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.
1.1 Purpose of the Safety Edge Treatment
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.
1.2 Research Objectives and Scope
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.
1.3 Summary of Evaluation Plan
The evaluation plan for the safety edge treatment was based
on the following types of sites:
-
Sites that were resurfaced and treated with the safety
edge (referred to as treatment sites).
-
Sites that were resurfaced but not treated with
the safety edge (referred to as comparison sites).
-
Sites that were similar to the treatment and
comparison sites but were not resurfaced (referred to as reference 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.
1.4 Organization of Report
The remainder of this report is
organized as follows:
-
Chapter 2 documents the project database,
including a summary of the length of the sites studied, the crash data
analyzed, traffic volumes and characteristics of the sites, and field
measurements of the pavement-edge drop-offs.
-
Chapter 3
presents the analysis results for the field measurements of pavement-edge drop‑offs.
-
Chapter 4 presents the safety effectiveness
evaluation.
-
Chapter 5 presents project cost comparisons for sites
resurfaced with and without the safety edge.
-
Chapter 6 presents the benefit-cost economic
analysis.
-
Chapter 7 presents conclusions drawn from the analysis
results.
-
Chapter 8 presents recommendations based on
results of the 3-year evaluation.
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