Safety Evaluation of Centerline Plus Shoulder Rumble Strips

Chapter 9. Summary and Conclusions

The objective of this study was to undertake a rigorous before-after evaluation of the safety effectiveness, as measured by crash frequency, of SRS and CLRS applied in combination on two-lane rural roads. The study used data from three States (Kentucky, Missouri, and Pennsylvania) to examine the effects for specific crash types, including total, FI, run-off-road, head-on, and sideswipe-opposite-direction crashes. Crashes occurring at or related to an intersection and animal-related crashes were not included. Based on the combined results, the CMFs shown in table 22 are recommended for the various crash types. The benefits indicated by these CMFs may be regarded as conservative for two reasons. First, the sites in Kentucky already had SRS, although those at the retrofit sites had already exceeded their useful lives. Second, the results include Pennsylvania sites, which experienced fewer benefits (likely because they were lower priority sites for the strategy).

Table 22. Recommended CMFs.

CMF = Crash modification factor.
HO = Head-on.
ROR = Run-off-road.
S-OD = Sideswipe-opposite-direction.

To date, the most comprehensive and reliable study of both SRS and CLRS individually applied is published in NCHRP Report 641-Guidance for the Design and Application of Shoulder and Centerline Rumble Strips. When compared with the recommended CMFs from that study, the results suggest that the effect of combining CLRS and SRS further reduces run-off-road crashes versus applying SRS alone. It also appears that SRS do not further reduce head-on plus sideswipe-opposite-direction crashes further than applying CLRS in isolation.

A disaggregate analysis of the results indicated that larger percentage crash reductions were found for run-off-road crashes for sites with higher AADTs. For head-on+sideswipe-opposite-direction crashes, smaller percentage crash reductions were found for higher AADTs. For the expected crash frequency per mile-year without treatment, larger percentage crash reductions were found for run-off-road crashes for higher crash frequencies. For head-on+sideswipe-opposite-direction crashes, smaller percentage crash reductions were seen at higher crash frequencies. Caution should be used in interpreting and applying these disaggregate results because they are not robust enough to develop CMFunctions that would allow the estimation of CMFs for different levels of AADT and expected crash frequency. However, they may be used in prioritizing treatment sites.

B/C ratios are estimated to range from 20.2 for a higher cost/higher service life assumption based on Kentucky information to 54.7 for a lower cost/lower service life assumption based on information from Missouri. These results, which are based on conservative service life assumptions, suggest that the treatment, even in its most expensive variations, can be highly cost effective.