Direction: For a portion of national highway where a descending gradient of 1 in 25 meets with an ascending gradient of 1 in 20, a valley curve needs to be designed for a vehicle travelling at 90 kmph based on the following conditions.
1. head light sight distance equal to the stopping sight distance (SSD) of a level terrain considering length of valley curve > SSD.
2. comfort condition with allowable rate of change of centrifugal acceleration = 0.5 m/sec³. Assume total reaction time = 2.5 seconds; co-efficient of longitudinal friction of the pavement = 0.35; height of head light of the vehicle = 0.75 m; and beam angle = 1°.