Search

Connected and automated vehicles (CAVs) will undoubtedly transform many aspects of transportation systems in the future. In the meantime, transportation agencies must make investment and policy decisions to address the future needs of the transportation system. This research provides much-needed guidance for agencies about planning-level capacities in a CAV future and quantify Highway Capacity Manual (HCM) capacities as a function of CAV penetration rates and vehicle behaviors such as car-following, lane change, and merge. As a result of numerous uncertainties on CAV implementation policies, the study considers many scenarios including variations in parameters (including CAV gap/headway settings), roadway geometry, and traffic characteristics. More specifically, this study considers basic freeway, freeway merge, and freeway weaving segments in which various simulation scenarios are evaluated using two major CAV applications: cooperative adaptive cruise control and advanced merging. Data from microscopic traffic simulation are collected to develop capacity adjustment factors for CAVs. Results show that the existence of CAVs in the traffic stream can significantly enhance the roadway capacity (by as much as 35% to 40% under certain cases), not only on basic freeways but also on merge and weaving segments, as the CAV market penetration rate increases. The human driver behavior of baseline traffic also affects the capacity benefits, particularly at lower CAV market penetration rates. Finally, tables of capacity adjustment factors and corresponding regression models are developed for HCM implementation of the results of this study.

Unnecessary traffic delays and vehicle emissions have adverse effects on quality of life. To solve the traffic congestion problem in the U.S.A., mitigation or elimination of bottlenecks is a top priority. Agencies across the U.S.A. have deployed several congestion mitigation strategies, such as lane and shoulder width reduction, which aim to adding lanes 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 Although the Highway Capacity Manual does account for lane and shoulder widths, the adjustments that it provides are outdated.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 from across the U.S.A. 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.

The paper provides methodologies for microscopic simulation modelling of freeway work zones with lane closures in the proximity of interchanges. In addition, the research presents simulation results using calibrated modelling parameters to determine the capacity available for mainline flow under various operating conditions. The results will help researchers or practitioners with designing and studying freeway work zone sites under various lane configurations using better estimates of delays and queue sizes in the proximity of interchanges. Additionally, a 2 to 1 on-ramp merge condition was explored to predict the merge ratios under both demand and service flow conditions. The paper illustrates how ramp demand affects the share of segment capacity available to mainline traffic to reveal strategies for controlling this demand (via ramp metering for example) in order to maintain a prescribed freeway capacity through the work zone. The overall study result indicates the degree to which the presence of a work zone with lane closures decreases the proportion of mainline flow and shows the degree to which differences in work zone lane configurations affect merge and weave scenarios.

The diverging diamond interchange (DDI), also known as the double crossover diamond interchange, has been a successful, if unconventional, solution used in many parts of the United States ever since its first installation in Springfield, Missouri, in June 2009. One of the challenges to agencies in planning and operating DDIs is to apply to them methodologies developed for a conventional diamond interchange. The Highway Capacity Manual 2010 (HCM 2010) provides lane use analysis models for various lane configurations. However, there is no guarantee that these models will work for DDIs. For this reason, 11 DDIs were studied nationwide ( a) to examine whether the current HCM lane use models provided accurate results for DDIs and ( b) to develop new lane use models for them if the HCM models did not work. As a result of the study, unique multiregime lane utilization models were proposed, separated by the number of approach lanes and validated by field data not used in the model development.

Work zones and lane closures on urban arterials can cause significant disruptions to the traveling public, and methods are increasingly needed to estimate the reductions to saturation flow rates that result from work zones at signalized intersections. A set of statistical models that estimate saturation headways as a function of the presence and configuration of the work zone on signalized arterial streets is presented. More than 10,000 individual vehicular headway observations were collected from video observations in and after work zones at six study sites in North Carolina. Conventional multiple-regression and path-based-regression models (structural equation model) were used to develop the saturation headway models. Three models are provided at different aggregation levels of the collected data with identical work zone configurations. The models developed at cycle-length, 15-min, and full aggregation produced adjusted R-squared values of .3259, .7209, and .895, respectively. The proposed model incorporates the effects of lane configuration, pavement condition, turning percentage from shared lanes, work intensity, and number of closed exclusive turning lanes. Based on path analysis, the structural equation model satisfies all the rule-of-thumb criteria for goodness-of-fit indices. The model uses Highway Capacity Manual default values for turning-vehicle headway effect as its intercept coefficient value.

Driver behavior analysis has recently been the focus of research to enhance traffic simulation modeling. Previous research has developed algorithms to model different driver types in both urban networks and freeways, and defined driver behavioral parameters based on pre-categorized driver groups. However, driver behaviors within the more confined, low speed, high pedestrian flow environment (such as CBD of a major city, urban downtown, campus) may be quite different and research concentrating on such locations is limited. The objective of this study is to assess and evaluate the relation between driver type and driver behavior within a relatively restrictive environment with high levels of pedestrian–vehicle interactions and develop suitable schemes to categorize driver types. To meet this objective, the University of Florida campus environment was used as the starting point of analyzing such driver types and behaviors, and a diverse pool of drivers was selected based on age, gender, driving experiences, ...

Diverging diamond interchanges (DDIs) are relatively new in the United States, and signal coordination between the crossovers and adjacent intersections is challenging. This paper provides a method for remotely fine-tuning offsets for a DDI and its adjacent intersections. The proposed method uses the dynamic bandwidth analysis tool (DBAT). The tool uses actuated phase times from the signal controller to optimize the dynamic bandwidth on the basis of that entry data set. Four performance measures evaluated the proposed method: delay, stop severity index, maximum queue, and vehicle trajectory plots. The test results confirmed that DBAT provided a better offset solution than other bandwidth optimization tools that generally optimized programmed bandwidth only and did not account for early return to green caused by skipped or gapped-out movements. Under the DBAT offsets, delay for the through movements on the corridor decreased by 52.8% for northbound vehicles and 46.83% for southbound vehicles. The average delay reduction over all measured paths for uncongested and congested scenarios was 13.88% and 3.50%, respectively. The proposed method and workflow can significantly reduce the offset retiming work process. Normally, this manual process takes more than a day, but the proposed method can be completed in less than an hour without visiting the study site. Furthermore, the proposed method can coordinate any set of movements, as well as multiple travel paths. The authors believe that the proposed method and workflow will significantly help both retiming and new timing of arterial signal coordination along DDI corridors and other signal systems.

