month={Sep}, pages={1186–1188} }
 @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{song_williams_rouphail_2018, title={Data-driven approach for identifying spatiotemporally recurrent bottlenecks}, volume={12}, ISSN={["1751-9578"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85053217294&partnerID=MN8TOARS}, DOI={10.1049/iet-its.2017.0284}, abstractNote={Identification of recurrent bottlenecks is an effective way to hone an appropriate investment in current facilities to relieve congestion. Furthermore, it would enable the ranking or prioritisation of bottlenecks since bottleneck removal and its associated impact alleviation are hampered by limited sources. It is imperative that transportation jurisdiction understand and identify the basis for ranking bottlenecks by exploring: how often they are active; how long it takes the congestion to disappear; and how many miles of road are affected. Previous bottleneck identification schemes have focused on identifying congestion with little attention to distinguishing the recurrent level at the same ‘bottleneck’ location. In contrast to traditional schemes, a data-driven approach for identifying recurrent bottlenecks is introduced, using probe vehicle speed reports. The historical spatiotemporal characteristics of bottlenecks are investigated through a comprehensive analysis of 2253 miles of all state-wide interstates in North Carolina. Using the characteristics determined the recurrent bottleneck locations with a historical time span of bottleneck activation are revealed and tested. The findings of the proposed identification schemes generate critical information in order to quantify and diagnose a bottleneck and its associated impact area.}, number={8}, journal={IET INTELLIGENT TRANSPORT SYSTEMS}, author={Song, Tai-Jin and Williams, Billy M. and Rouphail, Nagui M.}, year={2018}, month={Oct}, pages={756–764} }
 @article{huang_jia_guo_williams_shi_wei_cao_2018, title={Real-Time Prediction of Seasonal Heteroscedasticity in Vehicular Traffic Flow Series}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85038392558&partnerID=MN8TOARS}, DOI={10.1109/TITS.2017.2774289}, abstractNote={Over the past decade, traffic heteroscedasticity has been investigated with the primary purpose of generating prediction intervals around point forecasts constructed usually by short-term traffic condition level forecasting models. However, despite considerable advancements, complete traffic patterns, in particular the seasonal effect, have not been adequately handled. Recently, an offline seasonal adjustment factor plus GARCH model was proposed in Shi et al. 2014 to model the seasonal heteroscedasticity in traffic flow series. However, this offline model cannot meet the real-time processing requirement proposed by real-world transportation management and control applications. Therefore, an online seasonal adjustment factors plus adaptive Kalman filter (OSAF+AKF) approach is proposed in this paper to predict in real time the seasonal heteroscedasticity in traffic flow series. In this approach, OSAF and AKF are combined within a cascading framework, and four types of online seasonal adjustment factors are developed considering the seasonal patterns in traffic flow series. Empirical results using real-world station-by-station traffic flow series showed that the proposed approach can generate workable prediction intervals in real time, indicating the acceptability of the proposed approach. In addition, compared with the offline model, the proposed online approach showed improved adaptability when traffic is highly volatile. These findings are important for developing real-time intelligent transportation system applications.}, number={10}, journal={IEEE Transactions on Intelligent Transportation Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Huang, Wei and Jia, Wenwen and Guo, Jianhua and Williams, Billy and Shi, Guogang and Wei, Yun and Cao, Jinde}, year={2018}, pages={3170–3180} }
 @article{list_rouphail_smith_williams_2018, title={Reliability Assessment Tool: Development and Prototype Testing}, volume={2672}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85060926457&partnerID=MN8TOARS}, DOI={10.1177/0361198118798296}, abstractNote={ This paper presents a monitoring system that was developed to assess travel time reliability for observed operating conditions by utilizing traffic stream and non-transportation related data. A prototype was created for an interstate highway route in the Research Triangle region of North Carolina. It describes how the input datasets were obtained, the required data fusion procedures, how the data were analyzed to create the monitoring system outputs, and relevant insights obtained from the reliability monitoring system prototype. }, number={14}, journal={TRANSPORTATION RESEARCH RECORD}, author={List, George F. and Rouphail, Nagui and Smith, Russell and Williams, Billy}, year={2018}, month={Dec}, pages={29–38} }
 @article{karmakar_aghdashi_rouphail_williams_2018, title={Validation and Calibration of Freeway Reliability Methodology in the Highway Capacity Manual: Method and Case Studies}, volume={2672}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85060939515&partnerID=MN8TOARS}, DOI={10.1177/0361198118798723}, abstractNote={ Traffic congestion costs drivers an average of $1,200 a year in wasted fuel and time, with most travelers becoming less tolerant of unexpected delays. Substantial efforts have been made to account for the impact of non-recurring sources of congestion on travel time reliability. The 6th edition of the Highway Capacity Manual (HCM) provides a structured guidance on a step-by-step analysis to estimate reliability performance measures on freeway facilities. However, practical implementation of these methods poses its own challenges. Performing these analyses requires assimilation of data scattered in different platforms, and this assimilation is complicated further by the fact that data and data platforms differ from state to state. This paper focuses on practical calibration and validation methods of the core and reliability analyses described in the HCM. The main objective is to provide HCM users with guidance on collecting data for freeway reliability analysis as well as validating the reliability performance measures predictions of the HCM methodology. A real-world case study on three routes on Interstate 40 in the Raleigh-Durham area in North Carolina is used to describe the steps required for conducting this analysis. The travel time index (TTI) distribution, reported by the HCM models, was found to match those from probe-based travel time data closely up to the 80th percentile values. However, because of a mismatch between the actual and HCM estimated incident allocation patterns both spatially and temporally, and the fact that traffic demands in the HCM methods are by default insensitive to the occurrence of major incidents, the HCM approach tended to generate larger travel time values in the upper regions of the travel time distribution. }, number={15}, journal={TRANSPORTATION RESEARCH RECORD}, author={Karmakar, Nabaruna and Aghdashi, Seyedbehzad and Rouphail, Nagui M. and Williams, Billy M.}, year={2018}, month={Dec}, pages={93–104} }
 @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} }
 @article{list_williams_2017, title={Advances in travel time reliability monitoring and assessment}, volume={21}, ISSN={["1547-2442"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85041098125&partnerID=MN8TOARS}, DOI={10.1080/15472450.2017.1403536}, abstractNote={Travel time reliability has become a subject of increasingly intense interest for researchers and practitioners. Carriers and shippers have focused on reliability in terms of operational control ever since such services were first offered. As early as 1932, there were technical articles focused on assessing the reliability of alternate technologies (see Anonymous, 1932).With the advent of advanced technologies, it has become easier to track packages and vehicles, which has made reliability assessment all-themore feasible and interesting. And as probes have become more plentiful and sensing technologies have advanced, it has become increasingly easier to track the movement of individual vehicles within the highway mode. System performance assessment has become a reasonable objective. A market of private sector vendors of travel time-related data and associated analytics has grown in response. Reliability can be viewed from multiple perspectives. One is from the vantage of an individual driver who makes a trip repeatedly from an origin to a destination and who desires consistency in the travel time experienced. Another is as a carrier that promises service from airport A to airport B leaving at time tA and arriving at time tB. Here the concern is about the percentage of time that the departures and arrivals are “on-time”. A transit agency is interested in maximizing the number of transit stops that occur on-time, given tolerance windows. A highway agency is interested in ensuring that travel times, or travel rates, provided are consistent with promised performance targets during specific operating conditions, such as the AM and PM peak periods in the absence of disruptive events. Success is achieved if the travel rate is lower than policy targets. A shipper’s perspective might be that of ensuring that packages arrive on-time more frequently than a targeted “acceptable” percentage of the time.}, number={6}, journal={JOURNAL OF INTELLIGENT TRANSPORTATION SYSTEMS}, author={List, George F. and Williams, Billy}, year={2017}, pages={435–438} }
