A driverless taxi slows down on a dark San Francisco street and is quickly surrounded by a group of masked figures. One of them places a traffic cone on the hood of the car. Its hazard lights flick on, and the car stops in the middle of the road, disabled. This bizarre scene has been repeated dozens of times across the US tech capital this past week -- the work of activists protesting against the proliferation of robot cars, which they consider unsafe. IMAGES AND SOUNDBITES