The pedestrian mode is an important component of urban networks, and greatly affects the performance of the sidewalks and crosswalks, as well as the entire network traffic operations by interacting with other traffic modes (automobile, bicycle, transit). There have been many studies concerning different aspects of pedestrian behaviors, such as pedestrian walking speed, pedestrian delay, gap acceptance, signal compliance, route choice, etc. The Highway Capacity Manual (HCM) included the pedestrian mode in the HCM 1994 (update to the HCM 1985) and the HCM, 2010 provided several basic technical methodologies for evaluating the pedestrian level of service (LOS) of different urban street facilities. However, it does not fully cover the pedestrian operation in a great detail and it is missing some important findings in recent studies. The objective of this paper is to provide an overview of pedestrian operations in urban networks, to identify the important aspects of pedestrian operation analysis and to provide recommendations for pedestrian facilities in the HCM on the basis of a summary of available U.S. and international literature. The following topics will be discussed: pedestrian movement models, pedestrian delay estimation, pedestrian-vehicle interactions, and pedestrian travel time estimation. The major features of pedestrian movement along urban segments are the walking speed and available space, which were well examined in past studies. Pedestrian delay is one important measure of pedestrian operation, and its estimation models at signalized/unsignalized intersections are presented with respect to different scenarios. Modifications to the delay model in the HCM, 2010 are highlighted. Pedestrian-vehicle interactions highly affect traffic operations. Both driver and pedestrian behaviors in such an interaction were observed in the field and were examined quantitatively. Pedestrian jaywalking behavior was also observed and its impact on traffic is discussed. Pedestrian route choice at the strategic level and crossing location selection at the operational level jointly determine the pedestrian moving path and further affect the travel time at path-level. A model for estimating pedestrian travel time in an urban network is developed and presented. Such a model can be used to evaluate the facility performance as well as serve for pedestrian route selection. This paper identifies the important aspects of pedestrian operation analysis in an urban network and provides several recommendations to the HCM.

Diverging diamond interchanges (DDIs) have swept the country as an innovative interchange system since the original U.S. installation in Springfield, Missouri, in 2009. Although the DDI provides many benefits, prior research identified some operational challenges found at some of the first DDIs built around the country. This study explored the challenge of limited capacity for the right turn at the DDI ramp terminal intersection and associated queue spillback concerns that have been observed at several sites. The study investigated the impact of five geometric treatments at a DDI’s right-turn exit ramp, including a channelized turn lane with an auxiliary lane, dual right-turn lanes, right turn on red (RTOR), dual right-turn lanes with RTOR, and a slip lane. The results showed that any treatment that decreased the green-to-cycle length ratio for the right turn from the exit ramp simultaneously reduced queues at the exit ramp and the outbound through movement at the crossover. However, treatments that did not also increase the capacity of the outbound through movement at the downstream adjacent intersection resulted in increased queues at that movement. The study concluded that the design of the DDI must be considered in the context of the entire corridor. To provide the full extent of congestion relief that is possible with a DDI, consideration of the geometrics of downstream, closely spaced, adjacent intersections must be included from the earliest planning stages.

This study provides a methodology for calibrating freeway work zone capacity in a microsimulation environment and guidance for replicating field-observed freeway work zone capacity through simulation. From 81 field observations at 12 U.S. work zone sites, 90 work zone sites from literature archival sources, and a macroscopic capacity model developed in NCHRP Project 03-107, the authors show how to replicate field-observed or forecast capacity in the Vissim simulation tool under various scenarios of freeway work zone lane closure. With guidance from an in-depth literature review, key car-following and lane-changing parameters are proposed as a result of the calibration effort and extensive sensitivity tests of numerous combinations of parameter values. Lane configuration specific guidance is provided for two key car-following parameters in Vissim, cc1 and cc2. During the process of developing the guidance, every tested capacity scenario was verified by using a lane use volume balance that was obtained upstream of the lane closure point in order to replicate realistic freeway work zone conditions. The calibration methodology as well as default parameter guidance developed through this research are useful to practitioners who wish to model freeway work zone impacts accurately through microsimulation.

A diverging diamond interchange (DDI) is an innovative interchange design that is being used with increasing frequency in the United States because of the ability to use existing right-of-way and infrastructure frequently. This paper documents the state of the practice in DDI signal phasing with principles established in the Signal Timing Manual, namely, that it uses consistent phase numbering in a logical format to describe each of the phasing concepts being considered and discussed in this paper. Although some literature exists on several methods of signal phasing for DDIs, inconsistency in the reporting leaves practitioners scratching their heads when attempting to decipher what phasing scheme to use. In response, three fundamental phasing schemes are presented, one each for volume patterns with two, three, and four critical movements. Each scheme is described with a consistent naming convention, and, when possible, each is manipulated to provide varying coordination strategies. In addition, concepts such as use of actuation and barriers to improve coordination are described, and methods for reducing lost time are introduced. Other supplemental information related to preemption and pedestrians is discussed briefly. In conclusion, practitioners are given some basic guidance on when one phasing scheme may be more appropriate than another.

