One thing I've been wondering is whether self driving cars could result in lower emissions by never exceeding the speed limit, and by driving more smoothly than a human operator. Additionally, I wonder if having even say 10% of cars on the road that never exceed the speed limit would result in most other cars nearby driving the speed limit as well, since they would effectively block non-self-driving cars from speeding in many areas. Speeding is dangerous for all other road users, so there should be safety benefits as well. Why not require all self driving cars to drive at legal speeds?