U.S. Department of Transportation
Federal Highway Administration
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Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-11-024
Date: April 2011
Safety Evaluation of the Safety Edge Treatment
Chapter 1. Background and Research Objectives
This chapter describes the background and objectives of this research and the organization of the remainder of the report.
Many two-lane rural highways have unpaved shoulders immediately adjacent to the traveled way. Other two-lane highways and many multilane rural highways have narrow paved shoulders with widths of 1-4 ft. If roadway maintenance crews do not keep material against the pavement edge, a pavement-edge drop-off may form. The drop-off height can vary from less than 1 inch to 6 inches or more, even though maintenance performance standards usually require maintenance when the drop-off exceeds 1.5-2 inches.(1)
When a vehicle leaves the traveled way and encounters a pavement-edge drop-off, it can be difficult for the driver to return safely to the roadway. As the driver attempts to steer back onto the pavement, the side of the tire may scrub along the drop-off, resisting the driver's attempts. This resistance often leads the driver to overcorrect with a greater steering angle than desired to remount the drop-off. When the tire does remount the pavement, the increased tire angle may "slingshot" the vehicle across the road, resulting in a collision with other traffic or a loss of control and overturning on the roadway or roadside.
The safety edge is a treatment that is intended to minimize drop-off-related crashes. With this treatment, the pavement edge is sloped at an angle of 30 degrees to reduce the resistance of the tire remounting the drop-off (see figure 1). The reduced resistance is intended to allow a more controlled reentry onto the traveled way.
Figure 1. Diagram. Safety edge detail.
Research conducted by the Texas Transportation Institute (TTI) in the 1980s found that drivers rated a 45-degree wedge as a much safer pavement edge to remount than either the vertical or rounded edges normally found with portland cement concrete and asphalt pavements.(2) Because drivers in the study were instructed to go off the pavement edge, the TTI research has been criticized as not being representative of naïve drivers. Prior to this research, actual field evaluation of the safety edge had not been completed.
Selected highway agencies have begun to use the safety edge treatment as part of their pavement resurfacing projects. However, there has been no formal evaluation of the effectiveness of this treatment in reducing drop-off-related crashes on rural highways. Such an evaluation is needed to determine whether this treatment should receive more widespread use.
Eight State highway agencies joined the Federal Highway Administration (FHWA) in a pooled-fund study to implement and evaluate the safety edge treatment in conjunction with pavement resurfacing projects. Four State agencies provided study sites for this evaluation: the Colorado Department of Transportation, the Georgia Department of Transportation, the Indiana Department of Transportation, and the New York State Department of Transportation. The evaluation of the safety edge treatment extended over a 3-year period. Unpublished interim reports were prepared for the first and second years after implementation of the safety edge treatment. This final report presents the evaluation results for the entire 3-year study following implementation of the treatment.
The primary objective of the evaluation was to quantify the safety effectiveness of the safety edge treatment. An evaluation was performed to determine whether including the safety edge treatment as part of a pavement resurfacing project reduces crashes in comparison to pavement resurfacing without the safety edge treatment. The evaluation results are presented in terms of the percentage reduction in specific crash types that can be expected from the provision of the safety edge treatment. Other objectives of the study were to document the effectiveness of the safety edge treatment in reducing the presence of pavement-edge drop-offs and to perform an economic analysis of the safety edge treatment. The economic analysis used the safety effectiveness evaluation results and project cost data to define the types of roadways and traffic volume levels for which the safety edge treatment would be cost-effective.
The project scope included two-lane rural roads with no paved shoulder and with a paved shoulder no wider than 4 ft. Multilane roads with paved shoulders no wider than 4 ft were also studied.
The evaluation plan for the safety edge treatment was based on the following types of sites:
This final report is based on data for the characteristics and performance of treatment, comparison, and reference sites during the period before the treatment and comparison sites were resurfaced and for 3 years after resurfacing. Data collected and analyzed in this report include field measurements of drop-offs present on the treated sites before and during the 3 years after resurfacing; crash records for 2-5 years before the site was resurfaced and 3 years after resurfacing; traffic volumes and road characteristics for each site; and the date and cost of resurfacing the treatment and comparison sites.
This report presents the results of a comparison of the presence of pavement-edge drop-offs between the treatment and comparison sites for the period before resurfacing and during the 3 years after resurfacing.
The report also presents the safety evaluation results using traffic volume and crash data for the period before resurfacing of the treatment and comparison sites and the 3 years after resurfacing. Two statistical approaches were used to analyze these data: (1) a before-after comparison using the empirical Bayes (EB) technique and (2) a cross-sectional comparison of the safety performance of sites that were resurfaced with and without the safety edge treatment based on the after period only.
To estimate the safety performance of the safety edge treatment in the before-after EB analysis, safety performance functions (SPFs) were developed from the reference site data using negative binomial regression analysis.
The frequencies of specific target crash types were used as the dependent variables for the safety evaluation. The target crashes for the safety evaluation exclude at-intersection and intersection-related crashes because the safety edge treatment is targeted primarily at non-intersection crashes.
Safety measures used as dependent variables for this report included the frequencies of total non-intersection crashes, run-off-road crashes, and drop-off-related crashes. Run-off-road crashes included those crashes in which one or more involved vehicles left the road. Drop-off-related crashes were a subset of run-off-road crashes for which the crash data included specific evidence that a pavement-edge drop-off may have been involved, such as the inclusion of "low shoulder" or "shoulder defect" as a contributing factor. Separate analyses were conducted for each target crash type for fatal and injury crashes, property-damage-only crashes, and all crash severity levels combined.
Cost data for the resurfacing projects at the treatment and comparison sites are included in the report, and findings are presented concerning the cost-effectiveness of the safety edge treatment.
The remainder of this report is organized as follows: