I seriously hope that ALL autonomous systems will have a big off-switch. Said systems make mistakes: had a GPS telling me to go wrong way into one-way street; Mini tyre-pressure monitor got spurious reading because of low temperatures, refused to listen to my reset and tried to call International Rescue to send Thunderbird 2 to me. These functions are relatively minor and risk is low (although if the one-way-street issue had been on a dark rainy night with bad visibility it might not have been).
If a system can slam on brakes at 70mph on a busy motorway, well, do I need to go on..? Big off switches, please (not touch-screen!)