The A406 and A40 change the urgency
The North Circular and Western Avenue are fast, busy and largely unforgiving of a stopped vehicle. An incident in a live lane is not something to manage yourself — get everyone behind a barrier, call 999, and let the vehicle sit until it can be moved safely.
On these roads the recovery itself is also constrained. There is often nowhere to place a truck, loading may need to happen from a specific side, and in some sections it simply cannot be done until traffic is managed. That is why we ask exactly where a vehicle is before sending anything — 'the A406' covers a great deal of ground.
If the vehicle is driveable and you can get it off onto a side road or into a retail park, say so when you call. It usually makes the whole job faster and safer.
