under the bridge downtown, is where I drew some blood

This is some tremendously nerdy fun - you know those signs you get on low bridges to tell you that you, a lorry driver, are about to unzip the top of your vehicle like an old-fashioned can of sardines? Different countries have different signage to warn unsuspecting drivers of what's ahead - the yellow diamond in the photo on the right is from the USA, in particular the low railway bridge in Durham, North Carolina which is notorious enough to have its own website.

In Britain we operate a system of red signs, sometimes circles and sometimes triangles depending on some rather opaque rules, with clearance heights listed in both metric and imperial measurements. If you thought the rules governing sign shape were arcane, though, wait till you hear the rules governing the derivation of the clearance height figures. I mean, I won't go into it here but the end result is that - somewhat counter-intuitively - a single metric height can be associated with several different imperial heights, and vice versa. 

This opens up the possibility of a sort of sign-spotting subculture emerging, and, as you might imagine, this being the internet, it has. This page lists all the combinations of signs that eagle-eyed people (let's call them people, for the sake of argument) have spotted around the country.

Lest I get too snooty about others' intense weirdness and nerdery, though, I should disclose that the first thing I did, about half-way through watching Matt Parker's video, was think to myself: ooh, I bet I know where a good one is that might not be on the list. And it is in this interesting location near Bishton, just a few miles east of Newport, where a minor road crosses the South Wales Main Line via an interesting take-your-pick over/under arrangement. Head for the level crossing and you might have to wait for a train to pass; head under and you won't have to do that but beware if you've forgotten that you've got the bikes on the roof rack. I have been through the tunnel, a few years back; I can't remember which car it was in but I do remember stopping just in front of the entrance and getting out to visually inspect the clearance, just in case. I assume it can't have been the current family enormo-vehicle, a Seat Alhambra, because there's a good chance that might not have fitted under at all. Don't imagine that going over the top means you can take a fully-extended cherry-picker that way, by the way, as there is also a maximum height restriction of 5 metres to avoid getting entangled with some power lines. 

Anyway, it turns out this is already on the database as the type specimen for the 1.7m/5'6" height combination. Interestingly the type specimen for the 1.7m/5'9" height combination is only a handful of miles away in Caldicot, part of a similar choose-your-fighter under/over tunnel/level-crossing set-up. The lowest signed clearance on the list is, thankfully, not on a road but on the Bude Canal and would presumably require you to own a very low-profile boat (a punt, say) and lie down in it if you wanted to pass underneath.