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, the only way to alter the average force exerted on the passenger is to change the duration of the collisions.

Which of the following features of cars/roads act to increase the collision time directly (i.e. while a collision is occurring)?