Exploring the Impact of Connected and Autonomous Vehicles on Freeway Capacity Using Microscopic Traffic Simulation
Pengfei Liu, University of North Carolina, CharlotteShow Abstract
Wei (David) Fan, University of North Carolina, Charlotte
Connected and autonomous vehicle technologies are expected to change the driving behavior on freeway. This study investigates the impact of connected and autonomous vehicles on freeway capacity using VISSIM, a traffic microsimulation tool. A basic freeway segment is selected as the study area through the Caltrans Performance Measurement System (PeMS) located in the northwest Los Angeles. To obtain valid results, various driving behavior parameters are calibrated to the real traffic conditions for human-driven vehicles. In particular, the calibration is conducted using genetic algorithm for standstill distance and minimum headway between vehicles. After the calibration process, the simulation is conducted on the basic freeway segment under mixed traffic environment including regular human-driven vehicles and connected and autonomous vehicles. The results show that a lower penetration rate of connected and autonomous vehicles may have a negative impact on freeway capacity, while a higher penetration rate of connected and autonomous vehicles could increase the freeway capacity. The improvement of freeway capacity varies according to different connected and autonomous vehicles penetration rates.
A Procedure to Estimate the Effect of Autonomous Vehicles on Freeway Capacity
David Stanek, Fehr & PeersShow Abstract
Autonomous vehicles (AVs) are anticipated to improve traffic operations, but the magnitude of their effect on traffic flow is not yet known since the technology is still under development. However, transportation planners and engineers need to know how AVs will affect roadway capacity when developing projects to accommodate future transportation demand. The Highway Capacity Manual (HCM) provides the procedures to evaluate future conditions, but the methodologies are based on empirical research of existing facilities, which do not yet include AVs. To account for the capacity effect of AVs, the HCM procedure for calculating the demand flow rate under base conditions is proposed to be revised to add an adjustment factor for AVs. To develop the AV adjustment factor, traffic simulation analysis software can be applied since it provides the ability to model conditions that are not yet observable, such as a high percentage of AVs in a traffic stream. Given the complex nature of microscopic simulation tools, guidance for modeling AV capacity effects is needed. This paper presents a procedure for using traffic simulation software to develop capacity adjustment factors for AVs. A case study using a previously developed AV driving behavior is described to demonstrate the application of the procedure to estimate the effect of AVs on freeway capacity.
Comparing Alternatives of a 2+1 Highway with Two-Lane Highways Through Simulation
Manuel G. Romana, Universidad Politécnica de MadridShow Abstract
Marilo Martin-Gasulla, University of Florida
Ana Moreno, Technical University of Munich
The rural transportation system is composed mostly of two-lane highways. In recent year 2+1 highways have been built in many countries. 2+1 highways provide a continuous three-lane cross section with alternative passing. Thus, they reduce drivers stress to find a passing gap on the on-coming traffic, as well as minimize the likelihood of head-on collisions with opposing vehicles. 2+1 highways may offer advantages when traffic volumes may not be high enough to justify a four-lane highway, but still higher than the volume that can be served by a two-lane highway, and if a high number of head-on collisions or passing-related collisions happen. This paper summarizes a research seeking to estimate potential operational benefits of converting a Spanish two-lane highway into a 2+1 highway by using traffic simulation. Alternative designs for transition zones have be implemented. The results show that average travel speed was sustainably higher considering the 2+1 highway for traffic flows below 1,200 veh/h. From that point, average travel speed dropped dramatically. Similar results were obtained for PTSF. Transition zone design affected traffic performance: the longer the transition zone, the lower the quality of service. Comparing the Spanish and German designs, the German transition zone provided between 30 and 40 % better results, in both ATS and PTSF. Even though the results suggest modifying the current Spanish design, more research is required to verify that the German design is safe for Spanish drivers. The results and conclusions are valid within the simulation range and for the considered assumptions.
