A self driving car obviously needs to be aware of other cars on the road. I don't see any reason why the car couldn't observe other cars, see what speed they are going at, and refuse to go faster than the rest. A car that refuses to do 120mph when all the other cars are doing 60mph in a 50mph zone should be trivial.
(Trivial if the self driving tech works at all....)
You're getting downvoted for this comment apparently, but I'm still of the firm belief that we will never see full autonomous driving without some sort of P2P network among cars/infrastructure.
There's just too much shit that can't be "seen" with a camera/sensor in conjested traffic. Having a swarm of vehicles all gathering/sharing data is one of the only true ways forward IMO.
(Trivial if the self driving tech works at all....)