I also tend to question then ignore those signs, but sometimes ... On one of my forum rides we came to a road closure which we ignored. We proceeded a few hundred yards but then discovered that a section of the road was no longer there. In its place was a hole so big that it looked like a massive IED had gone off under the road! We had to shoulder our bikes cyclocross-stylee and clamber up a muddy slope to get round it. I'll see if I can find one of the photos of it ... Ah, here is one taken by @potsy: It would have been the mother of all detours if you'd had to turn back. Not quite on a 'Connel Bridge closure' scale***, but still not what you'd want to do! *** It involves a 140 km diversion to get 0.2 km!