Incorporating road safety into pavement management: maximizing surface friction for road safety improvements

Abstract

This research explored the relationship between asphalt mix design, skid friction, and roadway safety. Initial tasks attempted to find a relationship between pavement skid resistance (friction) and crash frequency, particularly wet weather crashes. Friction and crash data collected over 10 years at six study sites in Wisconsin were analyzed. The results of the analysis did not indicate a relationship between crash frequency and pavement skid friction. Although some evidence suggests that the number of wet pavement crashes increased as the pavement life increased (and skid friction values decreased), the frequency of crashes was not sufficient to statistically support. Nevertheless, the fact that the relationship seems to behave inversely proportional, that is to say more crashes occurred at low friction numbers (FNs), is an important indication that skid resistance may indeed be a factor affecting wet weather crashes. It was not possible to determine a skid friction threshold value that indicates the critical point where pavement maintenance would be needed. Although the data obtained in the research could not support a specific value, it is clear that friction values less than 35 are problematic from a safety standpoint. A possible indicator of friction on high-speed roadways is macrotexture. Therefore, macrotexture (measured as MTD) combined with friction data was of great interest in this research. Plots of MTD and FN values did not show a clear relationship between the two values, although it was evident that the larger FNs were concentrated in low MTD values. Skid resistance is an important feature which should be considered while evaluating roadway safety. An effective asphalt pavement asset management approach will include an annual testing program to monitor skid friction values. FN values less than 35 should trigger a safety monitoring program and those pavements scheduled for future rehabilitation or reconstruction.