Rig up streetlights to the traffic management system of a city/major road network.

Post simple electronic information signs on a suitable fraction of them (as is already happening at present).

Now when there is some sort of blockage on the main route, have the signs say "Accident on [xx] - Follow flashing streetlamps" and set the streetlamps on the diversion route to flash on and off, clearly guiding the way.

This could be set into action the moment a blockage is detected by traffic management, so would be much quicker than the time taken to get physical diversion signs to the scene.

It would be easier to follow the diversion route given that the whole route is highlighted, not just which way to turn at junctions, so it's probably less stressful too.

The route could also be dynamically adjusted depending upon flow of traffic, perhaps even setting up different routes a few miles back to stop traffic backing onto queues near the accident scene.