Pure speculation but I bet Waymo doesn’t do highways because they can’t safely reconcile the speed limit with the speed of traffic. In CA and AZ (where Waymo is currently operating) typical highway limits are 55 or 65 mph while traffic speeds in light traffic are easily 70+. Speed differential is dangerous; having a Waymo going 55 while everyone else goes 70+ sounds like a terrible idea but they can’t exactly teach the robot to speed the way a human does either. I’d love to be a fly on the wall during that discussion, ethics, human factors, law, you’d have to tackle the whole shebang.