With enough self-driving cars on the road (which of course there aren't now), rigid adherence to speed limits might change the safest-and-easiest driving speed for everyone and lead to everyone driving at the limit rather than everyone everyone driving 5mph (or 10mph or whatever) above the limit.
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.)
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.)