Joe Bared

ACCIDENT MODELS FOR TWO-LANE RURAL SEGMENTS AND INTERSECTIONS

Data collected from the states of Minnesota and Washington on rural two-lane highways are used to build accident models for segments and three-legged and four-legged intersections stop-controlled on the minor legs. The quantity, quality, and variety of data collected, together with the advanced techniques applied in the analysis, make this study of special interest. Variables include traffic, horizontal and vertical alignments, lane and shoulder widths, roadside hazard rating, channelization, and number of driveways. Models are of negative binomial and extended negative binomial form and yield R2 values from 0.42 to 0.73 and overdispersion parameters from 0.20 to 0.51. A segment model combining both states and including state as a variable, and intersection models derived from Minnesota data, are featured, along with summary statistics, goodness-of-fit measures, and cross-validation between the states. Segment accidents depend significantly on most of the roadway variables collected, while intersection accidents depend primarily on traffic. The study recommends development of adjustment factors for different regions and times and further development of extended negative binomial models.

Validation of FHWA Crash Models for Rural Intersections: Lessons Learned

A national-level safety analysis tool is needed to complement existing analytical tools for assessment of the safety impacts of roadway design alternatives. FHWA has sponsored the development of the Interactive Highway Safety Design Model (IHSDM), which is roadway design and redesign software that estimates the safety effects of alternative designs. Considering the importance of IHSDM in shaping the future of safety-related transportation investment decisions, FHWA justifiably sponsored research with the sole intent of independently validating some of the statistical models and algorithms in IHSDM. Statistical model validation aims to accomplish many important tasks, including (a) assessment of the logical defensibility of proposed models, (b) assessment of the transferability of models over future time periods and across different geographic locations, and (c) identification of areas in which future model improvements should be made. These three activities are reported for five proposed types of rural intersection crash prediction models. The internal validation of the model revealed that the crash models potentially suffer from omitted variables that affect safety, site selection and countermeasure selection bias, poorly measured and surrogate variables, and misspecification of model functional forms. The external validation indicated the inability of models to perform on par with model estimation performance. Recommendations for improving the state of the practice from this research include the systematic conduct of carefully designed before-and-after studies, improvements in data standardization and collection practices, and the development of analytical methods to combine the results of before-and-after studies with cross-sectional studies in a meaningful and useful way.

Design and Operational Performance of Double Crossover Intersection and Diverging Diamond Interchange

Transportation planners and traffic engineers are facing the challenge of inventing ways to mitigate congestion during the peak hours. Alleviating delays and improving safety for passengers and pedestrians are the primary motives. One way of achieving these objectives is to search for alternative intersection and interchange designs. This paper presents the results of a study on two new alternate designs: double crossover intersection and diverging diamond interchange. These designs were studied for different traffic scenarios with the use of traffic simulation, and the results showed better performance during peak hours than that of similar corresponding conventional designs. Better performance includes better level of service, shorter delays, smaller queues, and higher throughput.

Empirical Investigation of Interactive Highway Safety Design Model Accident Prediction Algorithm: Rural Intersections

One major gap in transportation system safety management is the ability to assess the safety ramifications of design changes for both new road projects and modifications to existing roads. To fulfill this need, FHWA and its many partners are developing a safety forecasting tool, the Interactive Highway Safety Design Model (IHSDM). The tool will be used by roadway design engineers, safety analysts, and planners throughout the United States. As such, the statistical models embedded in IHSDM will need to be able to forecast safety impacts under a wide range of roadway configurations and environmental conditions for a wide range of driver populations and will need to be able to capture elements of driving risk across states. One of the IHSDM algorithms developed by FHWA and its contractors is for forecasting accidents on rural road segments and rural intersections. The methodological approach is to use predictive models for specific base conditions, with traffic volume information as the sole explanatory variable for crashes, and then to apply regional or state calibration factors and accident modification factors (AMFs) to estimate the impact on accidents of geometric characteristics that differ from the base model conditions. In the majority of past approaches, AMFs are derived from parameter estimates associated with the explanatory variables. A recent study for FHWA used a multistate database to examine in detail the use of the algorithm with the base model-AMF approach and explored alternative base model forms as well as the use of full models that included nontraffic-related variables and other approaches to estimate AMFs. That research effort is reported. The results support the IHSDM methodology.

Design and Operational Performance of Crossover Displaced Left-Turn Intersections

Although the concepts of the crossover displaced left-turn (XDL) intersection (also called the continuous flow intersection) were developed approximately four decades ago, there is no simplified procedure to evaluate its traffic performance and to compare this intersection with conventional intersections. Several studies have shown the qualitative and quantitative benefits of the XDL intersection without providing accessible tools for traffic engineers and planners to estimate average control delays and queues. Modeling was conducted on typical geometries over a wide distribution of traffic flow conditions for three different design configurations or cases using VISSIM simulations with pretimed signal settings. Some comparisons with similar conventional designs showed considerable savings in average control delay, as well as average queue length and increase in intersection capacity. The statistical models provided an accessible tool for the practitioner to assess average delay and average queue length for three types of XDL intersections. Finally, a preferred signal setting was developed for each of the five intersections of the XDL network.

