We've acquired a lot of the Racer sets over the last few years. While only slightly more expensive than Matchbox/Hot Wheels cars they're better because you can change the cars' designs around and, of course, when they crash you can "damage" them.
The only downside is that you can't put a minifig in them... and their wheels are set too wide to match the rest of the Bricksburgh traffic. So, we had to adapt them.