Again heavy loads stress a pavement near to its elastic limit every time they pass over it. It is well established that any structural material when stressed near, to, or beyond the elastic limit will become fatigued with repeated stresses. The higher the stress the quicker they will fail by fatigue. It is not considered good practice to stress a material, especially a brittle material, repeatedly to a point half-way to its elastic limit. (The elastic limit is that stress that may be given to a body without producing a permanent set; below which it will return to its original shape upon the removal of the stress.) A walnut may not crack at the first blow but with a sufficient number of no harder repeated blows will crack. The higher the stresses the sooner the crack occurs. So it is important to know how many loads daily are stressing a pavement near to its elastic limit. From all the loads it has been the endeavor of road engineers to work out “importance factors” that will measure the relative damage done by the several classes. The Road Board of Great Britain has adopted the British ton as a unit and calculations are based on the traffic in tons per yard of width per year or per mile.[178] The British Road Board Unit Weights are as follows: -------------------------------------+--------------- |Assumed Average Classification of Vehicles |Weight in Tons. ------------------------------------+--------------- Ordinary cycles | 0.09 Motor cycles | .13 Motor cars (including motor cabs and| any other motor vehicles) | 1.6 Motor vans (covered) | 2.5 Motor omnibuses | 6.0 Motor lorries (rubber tires) | 6.0 Trailers to rubber tired lorries | 5.0 Motor lorries (steel tires) | 10.0 Trailers to steel tired lorries | 5.0 Light tractors | 5.0 Trailers to light tractors | 5.0 Traction engines | 12.0 Trailers to traction engines | 8.0 Light vehicles (one horse) | 0.4 Light vehicles (two or more horses) | 0.6 Heavy vehicles (one horse) | 1.25 Heavy vehicles (two or more horses) | 2.5 Omnibuses (two or more horses) | 3.0 Tram cars (electric, steam or horse,| as the case may be) | .... Horses (led or ridden) | 0.5 Cattle | 0.3 Sheep and pigs | 0.1 Hand carts and barrows | .... Horses drawing vehicles (to be | calculated from number of vehicles) | 0.5 ------------------------------------+---------------