Brit Pete, I think the wall idea is excellent. Probably wouldn’t even need to construct one especially, an existing building wall could be used.
However, in this day & age of ever cheaper technology, surely a lazer or infrared measuring device with a read out viewable from the driving seat would be no more expensive in the long run than the painting of the wall, provision of steps & walk way & maintainence of same?
Not to mention the possible HUGE cost involvements of a bridge bash.
Surely, where load heights are variable, such as vehicles on transporters, this is a must?
Do transporter drivers still carry the extendable measuring sticks?
I drove for a large company last year that did not have heights marked on trailers, it was pure guess work.