The rebounding velocities of the vehicles is determined by the coefficient of resitution, or COR, which is the ratio of the relative velocity of approach to the relative velocity of recession. The COR is determined by the material and structure of the vehicles, their relative velocity of impact, vehicle masses, and angle of impact. I have greatly oversimplified by assuming the following constant CORs for all the masses and speeds possible in this simulation:
http://www.collisionsafety.net/documents/System%20based%20collision%20analysis.pdf contains a nice treatment of the subject of car collisions, and contains references for car CORs. By the way, yes, I did make up "Elastoperfect"; no macroscopic collision has a COR of exactly 1.