The question becomes what do you wish to happen when 2 vehicles collide?
Gets kinda dicey.....momentum moving of two moving vehicles versus one moving the other not, angle of impacts and weights.....physical properties of various materials involved in ther actual colliding......metal thicknesses, rubber elasticity.......just to mention a few variables.....
Obviously do not need to get that complicated......I rather imagine in the game when one vehicle collides with another the collider should be stopped. If the collided vehicle was moving it too should be stopped. Keep it simple.