If you're on an A or B road where the local council has chosen not to signpost the number (ref), find a railway bridge, and look for the little Network Rail plate saying "If you hit this bridge...". These have the number handily written on them. This is helpful.

Except in Burton, where the local scrubbers have, er, scrubbed out the number.

Even the bridge over the Trent & Mersey near Stenson has one of these: "If you see any waterborne vehicle hitting this bridge..." How much damage do they think a narrowboat travelling at 3mph is going to do?!