However I can't imagine a car company accepting the legal liability of allowing their autonomous cars to go over posted speed limits to "match traffic".
In a trolley problem you have the option to refuse a decision. By participating in "speed matching" you become one of the trolleys and you have accepted the unsafe conditions.
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.
You'd have that range whether people went at the speed limit, above the speed limit, or below the speed limit. As long as speed limits are made with actual speeds taken into account (they are) that argument is irrelevant to whether you should speed. It just means that if you really want to be safe to hit someone you shouldn't go above 25mph, no matter what the speed limit is.
> The strangest thing about watching this story is that people who are apparently ignorant of this still feel sufficiently certain of the relative unimportance of speed differences in this range to sneer at other people for commenting on it.
The speed limit here was at least 40, wasn't it? Your own numbers say that the differences are unimportant above 40.
Of course, as others have pointed out, human tendency to ignore speed limits and react badly to vehicles driving slower than ambient traffic speed creates another potential hazard to trade increased accident fatality rates off against when deciding if and when self driving vehicles can speed. It's trolley problems all the way down.