Studies to Determine the Operational Effects of Shoulder and Centerline Rumble Strips on Two-Lane Undivided Roadways

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 two-lane, 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-in. 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.