several ways, one is to use physics. iF you can do that then you are pretty much done.

If you can't then you should use event_impact, that way you know when a car is hit. From there you have to determine where it was hit and what should happen. Check the "most asked questions" of the Lastest AUM, there is a block pushing code there, that should help.

Note: always be papared to rewrite all yoru code if you don't think ahead, this is especailly true when you are new to programming. Hey, we've all done it, it's how we learned.