This study developed a validation procedure for a dynamic traffic assignment–based mesoscopic simulator and identified its relevant data requirements. Model inconsistencies at the network level can be detected through route-based validation with probe vehicle travel time data. Simultaneously, link-based validation uses point detector–based volume and speed data. A procedure for filtering the validation data set to make it consistent and comparable with the simulation results is outlined in the paper. Two assignment methods were tested with a detailed validation data set through a case study. The no-diversion assignment approach yielded higher travel time than the field-observed average time. In contrast, simulated travel time with the optimal diversion method implied a more rational behavior by drivers than was observed in the field, along with more diversion of trips on arterial routes in the vicinity of a work zone. Finally, a link-based performance comparison revealed worse queue spillback effects on the upstream links of the work zone than in reality.

Conventional traffic engineering studies at two-way stop controlled (TWSC) pedestrian crossing locations have been largely interested in estimation of capacity and delay impacts due to pedestrian impedance. Review of the literature indicates that pedestrian gap acceptance behavior at TWSC locations has not been investigated in detail. The objective of this research was to obtain and use empirical observations from study sites in Alabama to develop pedestrian gap acceptance models at TWSC pedestrian crossings. Visibility due to daylight conditions was also explored. Discrete choice models with underlying normal and logistic distribution of error terms were developed. From the set of developed models, a probit model was found to be suitable for further empirical validation. Based on this model, gap length, presence of vehicle at minor street waiting for right of way, opportunity to cross the street in lag, and daylight visibility were found to affect pedestrian gap acceptance. This study broadens empirical pedestrian behavior model development to TWSC locations and is the first of its kind to elaborate on visibility impacts.

Pedestrian and driver behaviors as well as their interactions, are essential in planning, designing and operating highway facilities. Pedestrian crossing outside of a marked or unmarked crosswalk (i.e. jaywalking), is one of those pedestrian behaviors that may highly affect safety and operations. Unlike permissible crossings at crosswalks, jaywalking events are not often anticipated by drivers, which may result in less driver reaction time and different vehicle operation dynamics. It is important to understand pedestrian crossing behavior outside of crosswalks, as well as driver yielding behavior towards them. To date, limited quantitative and behavioral research has been conducted to investigate this interaction or simulate it microscopically. This paper aims to explore both pedestrian jaywalking behavior (gap acceptance and speeds) and the corresponding driver reactions (yielding behavior) for modeling the vehicle–pedestrian interactions (VPI) outside the crosswalks in a micro-simulation environment. The study also quantifies the differences between vehicle–jaywalker and vehicle-permissible crossing. An observational study and an instrumented vehicle study were conducted on the campus of the University of Florida to collect data from pedestrian and driver perspectives, respectively. Crossing speed, yield acceptance and delay of jaywalking crossings and permissible crossings were observed in the study and these attributed can be used for replicating pedestrian operations in simulators. Moreover, behaviors of driver approaching jaywalkers versus pedestrians crossing at designated crosswalks were compared on the basis of yield rates, and vehicle speed profiles. Vehicle yield dynamics were analyzed to model the driver reactions towards jaywalkers. Lastly, it was found that the locations of jaywalking events are highly concentrated and influenced by the crossing environment, such as pedestrian and vehicular volume, bus stops presence and crossing distance. This paper establishes several quantitative relationships describing interactions between pedestrians crossing outside of crosswalks and approaching drivers, which provide the basis and assumptions for modeling such interactions in a micro-simulation environment for traffic operational analyses.

In the Highway Capacity Manual 2010 (HCM 2010), Chapters 10 to 13 provide the methodology for analyzing freeway facilities in relation to level of service (LOS). Supplemental Chapter 25 contains additional details on the analytical methodology, which is tied to a computer program (FREEVAL) that implements the methodology and computer performance measures for oversaturated conditions. In the German Highway Capacity Manual (HBS), the methodology for oversaturated freeway facility analysis both assigns the complete facility with LOS F once the volume-to-capacity ratio on any of the component segments reaches 1.0 and terminates the analysis without further examination of the oversaturated segments. In German analysis practice, an alternative analysis should be performed next and represented, for example, by microscopic traffic flow simulation. But in practice, such simulation analyses are not often performed because of the complexity in the case of large freeway facilities. Therefore, a macro-scopic model able to handle the oversaturated traffic conditions is required. The FREEVAL implementation of HCM was tested for its usage in Germany but proved to be excessively U.S. specific and not applicable in Germany. The characteristics of the German facilities differ from those of the U.S. ones in driver behavior, capacity, and regulations and design standards. Therefore, a new approach was created to comply with German freeway design and operating standards, informed by lessons learned from the U.S. methodology. This paper presents the most important aspects of the German approach to freeway facility analysis and a direct comparison with implementation of the HCM software.

An empirically based macroscopic method is presented. It estimates and compares the pollutant emissions generated from signalized intersections and roundabouts. This method was built on a large sample size of real-world second-by-second vehicle trajectories, traffic volumes, and other traffic characteristics collected at signalized intersections and roundabouts in six U.S. states. The basis for predicting and estimating pollutant emissions was the concept of vehicle-specific power. The method enables inclusion of emissions standards and vehicle classes, such as Tier 1 (T1) and Tier 2 (T2) passenger cars (PCs) and passenger trucks (PTs). More than 1,980 vehicle trajectories were analyzed. Traffic variables including intersection capacity, demand-to-capacity ratio ( d/ c), cycle length, green-to-cycle length ratio, signal progression (i.e., arrival type), and number of lanes were included in the model for analysis and comparison between signals and roundabouts. Application of the method to a case study showed that on average under low d/ c ( 1), the amount of produced emissions increased steadily in roundabouts but increased a large amount at signals.

