At a crash test facility scientists run a car of mass m into a wall at a velocity v. The car comes to rest following the collision. The experiment is repeated several times at the same velocity for several different cars (all of mass m).

Given that the mass of the cars and the velocity of the cars are all the same, what aspect of the collision can a car designer try to change to reduce the average force applied to the car (and hence to the occupants of the car)?