Especially on a highway. I am talking about a situation where normal operation isn't possible such as due to hardware or software failure. Of course it isn't ideal, but wouldn't be much of a problem if the other vehicles on the road are driverless. (To be clear I mean they can just stop in the middle of the road if things are bad.)

A plane can't just let go of all control surfaces. Heck it can't even leave them where they are. It must do something and always must provide some degree of control. A flying plane is moving at several hundred miles an hour and cannot stop.