Like all structures, roads deteriorate over time. Deterioration is primarily due to accumulated damage from vehicles, however environmental effects such as frost heaves, thermal cracking and oxidation often contribute. According to a series of experiments carried out in the late 1950s, called the AASHO Road Test, it was empirically determined that the effective damage done to the road is roughly proportional to the Fourth power of axle weight.A typical tractor-trailer weighing 36.287 t with 3.629 t on the steer axle and 16.329 t on both of the tandem axle groups is expected to do 7,800 times more damage than a passenger vehicle with 0.907 t on each axle. Potholes on roads are caused by rain damage and vehicle braking or related construction works.