Friction Measurements Another important measurement of pavement performance is the surface friction. This is critical because low friction values can increase stopping distances and the probability of accidents. Given the variability of pavement surfaces, weather conditions, and tire characteristics, determining pavement friction over the range of possible values is not an easy task. To estimate friction, a standardized test is conducted under wet conditions using either a treaded or smooth tire. Although other speeds are sometimes used, the standard test is generally conducted at 40 mi/h using a friction-testing trailer in which the wheel is locked on the wetted road surface, and the torque developed from this wheel locking is used to measure a friction number. The friction number resulting from this test gives an approximation of the coefficient of road adhesion under wet conditions and is multiplied by 100 to produce a value between 0 and 100. The friction number with a treaded tire (FNt) attempts to measure pavement microtexture, which is a function of the aggregate quality and composition. The friction number with a smooth tire (FNs) provides a measure of pavement macrotexture, which is critical in providing a water drainage escape path between the pavement and tire.
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