When is the clear height considered sufficient so that this type of prohibitory sign is not erected?
Limited vehicle height (C17)
”This sign indicates that traffic with a vehicle over a certain height is prohibited if the clear height is less than 4.5. metres. The maximum permitted vehicle height is indicated on the sign.”
Clear height = the permitted height of a vehicle.
4.5 metres is very high in vehicle terms. It is not sensible to spend resources on erecting signs that will probably never be of use.