Exploring Lane-Based Vehicular Speed Characteristics in Freeway Work Zones with Drone Video
Zhongyu Wang, Shanghai Maritime UniversityShow Abstract
Wenya He, Public Transit Research Institute, Shenzhen Urban Transport Planning and Design Institute
Xi Zhang, California Department of Transportation (CALTRANS)
Yanli Wang, Tongji University
Bing Wu, Tongji University
Yinhai Wang, University of Washington
Work zones which are usually referred to as traffic bottlenecks or critical points may lead to reductions in both operational performance and safety. Researchers have conducted extensive studies on speed characteristics in freeway work zones. However, most of these studies are based on simulation data or are travel direction-based rather than lane-based. Through the use of drone’s video recording capabilities, this paper aims to explore the vehicular speed characteristics in one type of freeway work zones with considerations for lane and location deviations. A four-to-three lanes work zone on Huhang Expressway between Downtown Shanghai and Songjiang in China was selected for data collection. Then free flow speed variation pattern is analyzed, a speed prediction model is proposed, and the speed-flow relationship is investigated on each lane at nine virtual sections along the work zone. It is found that the merging area affects the speeds mainly in the segment from 200 m upstream of the taper to the start of the activity area and the speed characteristics depend on vehicle types (trucks vs automobiles) as well as lane locations.
Lane Changing in Freeway: A Stochastic Relation with Capacity
Marcel Sala, Universitat Politècnica de Catalunya (UPC)Show Abstract
Francesc Soriguera, UPC - BarcelonaTech
Lane changing activity is thought to play an important role in the capacity degradation of congested freeways. However, proofs of this negative impact are scarce due to the difficulties in obtaining suitable data. In this paper, the lane changing activity in the B-23 freeway accessing the city of Barcelona is analyzed. Lane changes (LC) were video recorded in six different stretches from where loop detector measurements were also available. The obtained database allowed finding a consistent relationship between LC activity and congestion. LC peaks in all analyzed sections when they become congested. This is particularly intense at the traffic breakdown, between congested and free flowing conditions. As an example, it is observed that LC activity peaks just downstream of a fixed bottleneck where free-flowing conditions are recovered. In addition, data show that the larger the lane changing rates, the smaller the maximum observable flows, supporting the hypothesis that LC is a key contributor to a capacity drop. A statistical analysis using Bayesian networks software is done to unveil the stochastic relationship between LC and average flow, including capacity. In spite of all these findings, this research highlights the difficulty in obtaining a suitable database to definitively answer most of the research questions formulated regarding freeway lane-changing. The spatial coverage of measurements is one of the major drawbacks. To this end, a careful planning of the data collection campaign is necessary in order to obtain meaningful conclusions.
Validation of Highway Capacity Manual’s Freeway Travel Time Reliability Prediction Using Probe and Sensor Data
Mohamad Shoaib Samandar, North Carolina State UniversityShow Abstract
Billy Williams, Institute for Transportation Research and Education
Ishtiak Ahmed, North Carolina State University
Nagui Rouphail, North Carolina State University
Travel time reliability is a key system performance indicator according to federal and state transportation agencies. The 6th edition of the Highway Capacity Manual delivered implementable methodologies and tools for predicting the reliability performance of freeways and arterial streets. Although the datasets employed in the development of the methodologies were chosen for robustness and generalizability, a detailed validation, assessing the accuracy and usefulness of the reliability performance predictions through application to multiple locations is important. The central objective of this paper is to provide this validation through real-world case studies on Interstates 540, 95 and 40 in North Carolina. The HCM-generated Travel Time Index (TTI) distribution and key reliability performance measures for the I-540 site closely matched estimates derived from probe and sensor-based data. However, the presence of weigh stations on the I-95 and I-40 sites led the HCM approach to generate shorter travel times in the upper tails of the distribution. An inherent secondary objective is to validate and study the impact of data availability on the freeway scenario generation (FSG) procedures that underlie the travel time distribution estimation. FSG reasonably predicted the number of incident events in the data-rich case but overestimated the number of incidents in both data-poor and data-moderate cases. Although the HCM approach significantly overestimated Heavy Rain and M/H Snow events due to discrete modeling of average weather durations as multiples of analysis periods, the methodology modeled the total duration of each weather severity to within 10% of the empirical data.
