Overview
The Reliability area of the second Strategic Highway Research Program (SHRP 2) has focused on the need to improve travel time reliability on freeways and major arterials. SHRP 2 Project L07 has focused specifically on design treatments that can be used to improve travel time reliability. The objectives of Project L07 were to (1) identify the full range of possible roadway design features used by transportation agencies to improve travel time reliability and reduce delays due to key causes of nonrecurrent congestion, (2) assess their costs and operational and safety effectiveness, and (3) provide recommendations for their use and eventual incorporation into appropriate design guides.
Three separate analyses of the design treatments were conducted in Phase 2 of Project L07: operational, safety, and benefit–cost. The traffic operational analysis methodology developed in Phase 2 built on work completed in SHRP 2 Project L03, Analytical Procedures for Determining the Impacts of Reliability Mitigation Strategies. As part of the traffic operational analysis, a spreadsheet-based Analysis Tool was developed to allow highway agencies to analyze and compare the effects of a range of design strategies on a given highway segment using the analytical procedures developed in Phase 2 of Project L07. Highway agencies can input data about a highway (e.g., geometrics, volumes, crash totals); the Analysis Tool computes delay and reliability indicators resulting from various design treatments, further translating those results into life-cycle costs and benefits.
In addition to the traffic operational benefits of reducing congestion, the potential safety benefits were explored. The reduction of congestion through application of design treatments or intelligent transportation system (ITS) improvements has been widely thought to have a positive effect on safety, but this relationship had not been well quantified in previous research. Congestion may result in stalled or slowed traffic, and the situation in which high-speed vehicles approach the rear of an unexpected traffic queue clearly presents a substantial risk of collision. The potential for collision within queues of stop-and-go traffic is also clear. Thus, on the one hand, the frequency of these conditions can be ameliorated by treatments that reduce nonrecurrent congestion. On the other hand, since collision severity is clearly a function of speed, the lower speeds on roadways during congested periods may reduce overall collision severity. This trade-off between crash frequency and severity in congested versus uncongested conditions has not been satisfactorily quantified in previous research.
Relationships between safety and congestion were developed in Phase 2 of Project L07 for application in the spreadsheet-based Analysis Tool. Safety-congestion relationships were developed from analyses of traffic operational and crash data for the freeway systems of two metropolitan areas: Seattle, Washington, and Minneapolis–St. Paul, Minnesota. Analysis of these data found that the crash rate on urban freeways varies with traffic density in a U-shaped relationship, with higher crash rates at very low traffic densities (due primarily to single-vehicle crashes), higher crash rates at very high traffic densities (due to multiple-vehicle crashes), and the lowest crash rates at medium traffic densities. This result was found for both fatal-and-injury and property-damage-only crashes. This finding implies that design treatments that are effective in reducing congestion levels on urban freeways should also be effective in reducing crashes.
Since the relationship between congestion and safety was based on only two metropolitan areas, SHRP 2 added a new task to Project L07—designated as Task IV-5—to further explore the relationship between safety and congestion using data from other metropolitan areas. The research in Task IV-5 was conducted to determine whether a similar U-shaped relationship between safety and congestion exists for the freeway systems of other metropolitan areas and how that relationship can be best generalized for broader application in the analysis of design treatments. The research also investigated whether the relationship applies to a full range of nonrecurrent congestion scenarios.
In Task IV-5, relationships between crash rates and level of service (LOS) were developed based on traffic operational and crash data obtained from instrumented directional freeway segments in five metropolitan areas: Seattle, Washington; Minneapolis–St. Paul, Minnesota; Sacramento, California; the Kansas portion of the Kansas City metropolitan area; and the Missouri portion of the Kansas City metropolitan area. The selection of these five metropolitan areas was based on the availability of relevant data. The Kansas and Missouri portions of the Kansas City metropolitan area were analyzed separately because the crash data were obtained from different sources.
The data for Sacramento freeways largely confirm the Seattle and Minneapolis–St. Paul results, showing a U-shaped relationship with minimum crash rates at about LOS C, slightly higher crash rates at lower densities (i.e., better LOS), and substantially higher crash rates at higher densities (i.e., poorer LOS). The data for freeways in both the Kansas and Missouri portions of the Kansas City metropolitan area show little variation in crash rate over the range of traffic density, although crash rates are substantially higher in the lowest traffic density category (LOS A+) and, for the Kansas portion of the metropolitan area, slightly higher in the highest traffic density category (LOS F+). Review of the data shows that the freeways in the Kansas City metropolitan area experienced a substantially lower proportion of LOS F conditions than the other metropolitan areas and, therefore, did not have much opportunity to show higher crash rates at higher traffic densities.
The most appropriate interpretation of these results is that the Seattle, Minneapolis–St. Paul, and Sacramento results show similar shapes for the safety-congestion relationships. The results for the Kansas City metropolitan area are not necessarily inconsistent with the other metropolitan areas but may not include sufficient congestion to show higher crash rates at the highest crash densities.
A combined safety-congestion relationship for the Seattle, Minneapolis–St. Paul, and Sacramento metropolitan areas was developed by translating the curves to the average freeway crash rate for the three metropolitan areas and then averaging the individual data points. With this translation completed, the results are representative of a freeway system with a total crash rate of 1.86 crashes per million vehicle miles of travel (MVMT), a fatal-and-injury (FI) crash rate of 0.42 crashes per MVMT, and a property-damage-only (PDO) crash rate of 0.82 crashes per MVMT. These are the average freeway crash rates for Seattle, Minneapolis–St. Paul, and Sacramento, giving equal weight to each metropolitan area.
Since the focus of Project L07 is on nonrecurrent congestion, a further analysis (using data for Sacramento freeways) was conducted to check whether the U-shaped relationship is specifically applicable to periods of nonrecurrent congestion. Of the more than 5 million site-periods (a 15-min period at a given site), 21% were classified as nonrecurrent congestion and 79% were classified as recurrent congestion or normal uncongested flow. Analysis of the data provided strong evidence that the general relationship between crash rate and traffic density is applicable to both recurrent and nonrecurrent congestion.
Thus, it is recommended that the safety-congestion relationship developed in this research be applied in the L07 Analysis Tool to compare the traffic operational and safety effects of design treatments on a given highway segment.