What is the design life period of a road?

It is a period considered appropriate to the function of the road. An arbitrary design period (say 25 years) needs to be chosen to enable the design of the road pavement to be developed. The determination of the total traffic the road will experience is a function of the design life chosen, and an estimate of the steady traffic growth expected over that period (say 2%). The resulting Design Traffic calculation is then used to determine what thickness the pavement should be to handle the stresses exerted on it from road vehicles over 25 years. It is accepted that after 25 years the pavement will likely need some sort of rehabilitation or construction, and the design process starts over again.