This paper presents a planning-level methodology for the analysis of freeway facilities. The proposed approach is based on and compatible with the operational method of the Highway Capacity Manual 2010 (HCM 2010). The approach is specifically constructed with the intent to minimize input data requirements. The method covers both under- and oversaturated flow conditions and produces estimates of travel time, speed, density, and level of service. The underlying methodology relies on developing a relationship between a basic freeway segment's delay rate per unit distance and its demand-to-capacity ratio. For weaving segments, the study develops capacity adjustment factors on the basis of volume ratio and segment length. With these factors, demand-to-capacity ratios on weave segments were adjusted and the segment was treated similarly to a basic freeway segment. For merge and diverge segments, a novel methodology is proposed to estimate their capacity on the basis of demand level, free-flow speed, and space mean speed. Subsequently, capacity adjustment factors are calculated on those segments and their demand-to-capacity ratios are adjusted accordingly. The proposed approach is applied to two examples in the HCM 2010 and produced very promising results. For undersaturated flow conditions, facility travel time is at most 3.4% and density was at most 1.1% at variance from the results found by applying the HCM 2010 operational methodology. The corresponding differences for oversaturated conditions are 6.7% and 13.0%, respectively.

Urban and suburban arterials with roundabouts in series are becoming more prevalent in North America. While the Highway Capacity Manual (HCM) provides a methodology for computing segment travel time of urban streets with various forms of intersection control, the manual does not provide a similar procedure for corridors with interdependent roundabouts. Such a methodology is necessary to evaluate performance of roundabout corridors and will allow the practitioner to compare both roundabout and signalized treatments for the same series of intersections. This paper presents a series of models intended to predict arterial travel time for a corridor with roundabouts, including a model for free-flow speed (FFS), a model to predict the length of the roundabout influence area (RIA, the area where geometric delay has incurred), a model for geometric delay, and models for impeded delay and average travel speed. These models were calibrated with data from seven roundabout corridors. The resulting models suggest that while FFS is a function of segment length, posted speed limit, and central island diameter, RIA length and geometric delay are functions of the FFS itself, as well as other geometric elements. The impeded delay and average travel speed are functions of traffic congestion and FFS. After validating the models with travel time data from two additional roundabout corridors not used in model development, the authors present a framework for incorporating the procedure for travel time prediction into the HCM analysis of urban streets.

Freeway work zone capacity has been the focus of significant research, but most studies were limited to specific geographic regions or work zone configurations or both. To date, no work zone predictive capacity model exists for U.S. freeways that is based on a geographically representative data set or sensitive to key geometric and operational attributes of the work zone. This paper proposes a new capacity model for freeway work zones, developed from nationwide field data fused with data obtained from an in-depth review of archived literature of prior work zone capacity studies in the United States. The proposed model estimates work zone capacity as a function of the lane closure severity index, barrier type, area type, lateral clearances, and daytime or nighttime work conditions. The model was successfully validated with 25% of the combined data that were not used for the model development. In addition, findings from literature archives suggested a 13.4% difference between prebreakdown, uninterrupted capacity and the ensuing queue discharge rate. The developed capacity model is intended to be incorporated into the next release of the Highway Capacity Manual and provides important insights on the relative effects of work zone configuration and other variables on the expected capacity of freeway work zones.

This paper presents a generic set of undersaturated speed–flow models for basic freeway segments on general purpose and managed lanes (MLs) consistent with the Highway Capacity Manual 2010 (HCM 2010). The proposed models predict segment space mean speed under a wide set of freeway operational conditions that can affect its free-flow speed (FFS) and capacity. Furthermore, the proposed models allow quantifying the impacts of nonrecurring events, such as severe weather conditions, incidents, and work zones on the speed–flow relationship. In addition, the model allows calibration of real-world facilities through adjustments to FFS and capacity. The incorporation of analyses of reliability and active traffic and demand management in the HCM context requires a set of speed–flow models capable of accounting for the effect of nonrecurring sources of congestion. Currently, the HCM 2010 provides a set of speed–flow models to predict space mean speed and consequently other freeway performance measures. This family of equations provides a limited adjustment to FFS. With guidance from NCHRP Project 3-96, separate speed–flow models are proposed for MLs through use of a different form from that in the HCM. The proposed generic equations describing the speed–flow relationship provide consistency between speed–flow relationships of managed and general purpose lanes and can incorporate any capacity or FFS adjustments to predict segment speed under different circumstances. The proposed generic equations are wholly consistent with the speed–flow models in the HCM 2010 and predict the same speed under any flow rate.

Trucks are not always addressed explicitly in planning studies and operational analyses, even though they are a very important part of the nation's economic engine. Most planning studies account for trucks as a percentage of the total traffic flow. The Highway Capacity Manual (HCM) converts trucks into autos through the use of a passenger car equivalency (PCE) factor. If the PCE is 2, then a truck is assumed to be equivalent to two autos. However, trucks are beginning to receive more attention, including that in the context of capacity assessments. A recent research effort focused on developing a level-of-service assessment for trucks and for enhancing the way that the impact of trucks is taken into account on freeways, arterials, and roundabouts. This paper focuses on the roundabout aspects of that research. The three roundabout-related objectives in the project were (a) to refine the PCE values used to convert trucks into auto equivalents for the flow on roundabout approaches, (b) to recalibrate the capacity equation so that it more appropriately accounts for trucks, and (c) to develop ways to estimate truck speeds so that travel times through the roundabout could be computed. The results of these analyses are presented as well as the methodology and data employed.

