Surely if they are self driving and the traffic is regulated. Then self driving cars could go a lot faster. They would basically become mini trains. So why not over 100mph.
I wouldn't trust the government to be able to get that past all the sandal wearers.
Cars have improved massively since the speed limit was made what it is - my car is probably safer at 155mph than a 15 year old micra at 70mph. Speed limits haven't moved though.