Google employees pay attention to the road as the cars drive because that's their job. Consumers will read the paper and go to sleep while the cars drive. Manual override is not a solution to this problem. A panic mode that alerts the driver to take over in uncertain situations could help.
The cars will be programmed to always ensure that there is enough room to stop the car safely when something unexpected happens. BMW already has a system to automatically stop the vehicle safely if the driver is suddenly incapacitated. And Volvo has one that stops you rear-ending someone. Pretty soon we'll have cars that are pretty tough to crash, even though the human will still drive it almost 100% of the time.
