If P is the number of vehicles per day at last census, r is the increase in traffic and n is the number of years passed after last census, number of vehicles A per day for design, is

Highway engineering (also known as roadway engineering and street engineering) is a professional engineering discipline branching from the civil engineering subdiscipline of transportation engineering that involves the planning, design, construction, operation, and maintenance of roads, highways, streets, bridges, and tunnels to ensure safe and effective transportation of people and goods. (Source: Wikipedia)