Impact of Ramp Metering Queue Override on the Capacity of an Isolated Freeway Merge
Xingan (David) Kan, University of California, BerkeleyShow Abstract
Xiao-Yun Lu, California PATH Program
Alexander Skabardonis, University of California, Berkeley
Freeway ramp metering is an effective control strategy to preserve freeway capacity, reduce freeway delays, and improve travel time reliability. The ramp metering operation may cause on-ramp queue spillback that interferes with the adjacent surface streets. In such situations, most ramp metering systems employ a “queue override” function, which temporarily suspends ramp metering to dissipate the on-ramp queues. A detailed field study was performed at a metered freeway on-ramp merge in San Jose, California using video recordings. The analysis of the collected field data over a two week period show that the freeway bottleneck discharge flow is reduced by 10% on the average when the queue override is activated. The paper provides suggestions for managing queue spillbacks at metered on-ramps.
Evaluating the Effect of Geometry and Control on Freeway Merge Bottleneck Capacity
Mohamadamin Asgharzadeh, University of KansasShow Abstract
Alexandra Kondyli, University of Kansas
Freeway capacity is considered a major factor for planning and operational analysis. This paper evaluated the impact of geometric and operational features on freeway capacity at merge bottleneck locations, by analyzing pre-breakdown flow rates and using parametric and non-parametric techniques. These features included number of lanes, free-flow speed or speed limit, length of acceleration lane, and presence or ramp meters. The analysis was carried out on seventeen freeway merge sites across the United States. The number of lanes showed negative relationship with the per-lane average pre-breakdown flow rate. Sites equipped with ramp meters also showed higher pre-breakdown flow rates than unmetered sites. In survival analysis, the number of lanes and presence of ramp meters were the only features found to have a statistically significant impact on the survival (or breakdown) probability. Capacities, defined at the flow rate that corresponds to 15 percent breakdown probability, were 2,048, 1,959, and 1,745 pc/hr/ln at three-lane, four-lane, and five-lane unmetered sites, respectively. Capacities were 2,248 pc/hr/ln at three-lane and 2,132 pc/hr/ln at four-lane metered sites. The effect of acceleration lane length and Free Flow Speed (FFS) on pre-breakdown flow rate and survival probability was not found to be significant.
Analyzing a Tandem Fluid Queueing Model with Stochastic Capacity and Spillback
Li Jin, Massachusetts Institute of Technology (MIT)Show Abstract
Saurabh Amin, Massachusetts Institute of Technology (MIT)
We study the joint impact of stochastic capacity and spillback on the long-time properties of fluid queueing systems. Our analysis focuses on the system of two fluid queueing links in series. The upstream link has a constant capacity (saturation rate) and an infinite buffer size. The downstream link has a stochastic capacity and a finite buffer size. Queue spillback occurs when the the downstream link is full. We derive a necessary condition and a sufficient condition for the total queue length to be bounded on average. The necessary (resp. sufficient) condition leads to an upper (resp. lower) bound for the throughput of the two-link system. Using our results, we analyze the sensitivity of throughput of the two-link system with respect to the frequency and intensity of capacity disruptions, and to the buffer size. In addition, we discuss how our analysis can be extended to feedback-controlled systems and to networks consisting of merges and splits.
An Efficiency-Based Methodology to Estimate Freeway Capacity: Analytical Properties and Practical Implications
Mohammad Rayeedul Kalam Siam, Oregon State UniversityShow Abstract
Changqiao Shao, Nanyang Technological University
Haizhong Wang, Oregon State University
Traditional capacity approaches focus only on the vehicle-carrying ability of a facility while ignoring the practical, well defined purpose of a freeway segment characterized by higher speed. Furthermore, these definitions lead to the two-capacity phenomenon which create confusion among practitioners and researchers over which one is the appropriate value. To address these issues, this paper puts forth a new understanding of freeway capacity by considering the optimal traffic state idea where the infrastructure is operating in the most efficient way. This paper endeavors to explain, based on simple, mathematically-sound rationale, the existence of the optimal traffic state that may lead to a substantial amelioration of traffic conditions on freeways. A case study of ramp metering is undertaken to demonstrate the incorporation of the efficiency-based capacity in real-time traffic control and evaluate the additional benefits that can be achieved. Results show that ramp metering control with efficiency-based capacity reduces the upstream congestion compared to when conventional capacity is used.
