So the safest reasonable thing for self-driving cars to do could depend on how many of them there are around.

(Of course there are other considerations of that sort. E.g., if all cars on the roads were self-driving then they could coordinate with one another in interesting ways and maybe go substantially faster than human-driven cars for a given level of safety. Maybe not, though, because of risks to pedestrians.)

Google’s driverless car is programmed to stay within the speed limit, mostly. Research shows that sticking to the speed limit when other cars are going much faster actually can be dangerous, Dolgov says, so its autonomous car can go up to 10 mph (16 kph) above the speed limit when traffic conditions warrant.

https://uk.reuters.com/article/us-google-driverless/look-no-...

I wonder how they decide over what warrants speeding.