that's the point isn't it. Driving down an mway the other day the signs were warning of long delays through three junctions, so when I hit near stationary traffic I took a chance and went into the services and, since fortune was smiling, found an 'authorised vehicles only' escape route onto a B road. Not having the faintest idea where it went or what I needed to do next, and knowing nothing about the locality, I was grateful for the box which, more or less seamlessly, got me through a couple of small towns and away, bypassing the whole jam. Of course it could have been done equally seamlessly by guesswork, or with a paper map, there's a much greater chance of that all going pearshaped.