Developing Procedures to Calibrate Travel Time Reliability Using NPMRDS Data
Mohammad Razaur Shaon, University of Wisconsin, MilwaukeeShow Abstract
Xiao Qin, University of Wisconsin, Milwaukee
Ambily Pankaj, University of Wisconsin, Milwaukee
Elizabeth Schneider, Wisconsin Department of Transportation
Benjamin Rouleau, Wisconsin Department of Transportation
Travel time reliability predicts the percentage of trips that may fail or succeed according to a pre-determined threshold. Reliable travel time is essential to travelers as they understand congestion is inevitable but they can plan for it. Because of the factors such as weather, accidents, work zones, special events, travel time varies between origin and destination and the variability is better to be presented by a probability distribution. In light of the Highway Capacity Manual (HCM) analytical procedures for computing freeway travel time, a suite of reliability methods and tools have been developed under the Strategic Highway Research Program 2 (SHRP 2) Reliability Area. In particular, a new analytical procedure and its accompanying tool, FREEVAL-RL, enables the incorporation of travel time reliability into the HCM. Although the approach is technically sound, calibrating the tool to match the real-world conditions is non-trivial. In this study, a general procedure has been developed to guide the calibration of FREEVAL-RL. Following the fundamentals of traffic flow theory, data-driven sensitivity analyses have been proposed to determine the importance of input variables according to their influence on travel time distribution. The calibration results were demonstrated in a case study in which the mean, 50th, 80th, and 95th percentile and Misery index of Travel Time Index (TTI) matched with what was reported in the National Performance Management Research Data Set (NPMRDS).
Evaluating the Impacts of the Road Marking Line Design on the Weaving Capacity in the Urban Area
Jingya Zhao, Southeast UniversityShow Abstract
Jie Bao, Southeast University
Pengying Ouyang, Southeast University
Chengcheng Xu, Southeast University
A special road marking line design has been common in the weaving segment between the expressways and urban roads, which separates the lane-changing positions of the merging and diverging flows within the weaving segment. This study evaluated the impacts of such road marking line design on the weaving capacity for seven sites in Nanjing, China. The simulation tool and gap acceptance theory were used for capacity estimation. The results were compared with the ordinary type A weaving segment. The findings indicated that the impacts of such road marking line design on the capacity is related to the volume ratio, the merging and diverging ratio. According to the sensitive analysis at the seven sites, it is unnecessary and unreasonable to implement such road marking line design in the following three cases: (1) when the volume ratio is larger than 0.7; (2) when the difference between the merging and diverging ratio is larger than 0.5.; (3) when the diverging ratio is larger than 0.6. Further, when the difference between merging and diverging ratio is around 0.2, such road marking line design can increase the weaving capacity in comparison with the ordinary type A weaving segment. Therefore, this study can provide some reasonable suggestions for the road designers to determine whether to implement such road marking line design in the weaving segment in the urban area.
A Data-Driven Approach for Estimation of Passenger Car Equivalents on Basic Freeway Segments Operating at Capacity
John Ash, University of WashingtonShow Abstract
kristian Henrickson, University of Washington
Yinhai Wang, University of Washington
Freeway capacity analyses are critical for transportation professionals to both assess the current state of a facility’s operations and plan for construction. Such analyses involve converting the demand volume into a demand flow rate in units of passenger car per lane per hour through an adjustment that uses a passenger car equivalent (PCE) factor. This PCE factor represents the equivalent effect heavy vehicles have on capacity in terms of a representative passenger car. Estimation of PCEs for basic freeway segments has been a topic of research for decades. Existing studies have often sought to estimate PCEs based on either small sets of data that are not readily available (e.g., individual vehicle headway measurements) or use of traffic microsimulation. Indeed, PCE values used in the 2016 Highway Capacity Manual (HCM) are based on the results of VISSIM simulation. While these approaches have their benefits (for example one can control and adjust many environmental and vehicular factors in a simulation to a very precise degree), they are not without their limitations. Further, such studies have often found results that do not directly agree with those in the HCM or other studies. In order to overcome some of the drawbacks of existing studies, a data-driven approach to estimate PCEs for basic freeway segments operating at capacity is developed herein. The method makes use of large quantities of readily available real-world data, namely, volume and other information obtained from dual loop detectors, in order to estimate PCEs via an equal impedance method.
