Under many circumstances, the odd ‘Road Closed’ sign is a challenge, as road folk often create a pedestrian route, which is quite passable by walking or slow cycling. So, when you do not know about a diversion, then you will have to wait for the rider to advise you of his ‘extra distance’ and authorise the longer route accordingly. Now, if it is something major (your Bridge) and you know about it, then I think it an Organiser’s responsibility to let potential riders know what the problem is and any suggested remedial/alternate solutions, other than wait for the problem to clear (new bridge).