Effects of Pavement Friction Improvement on Crash Rates on Spanish Two-Lane Rural Roads

This paper describes research conducted at the Technical University of Madrid to establish thresholds in Sideway-force Coefficient Routine Investigation Machine (SCRIM) skid resistance values that determine significant differences in wet pavement crash rates, and to assess the effects of resurfacing schemes on safety. Data from over 1,750 km of two-lane rural roads in the Spanish National Road System were used in the study. Wet pavement crash rates were significantly higher in curves than in tangents. The difference ranged from two to three times depending on the radius of the curves. Significant differences were found in mean wet pavement crash rates between segments with an average SCRIM coefficient above and below a threshold value of 55 both for tangents and for curves with a curvature radius equal or less than 500 m. A before-after evaluation of wet pavement crash rates with a comparison group showed significant reductions of wet pavement crash rates as a result of pavement friction improvement treatments for segments with a SCRIM value below 50. The average safety effect of improving pavement friction from a mean SCRIM value below 50 to a value above 60 resulted in an average reduction of wet pavement crash rates by 68 percent.