Rowley010:
If where the height marking is there are no lines specifying where the stated height is on the arch, does that mean the whole width of the carriageway is the height stated?Sometimes when your only a few inches off the stated height it looks a bit close for comfort as your approaching it with where that height on the arch actually is
On an arch bridge without width lines, I would assume the height marking refers to the clearance for a box van of 2.5m width approaching the arch within any of the marked lane(s) - or approaching centrally, if no lanes are marked.
No other principle would seem sensible or useful to drivers, and surely almost all victorian arches marked on any other basis will either have long since been knocked down by vehicles and rebuilt with girders, or else the responsible authority (usually the rail authority) will have taken action to mark the arch appropriately.
Vehicles with a narrow upper profile (like diggers on a flatbed) may be able to exceed the stated height by approaching the arch centrally (straddling marked lanes if necessary), but that is on their head to assess the bridge for themselves, having been forewarned of what the clearance is for a box van profile.