Using Simulation to Plan Capacity Models by Lane for Two- and Three-Lane Roundabouts

Recently, capacity models have been developed for single- and double-lane roundabouts by using U.S. data from NCHRP Report 572. The model for double-lane capacity was provided primarily for critical lane (usually right-lane) capacity as a function of total circulating volume. None of the existing international capacity models separate circulating volumes by lane. This study proposes planning capacity models for two-lane and three-lane roundabouts by separate entry-lane and separate circulatory-lane traffic volumes. VISSIM microsimulation software was used first to compare with the new NCHRP models as well as with Australian (SIDRA) and German (Tanner-Wu) models. Given that predictions from VISSIM for overall capacity of one- and two-lane roundabouts are consistent with U.S. data from NCHRP Report 572, new planning capacity models were developed by using VISSIM for the left lane and right lane of double-lane roundabouts that are functions of separate circulatory-lane traffic volume. For three-lane roundabouts, planning capacity models were developed by separate entry lane (left, middle, and right lane) as functions of inner, middle, and outer circulatory-lane volumes. All variable coefficients are statistically significant, and model fits are strong.

State-of-the-art design of roundabouts

In the United States roundabouts are becoming more popular. European improvements in design, operations, and traffic regulations have led to safer performance in roundabouts than in conventional intersections. Consequently, capacity and delay have also improved. More uniform practices have emerged, particularly the yield-at-entry traffic rule and vehicle path deflection through the roundabout. For the benefit of U.S. planners and designers, a synthesis and recommendations based on up-to-date international and domestic practices are provided. Those practical design issues are concisely discussed: justification; safety and accident prediction; capacity; delays; location; design considerations (design vehicle, design speed, sight distance, deflection, central island, circulating roadway, entries and exits, splitter islands, signing, and landscaping); bicycle considerations; pedestrian considerations.

Performance Analysis of Parallel Flow Intersection and Displaced Left-Turn Intersection Designs

The displaced left-turn (DLT) design, also known as the continuous flow intersection, has proved to be a superior alternative to the conventional intersection in terms of handling heavy volumes during peak periods. In locations where the availability of additional right-of-way and driveway access is not a major concern, the DLT design is a cost-effective and time-saving option compared with the expensive option of grade-separated interchanges. Recently, a new nontraditional intersection design called the parallel flow intersection (PFI) has been proposed. The PFI is also an at-grade design and operates with the same number of signal phases as a DLT. This research compares the operational performance of DLT and PFI designs based on the maximum through and left-turn movement throughputs for three different high-volume scenarios using traffic simulation. The results indicate that maximum throughput values of through movement in the PFI were very close to the values obtained for the DLT. The designs produced similar results, mainly because both operate as two-phase signals (at the main intersection) with equal green times for through movements. The left-turn movement throughputs in the PFI were found to be lower than those in the DLT. For two study cases, the DLT was able to process 180 and 80 more vehicles per hour per lane than the PFI. This is attributable to left-turning vehicles experiencing a greater number of stops, on average, in a PFI than they would in a DLT.

Design and Performance Analysis of Pedestrian Crossing Facilities for Continuous Flow Intersections

Although concepts of the continuous flow intersection (CFI) have been around for approximately four decades, minimal or no literature describing studies that have analyzed pedestrian traffic performance at these intersections is available. Several studies have reported on the qualitative and quantitative benefits for the vehicular traffic performance of CFIs in comparison with the benefits for the vehicular traffic performance of conventional intersections but have provided minimal or no discussion about pedestrian traffic performance. As a novel intersection design, many important considerations are required to design pedestrian accesses and crossings at CFIs without compromising pedestrian safety and vehicular traffic performance. In this paper, the design methodologies for providing pedestrian access and related pedestrian signal timings are discussed. Modeling was conducted on three typical geometries for CFIs with base signal timings optimized for vehicular traffic performance. The results indicate an acceptable pedestrian level of service of B or C on the basis of the average delay per stop experienced by any pedestrian for pedestrian crossings at the typical CFI geometries modeled. All pedestrians served at the CFIs are accommodated within two cycles for a typical signal cycle length ranging from 60 to 100 s.

Safety Analysis of Interchanges

As the U.S. freeway system ages and becomes more congested, many parts of the system, particularly interchanges, need reconstruction or rehabilitation. In addition, new development and expanding urbanization often require new interchanges to be constructed along existing freeways. Because interchange projects are typically among highway agencies’ highest-cost projects, it is important that they be planned wisely. Improved safety should be a key justification for interchange improvement projects, but engineers lack a suitable tool for examining the safety performance of existing interchanges and anticipating the safety performance of new or rebuilt interchanges. The objectives of this research are to synthesize the current state of knowledge concerning the safety assessment of interchanges, develop a spreadsheet-based computational tool for performing safety assessments of interchanges, and identify gaps in knowledge concerning interchange safety assessment and future research needed to fill those gaps. This paper summarizes safety data related to interchanges, discusses the use of safety performance functions within the Interchange Safety Analysis Tool (ISAT) (the spreadsheet-based tool developed in this research), describes the scope and capabilities of ISAT, and identifies substantive gaps in the current state of knowledge that limit the ability of ISAT to provide all of the capabilities desired by potential users.