I see these every time a bicycle path intersects with any kind of car or train infrastructure. It just seems really patronizing, as if we’re not capable of slowing down without getting off the bike; it also shifts responsibility onto the cyclists if there is a crash based on something arbitrary (mounted vs dismounted).
Even then, that usually is time-of-day dependent. Plenty of times when such places are mostly empty. Meanwhile, nearby areas also have brief peaks that people should dismount during those times.