This research considered the unique challenges for pedestrians and bicyclists at superstreet intersections and recommends crossing alternatives for both users. For pedestrians, the options included the diagonal cross, median cross, two-stage Barnes Dance, and midblock cross. For bicyclists, the options included the bicycle U-turn, bicycle use of the vehicle U-turn, bicycle direct cross, and midblock cross. These options were analyzed through microsimulation on the basis of average stopped delay, average number of stops, and average travel time per route. Various parameters were analyzed for each crossing configuration, including two signal cycle lengths (90 and 180 s), two signal splits (60/40 and 75/25), two signal offset designs (vehicle platoons arrive simultaneously and at various offset times), and two midblock distances (600 and 800 ft). The recommended pedestrian crossing was a combination of the diagonal cross with the midblock cross. The parameters that decreased travel time for pedestrians were a 90-s cycle length, a 60/40 signal split, and an offset signal design in which the vehicle platoons arrived at different times. For bicyclists, the bicycle direct cross resulted in the lowest average number of stops and the lowest average travel time, whereas the vehicle U-turn showed the lowest stopped delay. The parameters that decreased travel time for bicyclists included a 90-s cycle length and the vehicle platoons arriving at different times. The recommended options for bicyclists at a superstreet were a combination of the bicycle direct cross and the midblock cross.

The freeway reliability methodology proposed for the Highway Capacity Manual, which is based on SHRP2 L08 methodology, produces an approach to scenario generation that can result in several thousand scenarios to be evaluated to estimate travel time reliability. This large number of scenarios can result in cumbersome user input, a demanding computational burden, and more important, extensive challenges posed when trying to error-check and interpret individual scenarios or to calibrate the model on the basis of real-world observations. This paper presents a novel scenario-generating methodology that accounts for multiple operating conditions. The objective of the proposed approach is to increase the quality of each scenario to make it more representative of the expected congestion patterns on the freeway. This paper shows that the new approach estimates reliability performance measures more accurately than current methods, while reducing the number of scenarios significantly. Thus, the new approach results in a more direct interpretation of results, while simultaneously relaxing many assumptions in the present approach to scenario generation and decreasing biases and errors. The proposed approach uses three core mathematical schemes: (a) a deterministic mathematical model for demand generation and scheduled work zones, (b) a Monte Carlo simulation for incident and weather events, and (c) an optimization algorithm to maximize similarities between the generated set of scenarios and the population of all scenarios. A comparison of results between the proposed method and the SHRP 2 Project L08 approach confirms that the proposed approach yields a higher level of accuracy in matching observed freeway reliability performance measures.

Pedestrian gap acceptance has not been explored to the same degree as vehicle gap acceptance. Although the two are similar in concept, there are a variety of pedestrian characteristics and caveats in the interaction between the pedestrian and vehicle modes that require separate pedestrian gap acceptance models. The objective of this research was to analyze empirical observations collected at 27 sites in Alabama, Florida, and North Carolina and to develop pedestrian gap acceptance models at midblock crossings. Goodness-of-fit tests showed that a probit-based, single-lane gap acceptance model, drawn from a noncontrolled pedestrian crossing data set, yielded the best results (max-rescaled R2 = .69). This model involved only two parameters: the size of the gap length in seconds and a binary variable that distinguished between gaps and lag events (first-arriving vehicle, without a prior lead vehicle, to open the gap). An increase in gap length was associated with an increased probability that a pedestrian would cross, whereas a lag event had a negative coefficient, which meant that a pedestrian was less likely to accept a lag than a gap, given the same length in seconds. Other variables that described crosswalk characteristics as well as pedestrian and driver behavior did not emerge as significant factors in the model. The research reported in this paper offers a new, robust pedestrian gap acceptance model that can be used in traffic operational analysis. The model can be incorporated into microsimulation packages and thus improve the accuracy of pedestrian behavior modeling at midblock crossings in the future.

Urban traffic emissions have been increasing in recent years. To reverse that trend, restrictive traffic measures can be implemented to complement national policies. We have proposed a methodology to assess the impact of three restrictive traffic measures in an urban arterial by using a microsimulation model of traffic and emissions integrated platform. The analysis is extended to some alternative roads and to the overall network area. Traffic restriction measures provided average reductions of 45%, 47%, 35%, and 47% for CO2, CO, NOX, and HC, respectively, due to traffic being diverted to other roads. Nevertheless, increases of 91%, 99%, 55%, and 121% in CO2, CO, NOX, and HC, respectively, can be expected on alternative roads.

Double crossover diamond (DCD) interchanges, also known as diverging diamond interchanges, are popular and promising alternative interchanges that are increasingly being implemented nationwide. One unique feature of a DCD interchange is that through movements on the arterial road have to cross each other twice to complete their movements, while enabling left-turn movements from the arterial to the freeway to proceed without stopping at the downstream intersection. Consequently, interchanges with heavy left-turn movements are good candidates for DCD implementation. This unique feature of a DCD interchange means that there is a need to research lane utilization at the upstream approach intersection of DCD interchanges, as the lane use could be unbalanced. This unbalanced lane utilization could have a significant effect on operations at the first crossover and the interchange as a whole. This study examined lane utilization factors provided in the Highway Capacity Manual 2010 (HCM) for conventional diamond i...

This paper presents an optimization-based probability adjustment approach that enables an analyst to minimize the error and bias in estimating freeway reliability performance measures by using a small sample of reliability scenarios. The freeway facilities travel time reliability methodology proposed for the Highway Capacity Manual 2010 can produce up to 24,000 scenarios to be evaluated for estimating the travel time distribution. The methodology uses a deterministic scenario generation approach to account for any possible operational condition of freeway facilities. In addition to processing time considerations, the large number of scenarios poses analytical challenges because (a) a detailed assessment and scrutiny of individual scenarios cannot be performed and (b) the customized selection of active traffic management is infeasible. The proposed method allows the analyst to adjust the selected scenario probabilities to estimate the real-world freeway performance measure better with a manual biased sampl...

