I don't know if this will help, but we have our satnav set up as a "Lorry" and as yet has not taken us on any road that has a low bridge. I don't know if this is the norm or not. On the new mapping software we have recently bought there is a facility for putting in emergency vehicles but I think we will still stick as a lorry, at least if a lorry can get through we should be able to, well, that is our reasoning.