FHWA Publication No.: FHWA-HRT-15-064
Summary & Conclusions:
- The objective of this study was to perform a rigorous before-after evaluation of the safety effectiveness, as measured by crash frequency, of shoulder and centerline rumble strips applied in combination on twolane rural roads. The CMFs shown in table 3 are recommended for various crash types.
To date, the most comprehensive and reliable study of both shoulder and centerline rumble strips individually applied is published in NCHRP Report 641—Guidance for the Design and Application of Shoulder and Centerline Rumble Strips.
Compared to the recommended CMFs from that study, the results suggest that the effect of combining centerline and shoulder rumble strips further reduces run-off-road crashes compared to shoulder rumble strips alone and both total and fatal+injury crashes compared to centerline rumble strips alone. However, it appears that shoulder rumble strips do not further reduce head-on+sideswipe-opposite-direction crashes than applying centerline rumble strips 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 mi-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 were estimated to range from 20.2 for a higher cost/higher service life assumption to 54.7 for a lower cost/lower service life assumption. 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.