Tailpipe emissions from on-road vehicles have negative impacts on the air quality of a region and are influenced by several factors including driver behavior, road characteristics, and traffic conditions. Typically vehicles on arterial roads frequently stop and then accelerate, a behavior that produces more tailpipe emissions than does maintaining a steady speed. Microscopic simulation models generate synthetic vehicle activity that can be used to estimate emissions on a facility. For emissions to be estimated accurately from an arterial facility, the simulated activity must closely represent the activity observed in the field. This study investigated whether a selected set of simulation parameters that used speed trajectories observed in the field at high temporal resolution could improve the emissions estimated from a simulated signalized arterial corridor. The simulated trajectories and those observed in the field were characterized by vehicle-specific power (VSP). Emissions were estimated with VSP modal emission rates and the time spent by vehicles in each VSP mode. The emissions were compared for routes along an urban arterial facility and for shorter segments within routes. At both spatial scales, the emissions from parameter-adjusted simulation vehicles that had the same average number of stops and travel time distribution as vehicles observed in the field were closer to real-world emissions than were the simulation vehicles under default model parameters. Adjusting the parameters also allowed the model to capture emissions hot spots along the routes more accurately. Currently, microscopic simulation models are calibrated with traffic performance measures that are improving. The findings from this paper present the potential for using these improving emissions estimates from simulated vehicle activity for the additional calibration of microsimulation models.

Roundabouts generally provide safety and other advantages. During peak hours, however, even moderate demands on an upstream approach can result in long delays and driver frustration over downstream movements. A metering signal is one way to ensure that all demands at a roundabout are adequately served. A roundabout metering signal regulates flow into the circle from one approach and thereby creates larger gaps in the circle for downstream entrants. Although metering signals have been used successfully around the world, little guidance is available for U.S. designers. The goal of this research was to provide U.S. designers with some guidance on the use of metering signals. The authors developed a simple macroscopic model based on the Highway Capacity Manual, validated it by using a simulation model, and performed exercises with it on a number of demand combinations to see where a meter might help. The macroscopic model should be helpful as a quick screening tool. The results from application of the model p...

Calibration and validation results for modeling double-crossover diamond (DCD) interchanges in a microsimulation environment are presented. The VISSIM simulation tool and detailed field data collected at four operational DCDs in the United States help to describe modeling challenges, calibration steps, and validation results in the form of delay, travel time, and queuing estimates. DCD interchanges are rapidly being deployed across the United States, accelerated by their ability to process high volumes of especially left-turning traffic at interchanges at a greatly reduced construction cost as that of other interchange alternatives. In the absence of an analytical methodology for evaluating these interchanges, simulation currently represents the only option for evaluating the operational performance of DCDs. Although other research has applied simulation to DCD evaluation, this study is able to present detailed validation results from field data collected at four fully operational U.S. DCDs. The results show that the operations of DCDs can largely be replicated in a simulation environment but that care needs to be taken for properly setting speed and routing decisions throughout the DCD network. The analysis further show that validation is more readily achieved over an extended route analysis, with increasing difficulties for short segment validation. The validation of field-measured queues proved challenging because of definitional differences between simulated and field study results. Overall, the results demonstrate the feasibility of satisfactory calibration of simulation tools to enable the operational performance evaluation of DCD interchanges.

Transportation agencies have made significant strides to reduce collisions and increase capacity along strategic highway corridors. Efforts have led to the implementation of many different access management techniques along corridors, such as installing medians versus two-way left-turn lanes and closing median openings. Businesses along these corridors have indicated their concern that these new designs will have a negative economic impact on their business because of the lack of direct access to their properties. The objective of this study was to quantify the business perceptions of median treatments on businesses adjacent to multilane highways. Owners of businesses along treatment corridors viewed median installations with a more positive outlook following implementation of the median than they did prior to the installation. The performance of treatment sites in terms of reported impacts on business revenues indicates that there is no direct evidence of negative economic impacts due to median installations.

An auxiliary through lane (ATL) is a commonly used congestion relief measure at signalized intersections in which fiscal and right-of-way constraints do not allow for full widening between intersections. Previously developed models have predicted the flow in the ATL as a function of macroscopic elements, such as through-movement demand and the ratio of average green time to cycle length. However, these models explain neither driver behavior nor motivation to use the ATL, nor can such an approach be used in microscopic simulation models. This paper presents empirically developed models for driver lane choice at signalized intersection approaches with one ATL and one continuous through lane (CTL). These models were developed from a calibration data set that covers eight ATL approaches in three U.S. states, for a total of 12 h of observation. The results suggest that the utility of the ATL is a function of each through-movement driver’s arrival time (during either the effective red or green phase), a...

Unlike other types of controlled intersections, drivers do not always comply with the "yield to pedestrian" sign at the roundabouts. This paper aims to identify the contributing factors affecting the likelihood of driver yielding to pedestrians at two-lane roundabouts. It further models the likelihood of driver yielding based on these factors using logistic regression. The models have been applied to 1150 controlled pedestrian crossings at entry and exit legs of two-lane approaches of six roundabouts across the country. The logistic regression models developed support prior research that the likelihood of driver yielding at the entry leg of roundabouts is higher than at the exit. Drivers tend to yield to pedestrians carrying a white cane more often than to sighted pedestrians. Drivers traveling in the far lane, relative to pedestrian location, have a lower probability of yielding to a pedestrian. As the speed increases the probability of driver yielding decreases. At the exit leg of the roundabout, drivers turning right from the adjacent lane have a lower propensity of yielding than drivers coming from other directions. The findings of this paper further suggest that although there has been much debate on pedestrian right-of-way laws and distinction between pedestrian waiting positions (in the street versus at the curb), this factor does not have a significant impact on driver yielding rate. The logistic regression models also quantify the effect of each of these factors on propensity of driver yielding. The models include variables which are specific to each study location and explain the impact size of each study location on probability of yielding. The models generated in this research will be useful to transportation professionals and researchers interested in understanding the factors that impact driver yielding at modern roundabouts. The results of the research can be used to isolate factors that may increase yielding (such as lower roundabout approach speeds), and can feasibly be incorporated into microsimulation algorithms to model driver yielding at roundabouts.