Analysis of Urban Weaving Sections at Greater Cairo
Abdelrahman Awad, Cairo UniversityShow Abstract
Laila Radwan, Professor of highways
Ahmed Khalil, Benha University
Previous researches focused on freeway weaving, not urban weaving. Therefore, this research conducts a general analysis of urban weaving sections at Greater Cairo, especially urban collectors and locals. First, the research covers all weaving configurations in all selected sites. Then, speed and volume data are collected from each site by video observation at peak hours. These data develop volume models and speed models using linear regression. The volume models show that weaving section entrance volume affects predicted exit volume greatly. Moreover, the section geometry is the major factor for two-sided and rotaries, but urban weaving affects one-sided sections. The speed models show that predicted weaving speed is sensitive to entrance speed, but traffic condition affect two-sided speed. Finally, the study evaluates all sections using weaving delay or average density to state critical sections. The speed models solve the critical sections by optimizing major weaving parameters.
Analysis of the Longitudinal Variations in Traffic Capacity at Freeway Bottlenecks
Yasuhiro Shiomi, Ritsumeikan UniversityShow Abstract
Jian Xing, Nippon Expressway Research Institute
Hodaka Kai, Nippon Expressway Research Institute
Tomoya Katayama, Ritsumeikan University
In this paper, we analyze the longitudinal variation in traffic capacity at nine typical bottlenecks with various causes on intercity expressways in Japan. First, the evaluation indices of traffic capacity are rigorously defined based on three factors: the ease of phase transition from free flow to congested flow is represented by the fifth-percentile traffic volume of breakdown probability; its reliability is defined by the gap between the 50th- and fifth-percentile traffic volume of breakdown probability; and the smoothness of traffic flow after the capacity drop is evaluated as the mean of the queue discharge flow rate. The findings show that: i) the fifth-percentile traffic volume of traffic breakdown probability shows a longitudinal decreasing tendency, and the values in August and December are lower than the yearly average; and ii) the gap between the 50th- and fifth -percentile traffic volume of traffic breakdown probability and discharge flow rate tend to decrease, but the trends are not significant. Finally, we suggest that the decreasing trends might be attributed to slight changes in driving skills, driving characteristics or vehicle types in the past decade.
Sensitivity Analyses of Speed Limit, Truck Lane Restrictions, and Data Aggregation Level on the 2016 HCM PCE Estimation Methodology for Western U.S. Conditions
Jianan Zhou, University of Nebraska, LincolnShow Abstract
Laurence Rilett, University of Nebraska, Lincoln
Elizabeth Jones, University of Nebraska, Lincoln
In the 2016 Highway Capacity Manual (HCM), the impacts of trucks on freeway operation are measured by the passenger car equivalent (PCE). The PCEs are estimated by the “equal-capacity” methodology. The HCM-6 PCE values are based on the assumptions that passenger cars and trucks travel at the same free-flow speed, that they travel on freeways with three lanes per direction, and they travel on freeways with no more than 25 percent trucks. On I-80 in western Nebraska, it is observed the interaction of the high truck percentages and large speed differences between passenger cars and trucks may result in moving bottlenecks. It was hypothesized that the current HCM-6 PCEs may be not appropriate for these conditions. A companion paper showed this was true and that the major cause was speed differentials between trucks and passenger cars. In essence, when slow moving trucks pass each other they create moving bottlenecks which results in increased PCEs. This paper is an extension to a companion paper and examines a number of issues related to PCE estimation. The paper examines the effect of speed limit, truck passing restrictions, and data aggregation interval on PCEs. The results show that 1) if a higher speed limit is implemented, the trucks will affect the passenger cars more severely; 2) if truck passing is restricted by lane restrictions, the negative impacts of trucks on passenger car operation may be mitigated; and 3) using a longer data aggregation interval results in lower PCE values, all else being equal.
