Safety Evaluation of the Safety Edge Treatment
Chapter 7. Conclusions
Conclusions from the analysis of pavement-edge drop-off field
measurements and crash data, based on 3 years
of data for the period after resurfacing and installation of the safety edge
treatment, are as follows:
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The EB evaluation for the safety edge treatment
with 3 years of crash data for the period after resurfacing found that 56 of
the 81 comparisons showed a positive safety effect for the safety edge treatment. However, only 11 of these comparisons were
statistically significant, which may be due in part to the small
magnitude of the safety edge effect.
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The EB evaluation
results indicated that the best estimate of effectiveness of the safety edge treatment for all two-lane highway sites in
two States is an approximately 5.7 percent reduction in total
crashes. While this result was not statistically significant, the evaluation
results obtained for total crashes were nearly always in the positive
direction. The results of separate evaluations for fatal and injury crashes and
PDO crashes were too variable to draw conclusions.
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Benefit-cost
analysis based on the estimated 5.7 percent crash reduction effectiveness found
that the safety edge treatment is so
inexpensive that it is highly cost-effective for application in a broad range of conditions on two-lane
highways. Computed minimum values for benefit-cost ratios ranged from 4
to 44 for two-lane highways with paved shoulders and from 4 to 63 for two-lane
highways with unpaved shoulders. The benefit-cost ratios are generally higher with
increasing traffic volume and where the cost of installing the safety edge
treatment is lower.
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The cost of
adding the safety edge treatment to a resurfacing project is minimal.
Comparisons of overall project costs and overall costs of HMA
resurfacing material did not show an increase
for resurfacing projects with the safety edge when compared to normal
resurfacing projects without the
safety edge. However, computations based on the volume of asphalt required to form the safety edge
suggested that the cost of the safety edge treatment is approximately
$536-2,145 per mi for application to both sides of the roadway.
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Resurfacing
with or without the safety edge treatment was found to decrease the proportion
of drop-off-heights exceeding 2 inches, at least in the short term. However,
there is little evidence that resurfacing with the safety edge treatment
creates more high drop-offs than resurfacing without the safety edge treatment.
Data for drop-off heights showed that the proportion
of drop-offs on both treatment and comparison sites increased in the second and
third years after resurfacing. There is no
evidence that the safety edge treatment sites have more high drop-offs
than comparison sites that did not have the safety edge treatment.
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Evaluation results for the effect of the safety
edge treatment on run-off-road crashes and drop-off-related crashes on two-lane
highways were variable and inconsistent. More sites and higher crash frequencies are needed to obtain consistent,
statistically significant results. Two trends were evident in the EB
analysis of run-off-road and drop-off-related crashes. First, the safety edge
treatment generally appears to have a positive effect on safety for all site
types except for sites with unpaved shoulders in Indiana. This variability in
results has not yet been fully explained. Second, however, the negative safety
edge effects for Indiana sites with unpaved shoulders may be explained by low
frequencies of drop-off-related crashes on comparison sites in the period
before resurfacing.
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There were not
enough sites at which the safety edge treatment was applied on rural multilane highways to obtain meaningful evaluation
results. However, the physical role of the
safety edge treatment is no different on multilane highways than on two-lane
highways. Results of the cross-sectional analysis, while not definitive,
suggested that the safety edge treatment is effective on multilane highways.
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An increase in total crashes for the first 12-30
months after resurfacing has been noted in
previous studies of the effect of resurfacing on crashes.(4) The
observed increase in crash frequency for the period immediately after
resurfacing may have resulted from this effect. The use of 3 years of crash data after resurfacing resulted in more
realistic estimates of the safety effectiveness of the safety edge than
analysis using 1-2 years of data.
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A test of the proportion of fatal and injury
crashes after resurfacing indicated that the proportion of fatal and injury
crashes decreased significantly after resurfacing. However, there was no apparent shift in crash severity
distributions between sites that were resurfaced with and without the
safety edge treatment.
- Resurfacing appears to increase crash
frequencies, at least in the short term, and to reduce crash severities. Incorporating
the safety edge treatment in a resurfacing project appears to reduce crash
frequencies slightly but to have no effect on crash severities.
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