There is some percentage of driverless cars on the road that will force everyone to drive the speed limit.

Once driverless cars get to a certain percentage of adoption, there will be enough of them on the road, in all available lanes, that it will force everyone else to drive like them. Assuming driverless cars are programmed to go the speed limit, it will be really hard to go any faster. I'm guessing that number is not higher than about 30%, but it could be a lot lower.