Self driving haul trucks are already at work at a few open pit mine sites around the world. These are vehicles driving relatively short, prescribed routes, with well defined patterns. The few articles I've read seem to indicate they're operating just fine, although I'd be interested to learn more. If the cost savings are there, I'd bet the entire mining industry will move in that direction rather quickly.

http://www.riotinto.com/ourcommitment/spotlight-18130_18328....