Crashing would be interesting, but would very much change the nature of the game. Not to say that's a bad thing, could be fun and add tension as it would be another possibility to lose the motors.
Causing traffic seems like the obvious 'solution', i.e. not letting motors drive onto occupied tiles. This would increasingly lead to situations where you'd have more than 1 motor trying to enter the same tile and being on stand-by, as their timers ran out. When the tile would be vacated by the previous motor, you'd then have 2 or more motors trying to enter it at the same time.
Maybe prioritize them according to who's been waiting the longest? Randomize? Maybe go back to the crashing option? :D