Note: By this its benit13 means "Negative kinetic energy is impossible." Negative energy itself is very possible and always used in physicNegative energy is impossible.
I'm not 100% clear what TWq means by "energy of crash." The total energy before the collision is the sum of the kinetic energy of each car. The total energy after collision is the sum of sound energy and thermal energy, the sum of which equals the totals total energy before the collision.