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Life period is the time during which the road under study is planned to be functionally adequate. The functional adequacy is manifested through the road's ability to serve traffic demand at an acceptable level of service. A life period usually terminates when there is no or little traffic to use the road, or when there is so much new traffic that the road must be widened and possibly re-constructed on a different alignment. Life period is one of the input constants that are to be entered by the user. A typical value of 40 years is often used.