Using Estimated Available Friction at Posted Speed as Pavement Safety Performance Measure on Freeway

ABSTRACT

Highway safety is impacted by pavement surface characteristics, such as pavement microtexture and macrotexture. The study proposed a new approach to consider the combined effects of surface macrotexture and low-slip friction (mainly impacted by microtexture) to analyze pavement safety performance. The study investigated the impact of combining microtexture and macrotexture into a single pavement safety performance indicator, to allow a more comprehensive assessment of pavement safety. The implementation of the estimated available friction at different speeds, was investigated by using the speed correction equation used in ASTM E1960-07 to model the expected number of crashes by Negative Binomial regression (NB) on the Interstate and Primary freeway facility in the United States.

As vehicle speed increases, the interaction between macrotexture and friction at the tire-pavement interface becomes more complex. The interaction of the two parameters on higher speed should be considered to better analyze the frictional properties of the highway facility and enhance overall safety on high-speed highways. The research investigates the combined effect of macrotexture and friction on crash counts to provide insights and recommendations to develop safe driving on freeways under various weather conditions.

The analysis showed that the estimated available friction at 120 km/h (FRS120), significantly combines the effect of friction and macrotexture on freeway crashes. This index allowed to develop safety performance functions (SPFx) and crash modification factors (CMFs) that effectively capture the potential reduction in expected crashes due to changes in both, SFN40 (friction at a low slip speed) and macrotexture. Furthermore, the application of the index on a case study illustrated how the index can be used to evaluate alternative treatments and select the most cost-effective one.