can't be bad to offer some suggestions
maybe the vehicle class (weight) and the actual speed
=impetus
should be related to the amount of damage caused
along with accio=reaccio, the relative speed of the collision partners should play a role
then the cumulated damage amount according to impetus should be distributed between two partners, going with the type of Collision (elastic or unelastic)
as such, two equally heavy vehicles crashing against each other would distribute the damage by
(speed1+speed2) : 2 relation
wheras a vehicle crashing against a rock would absorb all the damage
as would a unit run over by a tank (eg: vehicle class x speed class = number of wounds inflicted)
or so