Vehicle collisions and increases in collisions rates during wet conditions are one of the major safety concerns for the NCDOT. Wet collision rates increase because skid resistance reduces under wet conditions. The precise amount of loss is dependent on many factors, but the consensus among experts is that pavement friction and macrotexture are important factors that affect the skid resistance and changes in this resistance under wet conditions. Although the NCDOT actively addresses skid resistance issues as they are identified, a recent study involving a small subset of North Carolina roadways has suggested that wet crash rates may increase after pavements are overlaid. That study concluded that NCDOT needed to consider characterization of both friction and macrotexture as part of its pavement friction measurement and management plan. While the current studies have successfully identified the potential for issues in recently overlaid projects, they did not identify whether these effects are temporary, and if so, how long they may last. In addition, past studies have not been able to identify any specific causative effects that may increase or decrease the impact of overlays on skid resistance. Since the primary change after an overlay is placed is the driving surface, there is a need to better understand how the asphalt mixture composition may affect the overall skid resistance of the roadway under wet conditions.
With respect to this need, the proposed research plan will achieve three objectives; 1) identify whether the observations from the initial study are systemic and quantify the initial findings on a larger basis, 2) determine how long potential impacts may last after the overlay is applied and what, if any, asphalt mixture characteristics contribute to the effect and longevity, and 3) develop a strategy for how to best monitor and manage the friction and surface characteristics of NCDOT pavements.
The primary outcome of the proposed research will be a dataset of continuous friction and macrotexture measurements for a range of NCDOT roadways. Two other key outcomes are also expected; 1) a method to identify problematic mixture designs prior to placement and 2) a laboratory experiment using QA cores that is capable of screening and identifying problematic mixtures after they have been placed (e.g., due to construction methods). These outcomes can be used by the Traffic Safety and Materials and Test Units of the NCDOT to improve the frictional characteristics and possibly reduce wet collision rates, which will improve roadway safety.