The car on terrain box2d example looks promising, but the question is, how does it perform in the context of a full game? Hillclimb Racing is the benchmark to aim for... great native app game. If we can get that performance happening in the browser with these engines then we're doing okay, but I can't help doubting these engines are up to Hillclimb Racing standards. That is, fast and smooth with lots of track loaded in memory. Even the native app suffers a slight stutter on occasion as it loads more track ahead, so I wonder how much track can be in memory in the browser. (I'm guessing these side scrolling games have a maximum window of game area available at any one time).