Really neat concept. Really hard to juggle everything going on at once, especially as the cars start going faster. Having the targets show where the rocks were going to land definitely made things easier, though that wasn't clear right off the start.
I couldn't tell what the percentage represented.
It'd be nice to either have the tombstones & rocks disappear after a while, or to have the map expand as you play so that you don't run out of room eventually (I didn't play for that long, but it definitely seems possible).