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This report describes the methodology and results of analyses performed to (1) evaluate the impact of shoulder rumble strips (SRS) and centerline rumble strips (CRS) on the placement of vehicles in the travel lane of two-lane, undivided roadways and (2) determine the minimum shoulder width required for drivers to correct errant vehicle trajectories once alerted by passing over SRS. Field studies indicated that CRS on twolane, undivided roadways with lane widths as narrow as 10 ft do not adversely impact the lateral placement of the vehicle in the travel lane. In fact, at locations with smaller shoulder widths (1 to 2 ft) drivers positioned the center of their vehicles closer to the center of the lane. Similar effects were found at locations with both edgeline rumble strips (ERS) and CRS. The impact of SRS located within 7 to 9 inches of the edgeline on the lateral placement of vehicles in the travel lane was less clear. However, it does appear that SRS located near the edgeline may shift vehicle travel paths closer to the centerline. In contrast, SRS located 35 inches from the edgeline did not seem to impact the lateral placement of vehicles in the travel lane. In addition, lateral offsets that position the center of 16-inch SRS in the middle of the shoulders at least 4-ft wide should provide enough remaining shoulder width for the 85th percentile distracted driver to correct their errant vehicle trajectory before leaving the paved roadway surface. Additional findings from all of the studies and detailed recommendations are discussed in the report.

An evaluation of shoulder rumble strip's effectiveness in reducing off-road and rollover crashes on Montana's highway system utilized 10 years of crash data on 393 miles of Interstate Highways and 213 miles of National Highway System (NHS) and Primary Highways. Statistical analysis of crash data involved rumble strip segments and control segments without rumble strips for 3 year periods before and after implementation of shoulder rumble strips. Descriptive statistics were developed from global data sets and comparative statistics on equivalent segments produced a finding of significant crash reductions on Interstate Highways, while the effect of shoulder rumble strips on NHS and Primary Highways was uncertain due to an inadequate sample size. Analysis of contingency tables indicate that the reduction in Interstate off-road crash rates attributable to shoulder rumble strips was 14.0% with a corresponding reduction of 23.5% in severity rates. The benefit/cost ratio for construction of shoulder rumble strips on Interstate highways was 19.5. A shoulder rumble strip driver survey involving questions related to knowledge of rumble strips, frequency of encounters, reaction to exposure, and general opinions on their use was incorporated as part of the study. The survey indicated that 95% of the driving public was familiar with rumble strips and their use with the majority of drivers encountering rumble strips on every highway trip. The overwhelming majority of motorists liked the benefits that shoulder rumble strips provide.