Off-Network Tactical Diversion Route

In the United Kingdom, emergency diversion routes, formally known as off-network tactical diversion routes, are permanently signed detour routes: planned road traffic routes that bypass the main trunk road network. Emergency diversion routes provide the public, where possible, with a planned, checked and agreed junction to junction diversion route that circumnavigates an incident which has resulted in the closure of the main carriageway.

Source: Wikipedia — Off-Network Tactical Diversion Route (CC BY-SA 4.0)