On most roads around where I live, if you are driving exactly the speed limit, you are a slow moving obstacle that is probably more of a danger due to the extreme speed differential with the other drivers who are 10-20 MPH over the limit. I’d hope self driving cars were programmed to deal safely within the bounds of reality. If there is nobody else on the road, sure drive the speed limit. If cars are whizzing past you on all sides you’re a hazard even if you’re obeying the law.