The greatest damage occurs when the soil is wettest. Adequate drainage will prevent most rainwater from doing any harm. The design - careful design will have taken into account the temperature changes which affect water vapour rising through dry soil. The variability of the soils on which the road is built contributes to its lifetime. Axle loads will be heavier when the economy is healthy.
Traffic volumes are time dependent.
The materials used will deteriorate with time. Bitumen loses viscosity and adhesion with the actions of oxygen and sunlight, particularly ultra violet light. The rougher the road the more expensive it is to run on and the faster it deteriorates.