If self driving is otherwise compelling, police and construction can use electronic signs (radio, networked notifications, whatever).
The robot drivers will do better than humans in that case, they will reliably obey the signs (imagine how easy it would be to detour a car that communicates with a regional traffic management system, compared to a human that thinks they know better).
The simplest form I can think of is a beacon broadcasting its location along with a stay back distance (with the routing system in the vehicle left to deal with the details of compliance).
That doesn't address security or authentication, but it should be fairly easy to track down a broadcasting radio, and it should also be easy to log and aggregate the active beacons that have been spotted by vehicles. With that context, traditional enforcement tools should probably work well enough.
The robot drivers will do better than humans in that case, they will reliably obey the signs (imagine how easy it would be to detour a car that communicates with a regional traffic management system, compared to a human that thinks they know better).