 @book{song_so_lee_williams_2017, title={Exploring Vehicle–Pedestrian Crash Severity Factors on the Basis of In-Car Black Box Recording Data}, volume={2659}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85057965576&partnerID=MN8TOARS}, DOI={10.3141/2659-16}, abstractNote={ This study investigated the main factors affecting the severity of injury to pedestrians in taxi–pedestrian crashes on urban arterial roads. Video data recorded by an in-car black box were used. Because the video data provided direct crash observation, they were more reliable than the crash data, and video images and speed profiles retrieved from the black box were advantageous for safety studies. For analysis of the black box data, this study defined new explanatory variables that affected injury severity; these variables could not have been identified by the conventional method, which was based on crash reports. A multiple-indicator and multiple-cause model was used to investigate the relationship between the explanatory variables and injury severity. A total of 484 taxi–pedestrian crash scenes over 2 years was used for the multivariate analysis in the city of Incheon, South Korea. The crash characteristics most strongly associated with increased crash severity were failure by the pedestrian to watch for approaching vehicles, jaywalking by the pedestrian, the pedestrian being elderly, excessive vehicle speed, failure by the driver to immediately stop, limited driver vision, and nighttime. This study emphasized the potential of individualized black box video recording data for crash severity analysis and investigation of the causal factors of crashes. }, number={1}, journal={Transportation Research Record}, author={Song, T.-J. and So, J. and Lee, J. and Williams, B.M.}, year={2017}, pages={148–154} }
 @inproceedings{clark_mclaughlin_williams_feng_2017, title={Performance in takeover and characteristics of non-driving related tasks during highly automated driving in younger and older drivers}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85042494534&partnerID=MN8TOARS}, DOI={10.1177/1541931213601504}, abstractNote={ This paper aims to examine the effect of age and various characteristics of non-driving related activities during highly automated driving on subsequent performance in notified takeovers among younger and older drivers. The paper presents new analyses of data collected in our earlier study (Clark & Feng, 2016). Non-driving-related activities that participants voluntarily chose to engage in during automated driving were categorized according to their cognitive dimensions in information processing. Using hierarchical multiple regressions, we analyzed the effect of driver age, total duration and number of engagement in non-driving-related activities, the duration and cognitive dimensions of the last activity prior to takeover on average speed during takeover and the response time to a takeover notification. We found that older drivers speed was negatively predicted by age while their response time to a notification was not predicted by any factor. In contrast, younger drivers showed a trend of positive relationship between age and average speed and the characteristics of the last task engagement explained a significant portion of the variance of response time to a notification. }, booktitle={Proceedings of the Human Factors and Ergonomics Society}, author={Clark, H. and McLaughlin, A.C. and Williams, B. and Feng, J.}, year={2017}, pages={37–41} }
 @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} }
 @book{yi_williams_2015, title={Dynamic traffic flow model for travel time estimation}, volume={2526}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84975795994&partnerID=MN8TOARS}, DOI={10.3141/2526-08}, abstractNote={ Travel time, as a fundamental measurement for intelligent transportation systems, is becoming increasingly important. Because of the wide deployment of fixed-point detectors on freeways, if travel time can be accurately estimated from point detector data, the indirect estimation method is cost-effective and widely applicable. This paper presents a modified dynamic traffic flow model for accurately estimating the travel time of freeway links under transition and congestion conditions with fixed-point detector data. The modified estimation model is based on a thorough analysis of the dynamic traffic flow model. The applications and the limitations of the model are analyzed for theory, equation derivation, and modifications. Through a simulation study and real traffic data, the (modified) dynamic models are compared according to performance measurements. A comparison of the estimated results and measurement errors shows the accuracy of the modified dynamic model for estimating the travel times of freeway links under transition and congestion traffic conditions. }, journal={Transportation Research Record}, author={Yi, T. and Williams, B.M.}, year={2015}, pages={70–78} }
 @article{yi_williams_2015, title={Dynamic traffic flow model for travel time estimation}, number={2526}, journal={Transportation Research Record}, author={Yi, T. and Williams, B. M.}, year={2015}, pages={70–78} }
 @article{guo_huang_williams_2015, title={Real time traffic flow outlier detection using short-term traffic conditional variance prediction}, volume={50}, ISSN={["0968-090X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85027920814&partnerID=MN8TOARS}, DOI={10.1016/j.trc.2014.07.005}, abstractNote={Outliers in traffic flow series represent uncommon events occurring in the roadway systems and outlier detection and investigation will help to unravel the mechanism of such events. However, studies on outlier detection and investigations are fairly limited in transportation field where a vast volume of traffic condition data has been collected from traffic monitoring devices installed in many roadway systems. Based on an online algorithm that has the ability of jointly predict the level and the conditional variance of the traffic flow series, a real time outlier detection method is proposed and implemented. Using real world data collected from four regions in both the United States and the United Kingdom, it was found that outliers can be detected using the proposed detection strategy. In addition, through a comparative experimental study, it was shown that the information contained in the outliers should be assimilated into the forecasting system to enhance its ability of adapting to the changing patterns of the traffic flow series. Moreover, the investigation into the effects of outliers on the forecasting system structure showed a significant connection between the outliers and the forecasting system parameters changes. General conclusions are provided concerning the analyses with future work recommended to investigate the underlying outlier generating mechanism and outlier treatment strategy in transportation applications.}, journal={TRANSPORTATION RESEARCH PART C-EMERGING TECHNOLOGIES}, author={Guo, Jianhua and Huang, Wei and Williams, Billy M.}, year={2015}, month={Jan}, pages={160–172} }
 @article{guo_huang_williams_2014, title={Adaptive Kalman filter approach for stochastic short-term traffic flow rate prediction and uncertainty quantification}, volume={43}, ISSN={["0968-090X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84902553625&partnerID=MN8TOARS}, DOI={10.1016/j.trc.2014.02.006}, abstractNote={Short term traffic flow forecasting has received sustained attention for its ability to provide the anticipatory traffic condition required for proactive traffic control and management. Recently, a stochastic seasonal autoregressive integrated moving average plus generalized autoregressive conditional heteroscedasticity (SARIMA + GARCH) process has gained increasing notice for its ability to jointly generate traffic flow level prediction and associated prediction interval. Considering the need for real time processing, Kalman filters have been utilized to implement this SARIMA + GARCH structure. Since conventional Kalman filters assume constant process variances, adaptive Kalman filters that can update the process variances are investigated in this paper. Empirical comparisons using real world traffic flow data aggregated at 15-min interval showed that the adaptive Kalman filter approach can generate workable level forecasts and prediction intervals; in particular, the adaptive Kalman filter approach demonstrates improved adaptability when traffic is highly volatile. Sensitivity analyses show that the performance of the adaptive Kalman filter stabilizes with the increase of its memory size. Remarks are provided on improving the performance of short term traffic flow forecasting.}, journal={TRANSPORTATION RESEARCH PART C-EMERGING TECHNOLOGIES}, author={Guo, Jianhua and Huang, Wei and Williams, Billy M.}, year={2014}, month={Jun}, pages={50–64} }
 @article{shi_guo_huang_williams_2014, title={Erratum for “Modeling Seasonal Heteroscedasticity in Vehicular Traffic Condition Series Using a Seasonal Adjustment Approach” by Guogang Shi, Jianhua Guo, Wei Huang, and Billy M. Williams}, volume={140}, ISSN={0733-947X 1943-5436}, url={http://dx.doi.org/10.1061/(ASCE)TE.1943-5436.0000687}, DOI={10.1061/(ASCE)TE.1943-5436.0000687}, number={9}, journal={Journal of Transportation Engineering}, publisher={American Society of Civil Engineers (ASCE)}, author={Shi, Guogang and Guo, Jianhua and Huang, Wei and Williams, Billy M.}, year={2014}, month={Sep}, pages={08014002} }
 @article{shi_guo_huang_williams_2014, title={Modeling Seasonal Heteroscedasticity in Vehicular Traffic Condition Series Using a Seasonal Adjustment Approach}, volume={140}, ISSN={0733-947X 1943-5436}, url={http://dx.doi.org/10.1061/(ASCE)TE.1943-5436.0000656}, DOI={10.1061/(ASCE)TE.1943-5436.0000656}, abstractNote={AbstractHeteroscedasticity modeling in transportation engineering is primarily conducted in short-term traffic condition forecasting to generate time varying prediction intervals around the point forecasts through quantitatively predicting the conditional variance of traffic condition series. Until recently, the generalized autoregressive conditional heteroscedasticity (GARCH) model and the stochastic volatility model have been two major approaches adopted from the field of financial time series analysis for traffic heteroscedasticity modeling. In this paper, recognizing the pronounced seasonal pattern in traffic condition data, a simple seasonal adjustment approach is explored for modeling seasonal heteroscedasticity in traffic-flow series, and four types of seasonal adjustment factors are proposed with respect to daily or weekly patterns. Using real-world traffic-flow data collected from highway systems in the United Kingdom and the United States, the proposed seasonal adjustment approach is implemented...}, number={5}, journal={Journal of Transportation Engineering}, publisher={American Society of Civil Engineers (ASCE)}, author={Shi, Guogang and Guo, Jianhua and Huang, Wei and Williams, Billy M.}, year={2014}, month={May}, pages={04014012} }