Comparison of Stochastic Estimates of Capacity and Critical Density for U.S. and German Freeways
Justin Geistefeldt, Ruhr University, BochumShow Abstract
Siavash Shojaat, Arcadis
The randomness of freeway capacity was analyzed in a large number of studies. In contrast, rather little attention was drawn to the critical density at capacity and its relationship to breakdown occurrence so far, although density is being used as quality of service criterion for freeways. In the paper, distributions of freeway capacity and critical density are estimated and compared based on traffic data samples from 38 freeway bottleneck sections in the U.S. and Germany. It is shown that the well-established methods for stochastic capacity analysis can be applied to estimate critical density distributions by replacing volume with density in the corresponding mathematical models. The comparison of the estimated capacity and critical density distribution functions reveals that the relative variability of the capacity is lower than the variability of the critical density. This suggests that the traffic volume is the more appropriate parameter to represent the trigger of traffic breakdowns than the traffic density. The empirical results also show that the rather smooth traffic flow on U.S. freeways leads to a smaller variance of both the capacity and the critical density distribution compared with German freeways, whereas the average capacity per lane is roughly the same.
Lane Change Rates at Freeway Weaving Sites: Trends In HCM6 and from NGSIM Trajectories
Ishtiak Ahmed, North Carolina State UniversityShow Abstract
Dezhong Xu, North Carolina State University
Nagui Rouphail, North Carolina State University
Alan Karr, RTI International
Concerns have been raised regarding the HCM6 weaving method lack of sensitivity to the weaving segment length. This study explores the trends in HCM6 as they relate to lane change estimates and their impact on the segment speed and LOS. The study also compares the HCM6 estimates of lane changes against empirical data from a congested NGSIM weaving site. Thus, the objectives of this study are twofold: (a) critically investigate the effect of weaving length on lane change and associated speed model estimates in HCM6, and (b) analyze trends in lane changes against congestion levels using detailed NGSIM trajectory data, and compare against HCM6 estimates. Regarding the first objective, it was found that the lack of sensitivity to weave length is the absence of this parameter in the non-weaving lane change and speed models. Regarding the second objective, a comparison of HCM6 lane change rates with NGSIM, US-101 data confirmed that the HCM6 estimates for weaving vehicles are fully consistent with those at the NGSIM site, controlling for density. On the other hand, non-weaving lane change estimates in HCM6 did not deliver the expected trends, with more discretionary lane changes predicted as congestion increased. Finally, the analysis of lane change patterns at the NGSIM site revealed a tendency for early merging for freeway-to-ramp-traffic and uniform merging for ramp-to-freeway traffic over the length of the weave. Interestingly, a speed analysis showed that in most cases, a higher frequency of discretionary lane changes yielded lower travel times for those drivers executing them.
Capacity Modeling of Weaving Areas on an Urban Expressway with the Exclusive Bus Lanes Based on Gap Acceptance Theory
Xumei Chen, Beijing Jiaotong UniversityShow Abstract
Xiaomi Han, Beijing Jiaotong University
Xianchao Jia, China Railway SIYUAN Survey & Design Group Co., Ltd
Lei Yu, Texas Southern University
Intense lane-changing maneuvers at weaving sections often cause traffic turbulence along expressway, especially in the presence of concurrent medium exclusive bus lane (XBL) and General Purpose (GP) Lanes. Such intense lane changing activity usually affects the operation and reduces the capacity of weaving sections in relation to their equivalent basic expressway segments. In this context, a study on the capacity model of weaving areas on the expressway with a median XBL is conducted based on the analysis of lane change behaviors using gap acceptance theory. Two weaving sections on expressway with a median XBL are selected as the case studies to obtain the estimated capacity as well as the maximum traffic throughput under a certain bus saturation on the XBL. The results show that estimated capacity is larger than maximum traffic throughput due to low utilization rate of buses on the XBL, and capacity is significantly affected by weaving demand. Besides, error measures based on the estimated and observed maximum traffic throughput are analyzed to verify the validity of the proposed model. A sensitivity analysis shows that, compared with the increase of on-ramp bus flow ratio, the increase of off-ramp bus flow ratio results in a more obvious trend of the reduction of capacity and maximum traffic throughput.
