We have one place like that on our MUP and it makes some sense because the crossing is too narrow to pass another cyclist or pedestrian safely, even for skilled riders. It is common to ignore the dismount sign when there is no other traffic on the bridge but very bad form to not dismount and wait when someone is coming the other way. OTOH there are several bridges 15 feet wide and 30 feet long around one of our lakeside hike/bike trails that actually have STOP signs. These are ridiculous and everyone just ignores them. There is so much foot traffic out there that it isn't much of a place to cycle anyway.