 @article{shi_guo_huang_williams_2014, title={Modeling seasonal heteroscedasticity in vehicular traffic condition series using a seasonal adjustment approach (vol 140, 04014012, 2014)}, volume={140}, number={9}, journal={Journal of Transportation Engineering}, author={Shi, G. G. and Guo, J. H. and Huang, W. and Williams, B. M.}, year={2014} }
 @book{xu_williams_rouphail_chase_2013, title={Development of an oversaturated speed-flow model based on the highway capacity manual}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897034574&partnerID=MN8TOARS}, DOI={10.3141/2395-05}, abstractNote={ Oversaturated speed, flow, and density relationships are of key importance to studies of freeway operations. The Highway Capacity Manual (HCM) oversaturated model, which is defined by a linear transition from the flow and the density at capacity to a zero flow at jam density in the flow–density space, provides a reasonable representation of this relationship but does not provide an unbiased representation for all freeway facilities with different road conditions or driver behavior. This study proposes a method for fitting the HCM model to oversaturated flow and density. Fifteen-minute aggregated flow rate and speed data were collected in 2010 from Traffic.com fixed-location sensors at three sites on North Carolina urban freeways. Density was calculated as the flow rate divided by the speed. The fitted models for these sites were compared with the default HCM model. A set of thresholds was defined to identify eligible sensor observations that represented the steady-state congested traffic conditions. The results revealed that data observations during inclement weather, lane closures, or incidents biased the model-fitting results and therefore needed to be filtered out. The steady-state congestion data identified in the manner proposed in this study fit well with the HCM-based linear flow–density oversaturated model. This method avoids possible bias caused by capacity and jam density differences between the default HCM model and the site-specific models; therefore, the fitted models represented the actual traffic characteristics relationships better than the default HCM models did. Fitting a site-specific HCM-based model is recommended for sites with sufficient speed and flow data. }, number={2395}, journal={Transportation Research Record}, author={Xu, Y. and Williams, B. and Rouphail, N. and Chase, R.}, year={2013}, pages={41–48} }
 @article{xu_williams_rouphail_chase_2013, title={Development of an oversaturated speed-flow model based on the highway capacity manual}, number={2395}, journal={Transportation Research Record}, author={Xu, Y. L. and Williams, B. M. and Rouphail, N. M. and Chase, R. T.}, year={2013}, pages={41–48} }
 @article{isukapati_list_williams_karr_2013, title={Synthesizing Route Travel Time Distributions from Segment Travel Time Distributions}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897082752&partnerID=MN8TOARS}, DOI={10.3141/2396-09}, abstractNote={ This paper examines a way to synthesize route travel time probability density functions (PDFs) on the basis of segment-level PDFs. Real-world data from I-5 in Sacramento, California, are employed. The first finding is that careful filtering is required to extract useful travel times from the raw data because trip times, not travel times, are observed (i.e., the movement of vehicles between locations). The second finding is that significant correlations exist between individual vehicle travel times for adjacent segments. Two analyses are done in this regard: one predicts downstream travel times on the basis of upstream travel times, and the second checks for correlations in travel times between upstream and downstream segments. The results of these analyses suggest that strong positive correlations exist. The third finding is that comonotonicity, or perfect positive dependence, can be assumed when route travel time PDFs are generated from segment PDFs. Kolmogorov–Smirnov tests show that travel times synthesized from the segment-specific data are statistically different only under highly congested conditions, and even then, the percentage differences in the distributions of the synthesized and actual travel times are small. The fourth finding, somewhat tangential, is that there is little variation in individual driver travel times under given operating conditions. This is an important finding, because such an assumption serves as the basis for all traffic simulation models. }, number={2396}, journal={TRANSPORTATION RESEARCH RECORD}, author={Isukapati, Isaac Kumar and List, George F. and Williams, Billy M. and Karr, Alan F.}, year={2013}, pages={71–81} }
 @article{chase_williams_rouphail_kim_2012, title={Comparative Evaluation of Reported Speeds from Corresponding Fixed-Point and Probe-Based Detection Systems}, ISSN={["0361-1981"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84874037734&partnerID=MN8TOARS}, DOI={10.3141/2308-12}, abstractNote={ Point-based traffic sensors, such as microwave radar and acoustic sensors, provide the valuable capability of sampling the entire traffic stream. However, full network coverage with point sensors requires a significant initial capital investment and ongoing maintenance expenditures. Probe-based sensors can cover an extensive roadway network at a much lower cost because roadway-based field equipment is not required. Decisions regarding the relative level of point sensor- versus probe-based deployment for traffic monitoring involve evaluating the trade-off between the value of comprehensive detection versus total system costs. An essential step in evaluating this trade-off involves directly comparing collocated point sensor and probe vehicle systems to understand how the derived traffic stream measures from the two approaches differ. This study compared 5-min speeds from microwave radar and acoustic sensors with link speeds from Global Positioning System (GPS) probes for both directions at five freeway locations. Systematic differences were found at one location. Floating car GPS runs were performed to confirm that the systematic error lay in the point speeds. The speed differences at all sites were normally distributed, with three locations indicating a mean speed difference greater than 5 mph. Nonsystematic speed differences were identified; the difference was more than 1.5 standard deviations lower than the mean difference. This difference may indicate inherent inaccuracies in reported GPS speeds under heavy congestion, including instances of time lag in recovering from congested speeds. }, number={2308}, journal={TRANSPORTATION RESEARCH RECORD}, author={Chase, R. Thomas and Williams, Billy M. and Rouphail, Nagui M. and Kim, SangKey}, year={2012}, pages={110–119} }
 @book{al-ghandour_schroeder_rasdorf_williams_2012, title={Delay analysis of single-lane roundabout with a slip lane under varying exit types, experimental balanced traffic volumes, and pedestrians, using microsimulation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84872838055&partnerID=MN8TOARS}, DOI={10.3141/2312-08}, abstractNote={ A slip lane facilitates right-turning traffic flow, reduces approach delay, and reduces conflict points within a roundabout. In this paper the delay performance of a single-lane roundabout with an adjacent slip lane is modeled with the VISSIM microsimulation tool for three slip lane exit types (free-flow, yield, and stop) and the results are compared with a roundabout having no slip lane. The VISSIM assessment considers four experimental traffic percentage turning volume distributions as balanced flow scenarios (total traffic flow into and out of every roundabout approach is the same). Simulated slip lane right-turning traffic volumes range from 50 to 500 vehicles/h, and the four pedestrian volume levels range from 0 to 100 pedestrians/h. VISSIM results confirm that average delays in a roundabout with a slip lane are a function of circulating conflict volumes and are related exponentially to slip lane volumes regardless of the slip lane exit type. Results also indicate that a free-flow slip lane exit type best reduces total average delay in the roundabout and in the slip lane itself. Yield and stop slip lane exit types also reduce the roundabout total average delay but to a lesser degree. Finally, at a higher traffic volume, a free-flow slip lane exit type can increase roundabout delay from 6.6 to 34.7 s/vehicle if drivers must yield the right-of-way to high pedestrian traffic (100 pedestrians/h) crossing a free-flow slip lane (priority rule). }, number={2312}, journal={Transportation Research Record}, author={Al-Ghandour, M. and Schroeder, B. and Rasdorf, W. and Williams, B.}, year={2012}, pages={76–85} }
 @article{al-ghandour_schroeder_rasdorf_williams_2012, title={Delay analysis of single-lane roundabout with a slip lane under varying exit types, experimental balanced traffic volumes, and pedestrians, using microsimulation}, number={2312}, journal={Transportation Research Record}, author={Al-Ghandour, M. and Schroeder, B. and Rasdorf, W. and Williams, B.}, year={2012}, pages={76–85} }