Development and Calibration of VISSIM Models for Rural Freeway Lane Closures: A Novel Approach to the Modification of Key Parameters
Nicholas Jehn, Kimley-Horn and Associates, Inc.Show Abstract
Rod Turochy, Auburn University
The definition of freeway work zone capacity has been a topic of debate for several decades, leaving agencies with limited guidance on predicting the behavior of traffic flow at given volumes for various work zone configurations. The methodology presented in the recently published 6th edition of the Highway Capacity Manual is a substantial improvement over historical guidance and provides estimates of the mean queue discharge rate under a variety of prevailing site conditions. However, it is limited by the fact that its outputs are deterministic, while traffic flow and breakdown are stochastic phenomena. Recently, well-calibrated microsimulation models have shown promise as a freeway work zone traffic analysis tool, but most guidance is focused on site-specific modeling. This research aimed to address these shortcomings by presenting a novel approach to developing and calibrating generalizable microsimulation models for rural freeway lane closures in VISSIM. Specifically, it was determined that such models may best replicate field conditions at rural freeway work zones when time headway is described by a field-measured distribution and truck characteristics are representative of the U.S. fleet. The results suggested that the default desired acceleration for heavy trucks should be set between 2 and 3 ft/s2 and that separate time headway distributions should be constructed for passenger cars and trucks. The methodology presented herein may be extended to obtain stochastic estimates of capacity for sites exhibiting a variety of geometric, traffic, and environmental characteristics.
A Lane-by-Lane Analysis Framework for Conducting Highway Capacity Analyses at Freeway Segments
Fabio Sasahara, University of FloridaShow Abstract
Lily Elefteriadou, University of Florida
Shen Dong, University of Florida
The Highway Capacity Manual methodology for freeway systems yields average speed values for each segment and does not consider lane-by-lane flow and operational conditions. However, flows are not equally distributed between lanes. During congested conditions and particularly when spillback occurs, flows and traffic conditions vary widely. For example, the rightmost lane may be blocked while the leftmost lane is free-flowing. The purpose of this research is to develop a model for estimating lane by lane speeds and flows under various freeway designs and demands. Speed and flow data from loop detectors at several locations around the US were collected, totaling 531,000 observations aggregated in 15-min intervals. The results show that lane flow distribution highly depends on the segment total flow, with different patterns for 4, 6 and 8-lane segments. The percent of heavy vehicles, presence of nearby ramps, day of week and time of day also affect the distribution of flow among freeway lanes. Theoretical lane-by-lane speed-flow curves were developed and results were compared to field data. Results showed that lane-by-lane speeds can be estimated accurately, as long as inputs for capacity and free-flow speeds can be provided for each lane in the segment.
Estimating Passenger Car Equivalent Using the 2016 HCM PCE Methodology on Four-Lane Level Freeway Segments in the Western United States
Jianan Zhou, University of Nebraska, LincolnShow Abstract
Laurence Rilett, University of Nebraska, Lincoln
Elizabeth Jones, University of Nebraska, Lincoln
The passenger car equivalent (PCE) of a truck is used to account for the presence of trucks in the Highway Capacity Manual (HCM). The HCM-6 employed an equivalency capacity methodology to estimate PCE. It is hypothesized in this paper that the HCM-6 PCE values are not appropriate for the western U.S., which consistently experiences truck percentages higher than 25 percent. Furthermore, the HCM PCE procedure assumes that truck and passenger cars travel at the same desired free-flow speed on level terrain. However, many heavy trucks in the western U.S. are governed through the use of speed limiters so that their speeds are considerably less than the speed limit. Thirdly, the HCM-6 PCEs are based on the freeways having three lanes per direction, which might not be appropriate for the freeways having two lanes per direction predominate in the rural sections of the western U.S. Lastly, the trucks used in the HCM-6 simulation might not be representative of the empirical trucks observed on rural freeways in western states. This paper examines these effects on PCEs using data from I-80 in western Nebraska. The PCEs were estimated using the HCM-6 equal-capacity method and VISSIM 9.0 simulation data under 1) the HCM-6 conditions and 2) the Nebraska empirical conditions. It was found that the PCEs recommended in HCM-6 underestimate the effects of trucks on four-lane level freeway segments that experience high truck percentages having large differences in free-flow speed distributions, and which have different truck lengths.
