The two car collision is not equal to one car hitting a solid steel wall at 120 mph. This is a common misconception. To calculate the impact, you need to calculate the force exerted when two cars collide. Force is equal to mass times velocity, so two cars travelling at 60 mph into one another would generate 120 mph of impact, but you would also need to factor in the mass of both vehicles, which would result in a smaller impact force than if one car was to hit a steel wall.