 @article{lee_williams_2012, title={Development and evaluation of a constrained optimization model for traffic signal plan transition}, volume={20}, ISSN={0968-090X}, url={http://dx.doi.org/10.1016/j.trc.2011.05.008}, DOI={10.1016/j.trc.2011.05.008}, abstractNote={In advanced traffic signal control systems, plan transition is the process of switching from one timing plan to another to accommodate changes in traffic demand. In traditional coordinated system operation, whether plans are set by time of day or by traffic responsive plan selection, traffic flow can be severely disrupted by the sudden changes in traffic signal timing. Although various transition methods have been developed and refined, existing transition methods are not based on optimizing operational measures of effectiveness during plan transition periods. As the first step in remedying this situation, this paper presents a non-linear mathematical model that provides constrained delay minimization through incremental and simultaneous adjustments in offset and cycle length during plan transitions. According to the simulation results, currently used transition methods tend to assign an excessive amount of green time to the main street, resulting in additional side street delay without performance improvement for the total transportation network. By contrast, the proposed transition method showed measurable improvement in delay performance under a broad range of traffic and geometric conditions.}, number={1}, journal={Transportation Research Part C: Emerging Technologies}, publisher={Elsevier BV}, author={Lee, Jisun and Williams, Billy M.}, year={2012}, month={Feb}, pages={185–198} }
 @article{guo_huang_williams_2012, title={Integrated Heteroscedasticity Test for Vehicular Traffic Condition Series}, volume={138}, ISSN={["1943-5436"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876721756&partnerID=MN8TOARS}, DOI={10.1061/(asce)te.1943-5436.0000420}, abstractNote={AbstractBecause of the growing awareness of the importance of the traffic condition uncertainty-related studies, traffic condition uncertainty modeling is gaining increasing attention from the transportation research community. In this field, traffic condition uncertainty, gauged mainly by the conditional variance of traffic characteristics, has been investigated primarily with two major approaches, generalized autoregressive conditional heteroscedasticity approach and stochastic volatility approach; however, both lack a thorough and sound test on the applicability of these approaches. To complete this modeling gap and hence lay the theoretical basis for traffic uncertainty-related studies, an integrated heteroscedasticity test, including an optimal transformation search and four statistical tests, is proposed in this study. By using real world data collected from 36 stations across four regions in both the United Kingdom and the United States and aggregated at 15-min interval as a typical representative,...}, number={9}, journal={JOURNAL OF TRANSPORTATION ENGINEERING}, author={Guo, Jianhua and Huang, Wei and Williams, Billy M.}, year={2012}, month={Sep}, pages={1161–1170} }
 @book{liu_williams_rouphail_2012, title={Temporal stability of freeway macroscopic traffic stream models}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897782260&partnerID=MN8TOARS}, DOI={10.3141/2315-14}, abstractNote={ A key assumption for the successful deployment of mesoscopic transportation network simulation to assess network behavior and performance at a future planning horizon is that the parameters of the constituent macroscopic traffic stream models remain stable at relatively long planning time scales. However, no research has been undertaken to test this assumption systematically. Assessing the validity of this implicit assumption requires rigorous analysis based on empirical traffic data covering a long period. Extensive freeway management systems have now been in place long enough to provide these empirical data. A method to investigate the issue of temporal stability of freeway macroscopic traffic stream models rigorously with 15 years of traffic data from the London Orbital Motorway is presented and implemented. Two forms of macroscopic traffic stream models, namely, a two-regime and a three-regime model form, were studied in this research. With a 13-week, outlier-corrected moving window sampling method and maximum likelihood parameter estimation, time series were developed for the model parameters. The temporal stability characteristics and correlation properties of the fitted parameters are analyzed and discussed. The parameters were found to be relatively stable over time and did not exhibit statistically significant seasonal correlation. Parameter variability was found to be governed primarily by the relative scatter of heavily congested observations and how well the heavily congested regime was represented in the individual data samples. Therefore, modelers should ensure that traffic stream model calibration data sets are free from outliers and well distributed across the model regimes. }, number={2315}, journal={Transportation Research Record}, author={Liu, C. and Williams, B. and Rouphail, N.}, year={2012}, pages={131–140} }
 @article{liu_williams_rouphail_2012, title={Temporal stability of freeway macroscopic traffic stream models}, number={2315}, journal={Transportation Research Record}, author={Liu, C. H. and Williams, B. M. and Rouphail, N. M.}, year={2012}, pages={131–140} }
 @article{williams_2012, title={‘Real-time road traffic forecasting using regime-switching space-time models and adaptive lasso’ by Y. Kamarianakis, W. Shen, and L. Wynter}, volume={28}, ISSN={1524-1904}, url={http://dx.doi.org/10.1002/asmb.1938}, DOI={10.1002/asmb.1938}, abstractNote={Applied Stochastic Models in Business and IndustryVolume 28, Issue 4 p. 319-321 Discussion ‘Real-time road traffic forecasting using regime-switching space-time models and adaptive lasso’ by Y. Kamarianakis, W. Shen, and L. Wynter PH.D. P.E. Billy M. Williams, PH.D. P.E. Billy M. Williams billy_williams@ncsu.edu Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908 418 Mann Hall, Raleigh, NC, 27695-7908 USASearch for more papers by this author PH.D. P.E. Billy M. Williams, PH.D. P.E. Billy M. Williams billy_williams@ncsu.edu Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908 418 Mann Hall, Raleigh, NC, 27695-7908 USASearch for more papers by this author First published: 13 August 2012 https://doi.org/10.1002/asmb.1938Citations: 1Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume28, Issue4July/August 2012Pages 319-321 RelatedInformation}, number={4}, journal={Applied Stochastic Models in Business and Industry}, publisher={Wiley}, author={Williams, Billy M.}, year={2012}, month={Jul}, pages={319–321} }
 @inproceedings{al-ghandour_rasdorf_williams_schroeder_2011, place={Reston, VA}, title={Analysis of Single-Lane Roundabout Slip Lanes Using SIDRA}, ISBN={9780784411674}, url={http://dx.doi.org/10.1061/41167(398)118}, DOI={10.1061/41167(398)118}, abstractNote={Roundabout intersections, increasingly used in the U.S., sometimes incorporate slip lanes to facilitate right-turning traffic flow and reduce delay, thereby increasing capacity and safety. Performance of a single-lane roundabout with an adjacent slip lane is modeled with the SIDRA Intersection analysis tool for three types of slip lane: free-flow, yield, and stop, and is compared to having no slip lane. The gap acceptance-based assessment considers four experimental traffic percentage distribution matrices representing flow scenarios. SIDRA results confirm that average delay and circulating conflict volumes in a roundabout with a slip lane are related exponentially to slip lane volumes. A free-flow slip lane exit type helps to reduce total average delay in the roundabout and the slip lane approach. Both yield and stop slip lane exit types also reduce roundabout total average delay but to a lesser degree than a free-flow slip lane. Finally, theoretical capacity threshold values for slip lane volumes are estimated to range from 150 to 350 vehicles per hour for traffic volume distribution scenarios.}, booktitle={T & DI Congress 2011 : integrated transportation and development for a better tomorrow : proceedings of the First Transportation and Development Institute Congress : March 13-16, 2011, Chicago, Illinois}, publisher={American Society of Civil Engineers}, author={Al-Ghandour, Majed and Rasdorf, William and Williams, Billy and Schroeder, Bastian}, editor={Al-Qadi, Imad L. and Murrell, ScottEditors}, year={2011}, pages={1235–1244} }
 @article{kittelson_rouphail_williams_zhou_2011, title={Analyzing Operational Improvements as an Alternative to Traditional Highway Construction}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80052930330&partnerID=MN8TOARS}, DOI={10.3141/2223-03}, abstractNote={ The primary objective of the study described in this paper was to develop methods and tools for analyzing operational improvements as an alternative to traditional highway construction. Twenty-five capacity-enhancing operational, design, and technological strategies were identified for use on freeways, arterials, or both. Methodological and analytic enhancements to existing dynamic traffic assignment models were developed to increase the realism and the sensitivity of the models in simulating the effects of one or more strategy applications. Diagnostic tools for identifying locations that may represent good opportunities to implement one or more operational strategies were created. An illustrative application of the procedure is presented to demonstrate its usefulness and the additional insights that can be gained to inform transportation investment decision making. The systematic procedure described in this paper can be used for diagnosing and remediating operational deficiencies at the link, corridor, and network levels. }, number={2223}, journal={Transportation Research Record}, author={Kittelson, Wayne K. and Rouphail, Nagui M. and Williams, Billy and Zhou, Xuesong}, year={2011}, pages={18–25} }
