Someone of my acquaintance was adamant (and still is) that the speed limit at the tolls for the Second Severn Crossing (M4) is 30mph. The only correctly signed speed limit is 50mph - the 30 only appears as a marking on the road in a circle (which is a reminder, not the beginning of a limit zone unless it coincides with the standard lollipop sign). It would actually be very stupid to try passing the toll gates at more than 2mph, even with a transponder tag.
Even the 50 limit is widely ignored, and I happily watch them approaching the queues too fast to be able to pick the shortest one... while I trundle through the tag lane and leave them all behind.