In the example given, a dilemma between veering off a bridge and hitting a bus, if the vehicles were automatically driven they would both have sensors and software that would be able to prevent this decision from needing to be made in the first place. In every instance I can think of, accidents happen due to driver carelessness, inability, or simply due to knowledge a driver could not have.
Take this scenario. In the world of driverless vehicles, the bus would either send a signal to notify other vehicles it was there, giving the other vehicles ample time to slow down safely or reroute. If it was incapacitated with no power to send a signal, the vehicle should still be equipped to know what obstacles are in the path (since the bus in that case would not be a moving target, the computer in the approaching vehicle should have no problem identifying it even without a beacon). This entire situation would simply not happen. I would imagine if we had the technology for an all driverless highway system, they would be sending and receiving tons of data... enough to know the status of every vehicle in the vicinity and the computing power to calculate how to respond to any changes.