 @article{al-ghandour_schroeder_williams_rasdorf_2011, title={Conflict Models for Single-Lane Roundabout Slip Lanes from Microsimulation Development and Validation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84855227767&partnerID=MN8TOARS}, DOI={10.3141/2236-11}, abstractNote={ Conflict patterns at single-lane roundabouts with and without slip lanes were evaluated and compared through VISSIM, the microscopic simulation program for multimodal traffic flow modeling, and surrogate safety assessment model (SSAM) analysis. From a sensitivity analysis of several volume distribution scenarios of the percentage of turning traffic, five zone-based conflict prediction models were developed through the use of Poisson regression. The models captured simulated conflict differences that resulted from the addition of a right-turn slip lane. The models were evaluated under three exit control scenarios (yield, stop, and free-flow merge). With SSAM analysis, the models predicted the occurrence of conflicts for roundabout zones with different R-squared values, which ranged from .69 to .97. The models were compared with national and inter-national crash prediction models for single-lane roundabouts and were further validated by actual crash data from 10 single-lane roundabouts in the city of Carmel, Indiana. The number of conflicts for a single-lane roundabout was predicted as a function of approach entry, circulation, and slip lane traffic flows and was determined to be sensitive to the slip lane exit type. Results confirmed that conflicts in the merge area were more frequent than in the roundabout approach area and that the installation of a free-flow slip lane exit type reduced overall conflict occurrence. The results demonstrated the usefulness of SSAM analysis for evaluating roundabout safety and developing an empirical relationship between simulated conflicts and field-observed crashes. }, number={2236}, journal={Transportation Research Record}, author={Al-Ghandour, Majed N. and Schroeder, Bastian J. and Williams, Billy and Rasdorf, William J.}, year={2011}, pages={92–101} }
 @article{lee_williams_2011, title={Development and evaluation of a constrained optimization model for traffic signal plan transition}, volume={17}, ISSN={["1877-0428"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79960078845&partnerID=MN8TOARS}, DOI={10.1016/j.sbspro.2011.04.528}, abstractNote={In advanced traffic signal control systems, plan transition is the process of switching from one timing plan to another to accommodate changes in traffic demand. In traditional coordinated system operation, whether plans are set by time of day or by traffic responsive plan selection, traffic flow can be severely disrupted by the sudden changes in traffic signal timing. Although various transition methods have been developed and refined, existing transition methods are not based on optimizing operational measures of effectiveness during plan transition periods. As the first step in remedying this situation, this paper presents a non-linear mathematical model that provides constrained delay minimization through incremental and simultaneous adjustments in offset and cycle length during plan transitions. According to the simulation results, currently used transition methods tend to assign an excessive amount of green time to the main street, resulting in additional side street delay without performance improvement for the total transportation network. By contrast, the proposed transition method showed measurable improvement in delay performance under a broad range of traffic and geometric conditions.}, journal={PAPERS SELECTED FOR THE 19TH INTERNATIONAL SYMPOSIUM ON TRANSPORTATION AND TRAFFIC THEORY}, author={Lee, Jisun and Williams, Billy M.}, year={2011}, pages={490–508} }
 @article{lee_williams_2011, title={Fundamental Insight into Signal Plan Transition Methods}, volume={137}, ISSN={["0733-947X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80155132543&partnerID=MN8TOARS}, DOI={10.1061/(asce)te.1943-5436.0000269}, abstractNote={This paper describes, in detail, the concrete operational mechanisms of the Corridor Simulation (CORSIM)–embedded signal transition algorithms. Through detailed analysis of these transition algorithms, this study found that controller configurations, such as minimum green time for main street and recall mode, are important factors affecting plan transition performance. Specifically, this study demonstrated that even when the same transition method is used, the operational result can vary significantly according to these controller configurations. A simulation study was performed to comparatively evaluate delay performance of plan transition algorithms under two operational situations, namely, transition into peak and out of peak traffic demands. Although the shortway method showed the best performance for most simulation scenarios, the three-cycle and immediate methods could be included in the best-performing group in the case of transition into peak through an adjustment of the main street minimum green ...}, number={10}, journal={JOURNAL OF TRANSPORTATION ENGINEERING-ASCE}, author={Lee, Jisun and Williams, Billy M.}, year={2011}, month={Oct}, pages={676–686} }
 @article{jia_zhou_li_rouphail_williams_2011, title={Incorporating Stochastic Road Capacity into Day-to-Day Traffic Simulation and Traveler Learning Framework Model Development and Case Study}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84863275292&partnerID=MN8TOARS}, DOI={10.3141/2254-12}, abstractNote={ A key foundation for developing strategies aimed at improving the efficiency and reliability of an urban transportation network is identifying the locations and impact of system bottlenecks. Although free-flow capacity and queue discharge rates at system bottlenecks have traditionally been modeled as fixed values, they are in fact random variables. Therefore, assessing the operational impact of network bottlenecks requires reliable and realistic tools that account for stochasticity in pre-breakdown flow rates and queue discharge rates. Focusing on methodological and analytic enhancements to existing dynamic traffic assignment models, this paper presents a method to seamlessly incorporate stochastic capacity models at freeway bottlenecks and signalized intersections and develops adaptive day-to-day traveler learning and route choice behavioral models under the travel time variability introduced by random capacity variations. To account for different levels of information availability and cognitive limitations of individual travelers, a set of bounded rationality rules are adapted to describe route choice rules for a traffic system with inherent process noise and different information provision strategies. A case study based on a real-world Portland, Oregon, subarea network is presented to illustrate the capabilities of the enhanced simulator and highlight the advantage of modeling stochastic capacity in a dynamic mesoscopic traffic simulator as compared with conventional tools that assume deterministic road capacity. }, number={2254}, journal={Transportation Research Record}, author={Jia, Anxi and Zhou, Xuesong and Li, Mingxin and Rouphail, Nagui M. and Williams, Billy}, year={2011}, pages={112–121} }
 @article{jia_williams_rouphail_2010, title={Identification and Calibration of Site-Specific Stochastic Freeway Breakdown and Queue Discharge}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79551607835&partnerID=MN8TOARS}, DOI={10.3141/2188-16}, abstractNote={ The stochastic nature of freeway bottleneck breakdown and queue discharge is investigated through a comprehensive analysis of sensor data collected at bottleneck sites in the San Francisco Bay Area, California, and San Antonio, Texas. A new procedure was proposed to define the stochastic variation of the onset of freeway breakdown and of queue discharge capacity on the basis of time-indexed field data of speed–flow profiles. The former was developed as a function of average vehicle time headways preceding observed conditions when both speed was below and density was above locally defined congested flow thresholds. A full-year 15-min data series was used in the demonstration and testing of the procedure and yielded a high degree of statistical confidence in the resulting estimates of headway distribution parameters. The statistical analysis indicated that the probability function of freeway bottleneck prebreakdown headways followed a shifted lognormal distribution. In addition, a recursive queue discharge model was proposed for bottleneck flows under congested (queued) conditions. The proposed queue discharge model was a simple autocorrelated time series recursion that was seeded with the corresponding prebreakdown flow and dampens to the mean queue discharge rate. The proposed stochastic models are robust and accurate and represent a significant improvement in the understanding and modeling of freeway bottleneck flow. The models were implemented in the mesoscopic network model DYNASMART-P to test the effects of stochastic freeway capacity on sustained service rates and network performance. }, number={2188}, journal={Transportation Research Record}, author={Jia, Anxi and Williams, Billy and Rouphail, Nagui M.}, year={2010}, pages={148–155} }
 @article{guo_williams_2010, title={Real-Time Short-Term Traffic Speed Level Forecasting and Uncertainty Quantification Using Layered Kalman Filters}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78651271213&partnerID=MN8TOARS}, DOI={10.3141/2175-04}, abstractNote={ Short-term traffic condition forecasting has long been argued as essential for developing proactive traffic control systems that could alleviate the growing congestion in the United States. In this field, short-term traffic condition level forecasting and short-term traffic condition uncertainty forecasting play an equally important role. Past literature showed that linear stochastic time series models are promising in modeling and hence forecasting traffic condition levels and traffic conditional variance with workable performance. On the basis of this finding, an autoregressive moving average plus generalized autoregressive conditional heteroscedasticity structure was proposed for modeling the station-by-station traffic speed series. An online algorithm based on layered Kalman filter was developed for processing this structure in real time. Empirical results based on real-world station-by-station traffic speed data showed that the proposed online algorithm can generate workable short-term traffic speed level forecasts and associated prediction confidence intervals. Future work is recommended to develop and test a proactive traffic control system in a simulated environment, to refine the uncertainty modeling through a stochastic volatility model, and to extend uncertainty modeling and forecasting to link level and network level. }, number={2175}, journal={Transportation Research Record}, author={Guo, Jianhua and Williams, Billy}, year={2010}, pages={28–37} }
