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Publication Number: FHWA-HRT-11-024
Date: April 2011

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:

  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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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