Neat concept. I liked building much more than driving actually. I don't want to repeat, what others said already, but the only thing that didn't really make sense to me, is how the car was at the very bottom on starting ramp, but when driving it was suddenly someplace entirely different, and I was like, "WHA?!" :D
But yeah, the idea is fine, but I'd make physics a bit more forgiving - so that it's possible to jump and do other weird stuff (and experimenting with the ramps), since right now - if you complete the puzzle in building mode, there's nothing to do really in driving mode, other than to drive (if you know what I mean).