 @article{vanajakshi_williams_rilett_2009, title={Improved flow-based travel time estimation method from point detector data for freeways}, volume={135}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57949084250&partnerID=MN8TOARS}, DOI={10.1061/(ASCE)0733-947X(2009)135:1(26)}, abstractNote={Travel time is an important parameter in evaluating the operating efficiency of traffic networks, in assessing the performance of traffic management strategies, and as input to many intelligent transportation systems applications such as advanced traveler information systems. Travel time can be obtained directly from instrumented test vehicles, license plate matching, probe vehicles etc., or from indirect methods such as inductance loop detectors. Because of the widespread deployment of loop detectors, they are one of the most widely used inputs to travel time estimation techniques. There are different methods available to calculate the travel time from loop detector data, such as extrapolation of the point speed values, statistical methods, and models based on traffic flow theory. However, most of these methods fail during the transition period between the normal and congested flow conditions. The present study proposes several modifications to an existing traffic flow theory based model for travel time ...}, number={1}, journal={Journal of Transportation Engineering}, author={Vanajakshi, L.D. and Williams, Billy and Rilett, L.R.}, year={2009}, pages={26–36} }
 @article{hu_williams_rouphail_khattak_zhou_2009, title={Modeling the Role of Transportation Information in Mitigating Major Capacity Reductions in a Regional Network}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76749103439&partnerID=MN8TOARS}, DOI={10.3141/2138-11}, abstractNote={ A practical method is presented for systematically evaluating the network impacts of advanced traveler information systems (ATISs) to support well-informed project decisions and well-founded funding priorities. Detailed comparative assessments of available evaluation tools are given on the basis of a common set of desirable criteria. A mesoscopic network modeling and dynamic traffic assignment (DTA) tool, namely, DYNASMART-P, was identified as a promising candidate model and was applied to the evaluation of case study scenarios in a subnetwork of the Triangle Regional Model in North Carolina. The case study involved planned work zone activities occurring during nonpeak time periods. The case study demonstrated that the DYNASMART-P DTA tool is capable of providing reasonable evaluation results, including realistic estimates of the effectiveness of ATISs in mitigating the congestion caused by work zone activities. The case study analyses further illustrated the high value of traveler information through an example benefit–cost analysis based on the scenario modeling. The simulation results were validated for reasonableness through comparison with field speed data. Finally, several potential DTA functionality enhancements are identified that will further support ATIS evaluation. }, number={2138}, journal={Transportation Research Record}, author={Hu, Hyejung and Williams, Billy and Rouphail, Nagui M. and Khattak, Asad J. and Zhou, Xuesong}, year={2009}, pages={75–84} }
 @article{khattak_pan_williams_rouphail_fan_2008, title={Traveler Information Delivery Mechanisms Impact on Consumer Behavior}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-61349195148&partnerID=MN8TOARS}, DOI={10.3141/2069-10}, abstractNote={ Advanced traveler information systems (ATISs) help individuals make informed travel decisions. Current ATIS applications encompass a variety of delivery mechanisms, including the Internet, telephone, television, radio, variable message signs, and in-vehicle navigation devices to support decisions about destinations, travel mode, departure time, routes, parking, and trip cancellation. It is important for researchers and practitioners to review the status of ATIS technologies and to understand travelers’ access and response to current ATIS deployment. Focusing on largely public-sector delivery mechanisms, this study answers two fundamental questions: whether accessing more information sources is associated with a higher likelihood of travel decision adjustments and which technologies are more likely to elicit substantive adjustments to routine travel. These questions are answered by using a comprehensive and recent behavioral data set, collected in the Research Triangle area of North Carolina. The study generates useful knowledge about how to operate existing traveler information systems more efficiently and how to improve them in the future. }, number={2069}, journal={Transportation Research Record}, author={Khattak, Asad J. and Pan, Xiaohong and Williams, Billy and Rouphail, Nagui and Fan, Yingling}, year={2008}, pages={77–84} }
 @article{shekhar_williams_2007, title={Adaptive seasonal time series models for forecasting short-term traffic flow}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-40449101916&partnerID=MN8TOARS}, DOI={10.3141/2024-14}, abstractNote={ Conventionally, most traffic forecasting models have been applied in a static framework in which new observations are not used to update model parameters automatically. The need to perform periodic parameter reestimation at each forecast location is a major disadvantage of such models. From a practical standpoint, the usefulness of any model depends not only on its accuracy but also on its ease of implementation and maintenance. This paper presents an adaptive parameter estimation methodology for univariate traffic condition forecasting through use of three well-known filtering techniques: the Kalman filter, recursive least squares, and least mean squares. Results show that forecasts obtained from recursive adaptive filtering methods are comparable with those from maximum likelihood estimated models. The adaptive methods deliver this performance at a significantly lower computational cost. As recursive, self-tuning predictors, the adaptive filters offer plug-and-play capability ideal for implementation in real-time management and control systems. The investigation presented in this paper also demonstrates the robustness and stability of the seasonal time series model underlying the adaptive filtering techniques. }, number={2024}, journal={Transportation Research Record}, author={Shekhar, Shashank and Williams, Billy}, year={2007}, pages={116–125} }
 @article{guo_williams_smith_2007, title={Data collection time intervals for stochastic short-term traffic flow forecasting}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-40449106442&partnerID=MN8TOARS}, DOI={10.3141/2024-03}, abstractNote={ The specification of time intervals for data collection is a fundamental determinant of the nature and utility of the resulting traffic condition data streams. In the context of short-term traffic flow forecasting, the establishment of the data collection time interval should play a key role in determining the corresponding appropriate forecasting approach. The data collection time interval provides the forecasting horizon for one-step-ahead forecasting. Nevertheless, the need for more rigorous understanding of the effects of data collection time interval specification within the context of short-term traffic flow forecasting is not well recognized. By contrast, it has been common practice in previous research to select the data collection time interval and forecasting approach without explicit consideration of time interval effects or systematic evaluation of available forecasting methods. A stochastic seasonal autoregressive integrated moving average plus generalized autoregressive conditional heteroscedasticity (SARIMA+GARCH) structure proposed in previous work holds promise in providing accurate point forecasts and reasonable forecasting confidence intervals. In this paper, a spectrum of data collection time intervals is tested with an online forecasting algorithm developed based on the SARIMA+GARCH structure to determine the applicable data collection time intervals for this structure. In this test, both the forecast accuracy and the validity of the forecasting confidence intervals are investigated. This work serves as an important step toward establishing a short-term traffic condition forecasting framework that identifies appropriate forecasting approaches for candidate data collection time intervals based on the functional requirements of specific applications. }, number={2024}, journal={Transportation Research Record}, author={Guo, Jianhua and Williams, Billy and Smith, Brian L.}, year={2007}, pages={18–26} }
