Water film on pavement surfaces entails a decrease of friction between the tire and the road. Nevertheless, only effects of water films above 1mm depth were investigated until now, considering hydroplaning risk. From these investigations, formulae were derived to predict the so-called hydroplaning speed at which happen hazardous situations for the driver because there is no more contact between the tire and the road. However, a significant number of accidents occurs on very thin water film such as when the road is drying after rainfalls. Indeed, during these periods, drivers have the feeling that the road is dry leading to inadequate behaviors. This work focuses on the effect of thin water film (< 1 mm) on pavement friction and aims at modeling the friction decrease. Experimental simulations were conducted in controlled conditions in laboratory using the Dynamic Friction Tester machine on various pavement surfaces. Pavement surfaces were wetted by a spray, which was weighted using a precision balance scale in order to estimate precisely the amount of sprayed water. The tire/road lubrication regime can be identified considering the Stribeck curve. A so-called critical water thickness, defined as the amount of water from which pavement friction collapses, was determined for each tested pavement surface. Critical water thicknesses are discussed as a function of surface macrotexture, microtexture and test speed.
