Maybe not in the U.S., but in Norway, as part of getting the driver's license, you'll have to take an ice driving course, where, amongst other things, a dummy is swung at your car while driving on an oiled lane in 30 mph. (It is practically impossible to steer away from the dummy, which is the meaning of the exercise; to realize the folly of driving too fast on icy roads)

Because we have a pretty good idea of how humans react in traffic, but we're still a bit unsure about robots.

Because it's assumed human drivers will at least have a desire to stop. If a machine isn't properly programmed, it will plow right through a crowd and never look back.

There is no inherent equality between humans and machines. Is it conceptually difficult to grasp that machines can be held to a different standard?