Wet pavement-related collisions represent a significant traffic safety concern, due in part to the lack of adequate friction between tire and pavement, known as skid resistance. State agencies employ a skid number (SN) system, based on a standard test procedure in which a locked wheel is towed at 40 mph and the skid number (SN40) is calculated from the measured resistance. SN40 is used as a reference value for speeds both greater than and less than 40 mph. For most Departments of Transportation (DOTs) in the nation, excluding California, pavements for which the SN40 is below 30 are deemed unacceptable and corrective actions are required. The main objectives of this study are (1) to evaluate and analyze skid test results from the test data inventory, and (2) to identify and analyze before-and-after collision data at sites where three experimental types of pavements (Open Graded Asphalt Concrete [OGAC], Groove Pavement [GP], and Rubberized Open Graded Asphalt Concrete [R-OGAC]) have been implemented. Study results suggest that a significant relationship exists between SN40 and seasonal effects (temperature, average monthly precipitation, and the number of dry months prior last precipitation). A significant relationship also exists in high-risk locations where average daily traffic (ADT) is higher, and in the more heavily-used shoulder lanes. If highway agencies wish to prioritize pavement improvements using SN40, SN40 must be standardized. The model developed in this study can provide the needed adjustment factors. In addition, while further research is needed, results suggest that new pavement types such as OGAC can improve safety performance of roadways.
April 1, 2010