<P> The damage a vehicle causes is proportional to the axle load raised to the fourth power, so doubling the weight an axle carries actually causes 16 times as much damage . Wheels cause the road to flex slightly, resulting in fatigue cracking, which often leads to crocodile cracking . Vehicle speed also plays a role . Slowly moving vehicles stress the road over a longer period of time, increasing ruts, cracking, and corrugations in the asphalt pavement . </P> <P> Other causes of damage include heat damage from vehicle fires, or solvent action from chemical spills . </P> <P> The life of a road can be prolonged through good design, construction and maintenance practices . During design, engineers measure the traffic on a road, paying special attention to the number and types of trucks . They also evaluate the subsoil to see how much load it can withstand . The pavement and subbase thicknesses are designed to withstand the wheel loads . Sometimes, geogrids are used to reinforce the subbase and further strengthen the roads . Drainage, including ditches, storm drains and underdrains are used to remove water from the roadbed, preventing it from weakening the subbase and subsoil . </P> <P> Good maintenance practices center on keeping water out of the pavement, subbase and subsoil . Maintaining and cleaning ditches and storm drains will extend the life of the road at low cost . Sealing small cracks with bituminous crack sealer prevents water from enlarging cracks through frost weathering, or percolating down to the subbase and softening it . </P>

Plastic asphalt is a mixture of cement and asphalt