This paper presents a methodology for incorporating freeway reliability analysis in the Highway Capacity Manual on the basis of Strategic Highway Research Program Project L08. The methodology uses a scenario-based approach in which each scenario represents a unique combination of traffic demands and facility (segment) capacities. Demand impacts consider variability in time-of-day, day-of-week, and month-of-year differences, and capacity impacts quantify effects of various nonrecurring congestion sources, such as weather, incidents, work zones, and special events. The method combines three key components: a data repository, a freeway scenario generator (FSG), and a computational software engine, FREEVAL-RL. The FSG determines the number of scenarios for the reliability analysis on the basis of the facility-specific combination of demand, incident, and weather events. For each scenario, it produces a set of inputs and adjustment factors that are passed on to the FREEVAL-RL engine, along with each scenario's probability. The engine is capable of batch-processing many scenarios and estimates the reliability distribution for the facility. At the core, the computational engine and underlying methodology are consistent with the freeway facilities method in the Highway Capacity Manual 2010, which is capable of estimating undersaturated and congested flow conditions in a multi-period analysis. The method is illustrated with a real-world case study freeway facility in North Carolina. The FSG for that facility resulted in 2,508 scenarios. The resulting travel time distribution is presented, and sensitivity analyses are presented to explore the contributing effects of weather and incidents on the overall reliability of the facility.

Auxiliary through lanes (ATLs) are a cost-effective means for reducing congestion at signalized intersections, but little guidance is currently available on how to model ATL operations in a microsimulation environment. Without calibration, microsimulation tools assign some vehicles to an ATL as a function of basic lane-changing algorithms; this process results in much higher ATL utilization than observed in the field. This paper presents empirical models that explain ATL use as a function of through-movement congestion. This level of ATL utilization can be achieved in simulation tools by altering certain parameters, chiefly the upstream decision distance (UDD) of the ATL. Peak-hour counts and signal-timing data were collected at 22 ATL approaches from across the United States and coded into simulation to compare field and simulated ATL utilization. Modification of the UDD enabled close modeling of ATL utilization at 19 of 22 sites; the remaining three ATLs could not be modeled accurately in simulation because they experienced extremely low field utilization (less than 10% of total through-movement flow). After calibration, a model that approximated the correct UDD as a function of the total entering through-movement flow and ATL length was empirically developed. This model will allow practitioners to have a starting point when modeling a planned ATL in microsimulation packages; practitioners can then use the ATL operational models for further calibration of the UDD to simulate the correct level of ATL use.

Managed lane facilities, including high-occupancy vehicle (HOV) lanes, high-occupancy toll lanes, and express lanes, have become attractive tools for managing today's transportation system. Although managed lanes, specifically, HOV lanes, have existed for several decades, there has been little documentation of their traffic flow behaviors. Because these facilities tend to take on a variety of configurations, including different numbers of managed lanes and separation types from abutting general purpose lanes, transportation engineers should understand the traffic flow differences among facility types. An understanding of the interaction between managed lanes and parallel general purpose lanes is also needed for assessing the performance of managed lanes. A study was done to investigate the performance and traffic flow behavior of managed lane facilities at sites across the country. Traffic flow behavior for five facility types, based on separation type and number of lanes, was analyzed. Factors such as frictional impact on the managed lane—caused by poor performance of the general purpose lanes and slow vehicle effects on managed lane facilities where passing is prohibited—were considered in development of a set of speed–flow curves for each of the five basic facility types. These speed–flow relationships are expressed similarly to basic freeway segments in the Highway Capacity Manual and can be readily incorporated into the manual for predicting the performance of combined general purpose and managed lane facilities.

Past research has documented that pedestrian crossings at modern roundabouts can result in an impedance effect on the available capacity of entering traffic. The magnitude of this impedance effect is intuitively linked to the allocation of rights-of-way at the crosswalk, where a greater likelihood of driver yielding is expected to affect capacity more severely. However, existing pedestrian impedance models in the 2010 Highway Capacity Manual and the FHWA Roundabout Guide are not sensitive to yielding rates. The principal objective of this paper is to quantify pedestrian impedance effects on the vehicular entry capacity at multi-lane roundabouts as a function of driver yielding behavior. A calibrated microsimulation model is used to develop the relationships and explore changes in volume and yielding parameters. Impedance models are further developed for the pedestrian hybrid beacon (PHB) treatment, which is gaining increasing attention at multi-lane roundabouts across the United States. The results confirm the expected effects that the pedestrian impedance is more severe with higher pedestrian flow rates but decreases in severity with greater conflicting flows. The results further show that the effect of varying yielding rates on the impedance effect is minimal for congested roundabout approaches, because the relative effect of yielding is small compared with that of pedestrian volumes and conflicting circulating flow. The analysis of the PHB treatment shows that the impedance effect of the same pedestrian flows is generally less than that for an unsignalized crossing environment.

The freeway facilities methodology in the 2010 Highway Capacity Manual (HCM) is the only HCM methodology that encompasses undersaturated and congested flow regimes over multiple periods. However, the methodology is limited by its assumption of a fixed capacity threshold between the two flow regimes. The method does not consider the two-capacity phenomenon, which suggests that a drop in the throughput from theoretical capacity is observed after breakdown has occurred. A summary of the available literature on empirical evidence of the capacity drop under queue discharge conditions offers a theoretical evaluation of the impact on queue discharge flow based on shock wave theory relationships that form the basis of the queuing model used in the HCM freeway facilities method. Examples illustrate incorporating queue discharge into a freeway facilities analysis, and implications for practice are discussed.

