Another reason that hasn't been mention is traffic. Now sure you say "what the fuck do you mean traffic? if some one stops im front of you, you just go up and around him" but think about it. we are used to drivin around in a 2 dimensional plane and have a lot of accidents as it is. now think about people drivin in a 3 dimensional environment there might not be as much "traffic", but believe me, the number of accidents would grow exponentially
"If you let your wife stand on your toe tonight, she'll stand on your face tomorrow morning" - Sir/Saint Thomas More