 @article{williams_tagliaferri_meinhold_hummer_rouphail_2007, title={Simulation and analysis of freeway lane reversal for coastal hurricane evacuation}, volume={133}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847207471&partnerID=MN8TOARS}, DOI={10.1061/(ASCE)0733-9488(2007)133:1(61)}, abstractNote={Following the unprecedented multistate evacuation for Hurricane Floyd in 1999, the North Carolina Department of Transportation and the Department of Crime Control and Public Safety jointly developed a lane reversal plan for Interstate 40 to facilitate evacuation of residents and tourists in southeastern North Carolina. Prior to the 2003 hurricane season, the NCDOT sponsored a two-year research to address areas of operational concern. The first year research findings resulted in modifications to the original lane reversal plan. The second year's findings showed the modified reversal plan to be effective in expediting evacuation from the coastal threat zone. However, modeling of the entire lane reversal plan revealed a potentially extensive queue formation at the contraflow termination point. This paper presents an overview of the project along with the principal research findings and resulting plan modifications. In addition, this paper highlights conclusions and recommendations applicable to the general emergency evacuation problem.}, number={1}, journal={Journal of Urban Planning and Development}, author={Williams, Billy and Tagliaferri, A.P. and Meinhold, S.S. and Hummer, J.E. and Rouphail, N.M.}, year={2007}, pages={61–72} }
 @article{williams_guin_2007, title={Trraffic management center use of incident detection algorithms: Findings of a nationwide survey}, volume={8}, ISSN={["1524-9050"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34249913940&partnerID=MN8TOARS}, DOI={10.1109/TITS.2007.894193}, abstractNote={The focus of this paper is the context in which the decision makers for traffic management centers (TMCs) choose whether to include and/or use automatic incident detection (AID) algorithms. A survey was conducted of TMC professionals in positions to make, influence, or provide input to decisions regarding TMC operational policies as well as decisions regarding priorities for future system enhancements. Analysis of the survey results not only provides an understanding of the reasons behind the limited implementation of AID algorithms but also allows a direct comparison between the conventional incident detection methods and the AID technology on the basis of measured and/or perceived performance. It was observed that 90% of the survey respondents feel that the current methods of incident detection are insufficient either at present (70%) or will be so in the future (20%). This finding alone motivates a need to redouble research efforts aimed at developing robust and accurate automatic detection methods. In this regard, this paper presents promising directions to overcome past AID algorithm deficiencies}, number={2}, journal={IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS}, author={Williams, Billy M. and Guin, Angshuman}, year={2007}, month={Jun}, pages={351–358} }
 @article{williams_amekudzi_folds_zureick_kim_2006, title={Comprehensive Evaluation of Guardrail Delineation Systems}, volume={11}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-43249161701&partnerID=MN8TOARS}, DOI={10.1177/1087724X06292289}, abstractNote={ The Georgia Department of Transportation (GDOT) sponsored a controlled evaluation of guardrail delineators. Candidate delineators were compared relative to durability, conspicuity, maintainability, and cost. Guardrail delineator samples from six manufacturers in various shapes, dimensions, and colors were evaluated. The study found no significant differences among the samples based on the environmental and mechanical degradation tests. However, installation and wear issues were observed and noted for one of the candidate systems. Current GDOT standard specifications call for 75-foot guardrail delineator spacing. The conspicuity tests indicate that 100-foot spacing will provide satisfactory conspicuity under certain conditions. Use of 100-foot spacing could lead to significant cost savings if the longer spacing is found to be acceptable in a broad range of situations. }, number={1}, journal={Public Works Management & Policy}, author={Williams, B.M. and Amekudzi, A.A. and Folds, D. and Zureick, A.-H. and Kim, Y.S.}, year={2006}, pages={49–62} }
 @article{baek_hummer_williams_cunningham_2006, title={Reasonable Speed Limits on Suburban Multilane Highways with Curbs}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33846881602&partnerID=MN8TOARS}, DOI={10.3141/1969-04}, abstractNote={When some two-lane roads with 55 mph speed limits are widened to four through lanes, curb and gutter is installed to address issues such as access control, difficult terrain, and limited right-of-way. Posted speed limits along such highway segments are typically decreased to 45 mph in North Carolina because of guidance in the AASHTO Green Book and elsewhere that vertical curbs should not be placed next to high-speed lanes. Although much money is spent to improve such roadways, the results may be viewed negatively by the public, design professionals, and law enforcement personnel. Drivers may be unhappy about getting tickets or driving more slowly, designers are unhappy about being blamed by the public, and police are unhappy about the increased enforcement burden. To help resolve such a dilemma, in this research the team collected relevant data such as speeds and collisions on four-lane road sections with curbs that have 45 or 55 mph speed limits and nontraversable medians or two-way left-turn lanes. The team found that the speed limit does not seem to make an important difference in collision rates or severities for the roads the team examined. The higher speed limit also made relatively small differences in the mean speeds and speed variances observed. Considering all results, the researchers recommended that the North Carolina Department of Transportation continue its current policy of allowing 55 mph speed limits on four-lane roads with curbs on a selective, case-by-case basis.}, number={1969}, journal={Transportation Research Record}, author={Baek, J. and Hummer, J.E. and Williams, B.M. and Cunningham, C.M.}, year={2006}, month={Jan}, pages={10–17} }
 @inbook{baek_hummer_williams_cunningham_2006, title={Reasonable speed limits on suburban multilane highways with curbs}, ISBN={0309099781}, number={1969}, booktitle={Highway safety: law enforcement; alcohol; driver training; safety planning and management; commercial vehicles; and motorcycles}, publisher={Washington: Transportation Research Board Natl Research Council}, author={Baek, J. and Hummer, J. E. and Williams, B. M. and Cunningham, C. M.}, year={2006}, pages={10–17} }
 @inbook{baek_hummer_williams_cunningham_trb_2006, title={Reasonable speed limits on suburban multilane highways with curbs}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000243925000002&KeyUID=WOS:000243925000002}, booktitle={Highway Safety: Law Enforcement; Alcohol; Driver Training; Safety Planning and Management; Commercial Vehicles; and Motorcycles}, author={Baek, Jongdae and Hummer, Joseph E. and Williams, Billy M. and Cunningham, Christopher M. and TRB}, year={2006}, pages={10–17} }
 @article{ni_leonard_williams_2006, title={The network kinematic waves model: A simplified approach to network traffic}, volume={10}, ISSN={["1547-2442"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33745003924&partnerID=MN8TOARS}, DOI={10.1080/15472450500455070}, abstractNote={Flow of traffic on freeways and limited access highways can be represented as a series of kinemetic waves. Solutions to these systems of equations become problematic under congested traffic flow conditions, and under complicated (real-world) networks. A simplified theory of kinematics waves (KWaves) was previously proposed. Simplifying elements includes translation of the problem to moving coordinate system, adoption of triangular speed-density relationships, and adoption of restrictive constraints at the on- and off-ramps. However, these simplifying assumptions preclude application of this technique to most practical situations. By directly addressing the limitations of the original theory, this article proposes a simplified Kwaves model for network traffic (N-KWaves). Several key constraints of the original theory are relaxed. For example, the original merge model, which gives full priority to on-ramp traffic, is relaxed and replaced with a capacity-based weighted queuing (CBWFQ) merge model. The original diverge model, which blocks upstream traffic as a whole when a downstream queue exceeds the diverge, is also relaxed and replaced with a contribution-based weighted splitting (CBWS) diverge model. Based on the above, the original theory is reformulated and extended to address network traffic. Central to the N-KWaves model is a five-step computational procedure based on a generic building block. It is assumed that a freeway network can be represented by the combination of some special cases of the generic building block. An empirical field study showed satisfactory results. The N-KWaves model is best suited for modeling traffic operation in a regional freeway network and has a strong connection to Intelligent Transportation Systems (ITS).}, number={1}, journal={JOURNAL OF INTELLIGENT TRANSPORTATION SYSTEMS}, author={Ni, Daiheng and Leonard, John D., II and Williams, Billy M.}, year={2006}, pages={1–14} }
 @article{ni_leonard_guin_williams_2004, title={Systematic Approach for Validating Traffic Simulation Models}, volume={1876}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-15544373118&partnerID=MN8TOARS}, DOI={10.3141/1876-03}, abstractNote={ Modeling processes and model testing processes are discussed as parts of the model life cycle, and the tasks of these processes and their relations are highlighted. Of particular interest is the model validation process, which ensures that the model closely simulates what the real system does. A collection of validation techniques is presented to facilitate a systematic check of model performance from various perspectives. Under the qualitative category, a few graphical techniques are presented to help a visual examination of the differences between the simulation and the observation. Under the quantitative category, several statistical measures are discussed to quantify the goodness of fit; to achieve a higher level of confidence about model performance, a simultaneous statistical inference technique is proposed that tests both model accuracy and precision. As an illustrative example, these validation techniques are comprehensively applied to test an enhanced macroscopic simulation model, KWaves, in a systematic manner. }, number={1876}, journal={Transportation Research Record}, author={Ni, D. and Leonard, J. and Guin, A. and Williams, B.}, year={2004}, month={Jan}, pages={20–31} }
