Ohhhh dear.... Know your height lol

[hextal]

I agree.

Many times I have seen height restrictions with curved arch structures and wondered just this. Just occasionally it is qualified by a horizontal marker showing the point at which the clearance has been measured. I think this should be mandatory.

There are guidance documents relating to the signing of substandard headroom (sub 5.03m), and from memory these include use of reflectorised horizontal chords, but I can't recall any mandatory docs. Based on conversations with numerous local authority bridge managers it's a relatively low priority unless there's a history of bridge strikes or it's a particularly susceptible structure.