Whenever my group comes across a situation that the rules don't explicitly lay out we ultimately always resort to 'real world' physics. What I mean is I would try and play out the situation to the best of your abilities to try and anticipate what would actually occur. What are the weights of the two vehicles? What are the speeds? Where do the vehicles collide into each other? Ball park how much damage would occur? Would the direction of the npc vehicle go off course? Et cetera, et cetera.
If the situation isn't completely linear, you can roll some dice to determine magnitude. You can use the groups effort (& creativity) towards an objective and roll against that. If the groups concept is solid, maybe it might take only a few turns or a few good rolls.
At the end of the day, its okay to wing it provided you're following the mood and general thinking of the table.