Auxiliary through lanes (ATLs) are often implemented at congested signalized intersections where fiscal and right-of-way constraints do not allow for full roadway widening. Previous research has suggested that ATLs are used significantly less than the theoretical capacity of a continuous lane. However, analysts have had little empirical basis for evaluating ATL operations in the context of the 2010 Highway Capacity Manual (HCM). Two empirical models that predict the flow in the ATL as a function of congestion and total through-movement flow are presented. A method is described for incorporating the model's findings into the HCM procedure for estimating approach capacity. The study addresses the challenges of modifying the HCM lane utilization factor and accounting for the iterative effect of actuated signal control on ATL use. The study suggests that a cyclic process can be used to balance the ATL flow with the green-to-cycle-length ratio. A method is presented for determining the upstream ATL length needed for efficient ATL operations. Two example problems illustrate the use of the new models in the context of HCM 2010.

With the increasing concerns about environmental effects and the sustainability of roadway capacity expansion, transportation agencies are seeking alternative solutions to mitigate congestion. Managed lanes (MLs) promote person throughput on freeways and manage congestion through improving efficiency. The ML concept therefore has been gaining popularity in past decades. However, the lack of guidance on evaluating the performance of the ML facility poses challenges for agencies wanting to design and implement the strategy effectively. Many MLs are designed to be left-lane concurrent; vehicles entering the freeway from general purpose (GP) lane on-ramps need to cross weave over multiple GP lanes to access the ML. These weaving vehicles will have a negative effect on the operating performance of the parallel GP lanes. This paper investigates this cross-weaving effect as a function of different roadway geometric configurations as well as traffic conditions. A microscopic simulation model was built and calibrated on the basis of video data collected along I-635 in Dallas, Texas. Multiple scenarios were tested to explore the effect of the following parameters: number of GP lanes, cross-weave demand, and cross-weaving length. A set of capacity adjustment factors was determined to account for this effect as a function of those parameters. Results showed that the capacity-reducing effect was higher with a reduction in cross-weaving length, an increase in the number of GP lanes, or a rise in cross-weave demand volumes. The results are important in evaluating the operational performance of freeway segments in the presence of concurrent GP lanes and MLs in a Highway Capacity Manual context.

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).

This paper evaluates the structural integrity and safety performance of weathered steel beam guardrail (WSBG) in North Carolina. WSBG is a popular alternative to galvanized steel guardrail (GSG) in that it is an aesthetic solution that blends in with the surrounding natural environment. A New Hampshire study, however, found that WSBG deteriorates more quickly. That study prompted an FHWA recommendation discouraging the use of WSBG across the country. This paper describes a comprehensive study of all WSBG installations in North Carolina, where weather conditions and deicing practices are generally much less severe than in New Hampshire. The research results did not show any trends of deteriorating thickness as a function of guardrail age (the oldest installation was almost 30 years old), elevation (the tallest installation was 4,200 ft), and annual average daily traffic (the highest traffic volume was 27,000 vehicles per day). The WSBG at all 25 test sites was shown to be above AASHTO minimum thickness specifications of 0.096 in. at a 95% confidence level. From a safety perspective, the research results suggested a lower percentage of injury collisions at WSBG installations relative to GSG installations at comparison sites, although low crash sample sizes made a more robust comparison difficult. Overall, this research suggests there are no structural concerns about using WSBG in North Carolina. The findings of this study may have broader policy implications for other states.

The building of managed lanes parallel to general purpose lanes is an increasingly common approach to optimizing freeway capacity. Managed lanes allow agencies to classify customers and assign a portion of the freeway capacity to them. With no methodology in the Highway Capacity Manual (HCM) for analyzing these facilities, analysts rely on more time-consuming simulation analyses. A methodology is presented for estimating the performance of a parallel system of general purpose and managed lane facilities in an HCM context based on NCHRP Project 3–96. The methodology defines new managed lane segment types to use in an HCM analytical framework and is associated with a new set of speed–flow curves. It is sensitive to the number of lanes and the type of separation between managed lanes and general purpose lanes. The method introduces the concept of parallel lane groups of general purpose and managed lanes and thus can account for speed reduction in managed lanes caused by congestion in adjacent general purpose lanes. The method was implemented in a computational engine, FREEVAL-ML, which was built on the freeway facilities method in HCM 2010 but which was updated to incorporate inputs and outputs of the managed lane components. The geometry of two existing managed lane facilities in Washington State is used to illustrate the method, demonstrating the applicability of the analytical framework to real-world facilities.

Arterials across the United States are operating inefficiently and are becoming increasingly congested because of growing traffic demand. Agencies tasked with improving these arterials are running out of good solutions. Superstreets, called restricted crossing U-turns by FHWA, are part of a menu of unconventional arterial designs that may provide promising solutions. Up to this point, little valid information exists on the operational effects of superstreets because study results have been from macroscopic analyses and simulations of hypothetical arterials. The purpose of this research was to determine the operational effects of the superstreet treatment on existing signalized arterials in North Carolina. The operational analysis involved calibrating and validating VISSIM models of three existing signalized superstreets in North Carolina; two isolated intersections and one five-intersection superstreet corridor. After some adjustments, the team produced VISSIM models that provided travel times within an acceptable range of the field data collected. Results from the three calibrated VISSIM models were compared with results from models of equivalent conventional sites at various volume levels with travel time as the primary measure of effectiveness. The superstreet outperformed the conventional design at each location studied and reduced the overall average travel time per vehicle traveling through the intersection. The travel time savings and extra capacity at higher volumes can buy agencies more years of acceptable operation before intersection improvements, or even interchanges, are necessary.

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.