 @inbook{ni_leonard_guin_williams_2004, title={Systematic approach for validating traffic simulation models}, ISBN={0309094704}, number={1876}, booktitle={Calibration and validation of simulation models 2004}, publisher={Washington, DC: Transportation Research Board}, author={Ni, D. H. and Leonard, J. D. and Guin, A. and Williams, B. M.}, year={2004}, pages={20–31} }
 @inbook{ni_leonard_guin_williams_tbr_2004, title={Systematic approach for validating traffic simulation models}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000227334900003&KeyUID=WOS:000227334900003}, booktitle={Calibration and Validation of Simulation Models 2004}, author={Ni, DH and Leonard, JD and Guin, A and Williams, BM and TBR}, year={2004}, pages={20–31} }
 @article{oliveira_williams_leonard_2003, title={Determining traffic stream impacts of radar detectors using microscopic simulation}, volume={129}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037246604&partnerID=MN8TOARS}, DOI={10.1061/(ASCE)0733-947X(2003)129:1(7)}, abstractNote={Police radar is known to have an effect on the speed of drivers. This effect derives from the presence of vehicles equipped with radar detectors in the traffic stream and the reactions that the drivers of these vehicles have to radar. The level of impact on a traffic stream exposed to radar transmissions is a function of the proportion of radar detector equipped vehicles in the traffic stream, i.e., the radar detector density. Microscopic simulation can be used to model this impact. A microscopic simulator extension is proposed that allows determinations of this type to be carried out by adding the capability to model driver response to radar transmissions. This approach can also be applied to the modeling of other driver warning technologies such as changeable message signs. A case study using the proposed methodology to analyze different congestion and radar detector density levels is presented and discussed. The case study shows that the efficacy of using radar as a speed reduction strategy is a function of congestion and radar detector density, with the strategy being most effective for volumes levels between 200 and 1,400 vehicles per hour per lane.}, number={1}, journal={Journal of Transportation Engineering}, author={Oliveira, M.G. and Williams, B. and Leonard, J.D.}, year={2003}, pages={7–15} }
 @article{williams_hoel_2003, title={Modeling and forecasting vehicular traffic flow as a seasonal ARIMA process: Theoretical basis and empirical results}, volume={129}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0344944192&partnerID=MN8TOARS}, DOI={10.1061/(ASCE)0733-947X(2003)129:6(664)}, abstractNote={This article presents the theoretical basis for modeling univariate traffic condition data streams as seasonal autoregressive integrated moving average processes. This foundation rests on the Wold decomposition theorem and on the assertion that a one-week lagged first seasonal difference applied to discrete interval traffic condition data will yield a weakly stationary transformation. Moreover, empirical results using actual intelligent transportation system data are presented and found to be consistent with the theoretical hypothesis. Conclusions are given on the implications of these assertions and findings relative to ongoing intelligent transportation systems research, deployment, and operations.}, number={6}, journal={Journal of Transportation Engineering}, author={Williams, Billy and Hoel, L.A.}, year={2003}, pages={664–672} }
 @article{smith_williams_keith oswald_2002, title={Comparison of parametric and nonparametric models for traffic flow forecasting}, volume={10}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036692982&partnerID=MN8TOARS}, DOI={10.1016/S0968-090X(02)00009-8}, abstractNote={Single point short-term traffic flow forecasting will play a key role in supporting demand forecasts needed by operational network models. Seasonal autoregressive integrated moving average (ARIMA), a classic parametric modeling approach to time series, and nonparametric regression models have been proposed as well suited for application to single point short-term traffic flow forecasting. Past research has shown seasonal ARIMA models to deliver results that are statistically superior to basic implementations of nonparametric regression. However, the advantages associated with a data-driven nonparametric forecasting approach motivate further investigation of refined nonparametric forecasting methods. Following this motivation, this research effort seeks to examine the theoretical foundation of nonparametric regression and to answer the question of whether nonparametric regression based on heuristically improved forecast generation methods approach the single interval traffic flow prediction performance of seasonal ARIMA models.}, number={4}, journal={Transportation Research Part C: Emerging Technologies}, author={Smith, B.L. and Williams, B.M. and Keith Oswald, R.}, year={2002}, pages={303–321} }
 @article{williams_2001, title={Multivariate Vehicular Traffic Flow Prediction: Evaluation of ARIMAX Modeling}, volume={1776}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035563672&partnerID=MN8TOARS}, DOI={10.3141/1776-25}, abstractNote={ Short-term freeway traffic flow forecasting efforts to date have focused on predictions based solely on previous observations at the location of interest. This univariate prediction is useful for certain types of intelligent transportation system (ITS) forecasts, such as operational demand forecasts at system entry points. In addition, data from upstream sensors should improve forecasts at downstream locations. This motivates investigation of multivariate forecast models that include upstream sensor data. A candidate model is transfer functions with autoregressive integrated moving average errors, otherwise known as the ARIMAX model. The ARIMAX model was applied to motorway data from France that had been the subject of previous traffic flow forecasting research. The results indicate that ARIMAX models provide improved forecast performance over univariate forecast models. However, several issues must be addressed before widespread use of ARIMAX models for ITS forecasts is feasible. These issues include the increased complexity of model specification, estimation, and maintenance; model consistency; model robustness in the face of interruptions in the upstream data series; and variability in the cross-correlation between upstream and downstream observations. The last issue is critical because ARIMAX models assume constant transfer function parameters, whereas the correlation between upstream and downstream observations vary with prevailing traffic conditions, especially traffic stream speed. Therefore, further research is needed to investigate model extensions and refinements to provide a generalizable, self-tuning multivariate forecasting model that is easily implemented and that effectively models varying upstream to downstream correlations. }, number={1776}, journal={Transportation Research Record}, author={Williams, B.M.}, year={2001}, month={Jan}, pages={194–200} }
 @inbook{williams_trb_2001, title={Multivariate vehicular traffic flow prediction - Evaluation of ARIMAX modeling}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000176595900025&KeyUID=WOS:000176595900025}, booktitle={Traffic Flow Theory and Highway Capacity 2001}, author={Williams, BM and TRB}, year={2001}, pages={194–200} }
 @article{williams_hoel_1998, title={Freight planning requirements for interstate corridors}, volume={52}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032021740&partnerID=MN8TOARS}, number={2}, journal={Transportation Quarterly}, author={Williams, B.M. and Hoel, L.A.}, year={1998}, pages={39–48} }
 @article{williams_durvasula_brown_1998, title={Urban Freeway Traffic Flow Prediction: Application of Seasonal Autoregressive Integrated Moving Average and Exponential Smoothing Models}, volume={1644}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032207514&partnerID=MN8TOARS}, DOI={10.3141/1644-14}, abstractNote={ The application of seasonal time series models to the single-interval traffic flow forecasting problem for urban freeways is addressed. Seasonal time series approaches have not been used in previous forecasting research. However, time series of traffic flow data are characterized by definite periodic cycles. Seasonal autoregressive integrated moving average (ARIMA) and Winters exponential smoothing models were developed and tested on data sets belonging to two sites: Telegraph Road and the Woodrow Wilson Bridge on the inner and outer loops of the Capital Beltway in northern Virginia. Data were 15-min flow rates and were the same as used in prior forecasting research by B. Smith. Direct comparisons with the Smith report findings were made and it was found that ARIMA (2, 0, 1)(0, 1, 1)96 and ARIMA (1, 0, 1)(0, 1, 1)96 were the best-fit models for the Telegraph Road and Wilson Bridge sites, respectively. Best-fit Winters exponential smoothing models were also developed for each site. The single-step forecasting results indicate that seasonal ARIMA models outperform the nearest-neighbor, neural network, and historical average models as reported by Smith. }, number={1644}, journal={Transportation Research Record}, author={Williams, B.M. and Durvasula, P.K. and Brown, D.E.}, year={1998}, month={Jan}, pages={132–141} }
 @inbook{williams_durvasula_brown_nrc_1998, title={Urban freeway traffic flow prediction - Application of seasonal autoregressive integrated moving average and exponential smoothing models}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000082030300014&KeyUID=WOS:000082030300014}, booktitle={Traffic Flow Theory}, author={Williams, BM and Durvasula, PK and Brown, DE and NRC}, year={1998}, pages={132–141} }