If self driving cars go the posted speed limit, it will not improve safety. The safest speed to go is the same speed as the rest of the traffic around you. On a highway where everyone is going 20 over the limit, one slowpoke going the speed limit will make a huge mess and in heavy traffic will trigger a compression wave that will stop the traffic dead a km behind him, though he'll never see it from the front of the mess he created.
If self driving cars go the posted speed limit, we will need politicians with the guts to increase speed limits.