While I don't doubt there will be exploits, we already have a great deal of computer automation in vehicles, and one way they prevent potential hacks of the system is by making it one-way only. Plus, car manufacturers are actually being proactive about protection. More info: http://www.caranddriver.com/features/11q2/can_your_car_be_ha...
With computer controlled cars we'll have zero day hacks and crashes, viruses, and near simultaneous accidents all due to the same unpatched bug.
I would never leave the house on Wednesdays in such a world.