While most people don’t give it a second thought, there are a variety of factors that go into designing a road.   The civil engineer’s role is ensure adequate structural design (asphalt thickness) for the soil type/drainage (soil subgrade strength), expected loads and speeds, pavement material characteristics and climate conditions.  In Alberta, asphalt thickness on roads ranges from 3 inches (light duty parking lots) to 10 inches (heavy duty highways) thick.