I like the horn brake idea
Excepting for engine braking/hand braking, that would reward (or punish less) good braking habits, i.e. brake quickly and early (or not at all).
There is certainly an appeal for a game-like aspect to efficiency. Might want to think about incorporating some of these "alerts" too.
Arcade Sound Clips i.e. if you don't beat your high score for a gps tracked leg of your trip or ???
Obviously a mute button is needed too
