In Carmageddon-speak, smashable track things are imaginatively called ‘Noncars’. They include objects like traffic lights, trees and signs. I’ve partially implemented them into PhysX. You should be able to bend them before knocking them over, but I haven’t figured out the bending part yet.
Once i’ve finished that part, I’ll do another video, then next up is hooking up a real engine/gearbox model, then animations :)