Evaluating the Operational Impact of Narrow Lanes and Shoulders for the Highway Capacity Manual
Alexandra Kondyli, University of KansasShow Abstract
David Hale, Leidos, Inc.
Mohamadamin Asgharzadeh, University of Kansas
Bastian Schroeder, Kittelson & Associates, Inc. (KAI)
Anxi Jia, Kittelson & Associates, Inc. (KAI)
Joe Bared, Federal Highway Administration (FHWA)
Unnecessary traffic delays and vehicle emissions produce adverse impacts on quality of life. To solve the U.S. traffic congestion problem, mitigation or elimination of bottlenecks is a top priority. Agencies across the U.S. have deployed several congestion mitigation strategies, such as lane and shoulder width reduction, which aims in adding one lane without significantly altering the footprint of the freeway. A limited number of studies have evaluated the operational benefits of lane narrowing. Although the Highway Capacity Manual does account for lane and shoulder widths, the adjustments that it provides are outdated. The goal of this research was to develop analytical models, compatible with the Highway Capacity Manual methods, to account for lane and shoulder width narrowing, using field data across the U.S. This paper presents a new free-flow speed regression model, which accounts for lane and shoulder widths, and capacity adjustment factors depending on the lane width.
Travel Time Reliability Analysis Using Microsimulation
Nima Haghighi Naeini, University of UtahShow Abstract
Xiaoyue Cathy Liu, University of Utah
Travel time reliability is a crucial factor in evaluating transportation systems performance. Estimating travel time reliability is important for individual travelers as well as transportation agencies. This study developed a framework to model and estimate travel time reliability using microscopic simulation. The framework consists of three major components: scenario manager, scenario processor, and traffic simulation model. The scenario manager captures exogenous sources of travel time variability by generating multiple scenarios composed of various events (e.g. weather, incidents) based on real-world disruption data. The scenario processor translates scenarios’ attributes to inputs that can be fed into microscopic simulation model and modifies simulation input parameters values in real-time. A microsimulation model is then utilized to simulate the generated scenarios and to estimate the effects on travel time reliability. The proposed framework is applied to VISSIM model of a corridor along Interstate 15 in Salt Lake City, Utah to show the impact of weather condition on travel time reliability.
Validating the Adaptability of Travel Time Reliability Measurements Using Probe Data
Zhuo Chen, University of UtahShow Abstract
Xiaoyue Cathy Liu, University of Utah
Grant Farnsworth, Utah Department of Transportation
Kelly Burns, Utah Department of Transportation
Travel time reliability (TTR) is considered a critical piece in highway performance evaluation. The L02 project from Strategic Highway Research Program 2 (SHRP2) has developed a holistic method using statistical probability functions of travel time as the TTR measure to build highway performance evaluation and monitoring systems. Compared with single-value reliability measures, the L02 measure is able to identify unreliability sources and quantity their associated impacts. To validate the adaptability of L02 measure, TTR analysis on I-15 freeway corridor in Salt Lake City, Utah using probe data has been conducted. The result is compared against output from the quadrant-based TTR measure that is currently used by the Utah Department of Transportation (UDOT). Through cross validation, it is determined that the two suites of measures demonstrate good consistency in terms of reliability assessment and unreliability source diagnoses. In addition, the study provides a method to calibrate the quadrant-based TTR measure, and new critical values were developed based on the cross validation.