please empty your brain below

Some of us actually need the 1700 pages of the Traffic Signs Manual to do our jobs. Some parts confusing, some hopelessly outdated, but still the best traffic signs advice in the world by far. AASHTO (American advice and standards) pale into insignificance in comparison.
'I speculated wildly like a man in a comments box'....DG bites back! Love it.
What MO said.

Surely none of your readers would speculate wildly without any evidence?
Proper research and top facts. Marvellous.

Also like trackbeds of closed railways which include PR and TFs.
This is what I come here for
I love posts like this. Will there be a follow up, possibly including FoI requests, and/or disappointed letters to the highways authority, in which you get to the bottom of the mystery of the original sign?
Surely a metric measurement ending in 0 is rounded down twice, not once as you summarise - the rules said ending in 8 or 9,or have I misunderstood something?

dg advises: Never risk a 'surely'.
Interesting. We have a yellow box junction near us, which I now see may be breaking the rules. If all traffic did not enter before their exit was actually clear, there would be massive jams.
Thank you for the link to the manuals!
I wonder who the psychologist was who decided that we can cope with inches only in threes. Height indicators in UK vehicles must be in both metric and feet and inches, but there is no requirement to round up to the nearest three inches, so drivers have to consider inches in ones anyway.

There is a cost to too much rounding down of bridge heights, in extra vehicle miles, which could sometimes be a lot.
Is it really only men who speculate wildly, DG? 😉
Btw, I found your post today quite interesting. I shall take more notice of the warning signs when next I'm allowed out in a high hat. 😊😊
Fascinating read!
Perhaps this explains some bridge strikes? - drivers assuming there has been more rounding down at a location, than there actually is.
Bridge signage in Wick Lane is in a sorry state.
The advance sign at Tredegar Road is a triangle 13' 0" 285yds.

At the other end under A12, (a bridge that has had a number of serious accidents), there is a 12' 9" triangle, next to a 3.8m 12'6" circle northbound. Advance sign is a 12'9" triangle.
Southbound 3.88m 12'9" circle southbound.

Might as well put up a generic sign, "lucky dip, duck"
Could you tackle weight limits on bridges - especially canal bridges -next please....?
Classic DG post. Fascinating, again.
Interesting! I noticed this exact thing on on Hemming St in Whitechapel yesterday!
Oddly, I've never been conscious of the hyphen between feet and inches. Just two of your three examples comply with the manual in this respect.
Must get out more, if only to inspect local signage!
Brilliant article. The sort of stuff I idly speculate about but never bother to do the research you do. Thanks!
Good work - perseverence, tenacity, gritty detail.
I love that Coppermill bridge. It always amuses me to see people ducking to cycle through, it's quite a long way to crouch. I struggle to walk through stooped over so much.
I like the old imperial only low bridge sign on Coborn Road, E3 as there's two bridges - the first has the correct metric/imperial signs, and you can't hit the imperial only bridge without hitting that one first! [photo]
In real life I doubt if anyone actually measures the bridge twice, since an inch is 2.54 cm by definition.
However the actual measurement will be a bit tricky, as the road surface, and the underside of the bridge, will never be exactly horizontal nor exactly planar. Measuring into a dip in the surface could have unfortunate results.

dg writes: Perhaps read the guidance.
@MO The best in the world..? Meaningless exceptionalism I'd say. Mostly British signs, I find, are over large and often very clumsy.
How it should be done: [photo]
Metric signs in a metric land. 2,7 without an M, €, $. etc., etc.

Love this article.
I don't think that MO claimed that UK signs are the best in the world. It was the relative clarity and precision of the document describing them which MO praised.
Steve's German example is a cycle direction sign, and those in the UK are of a similar size.

Signs for road vehicles in both countries are generally larger because speeds are higher than for cycles and so they need to be read from further away.

The difference between the triangular and circular signs mentioned by scrumpy are that the former are advisory and the latter are mandatory.
Steve I can't be doing with that example - different sizes / fonts / parsing of text, use of comma instead of decimal, etc.

I do think UK signage is very high quality and Jock Kinnear / Maggie Calvert did a great job in effectively "designing" the UK's. Comprehensive standards have worked well. We are probably well overdue a bit of an update though.

I'm speculating wildly as to why the sign is a circle (a sign giving an order) when I thought warning signs were supposed to be triangles?!
Malcolm, thank you that was exactly what I was saying. Having worked in various countries as a traffic engineer, I have found that UK Department for Transport advice, whilst not perfect, far exceeds that of anywhere else.
On a fenland road near March there's a level crossing with manual gates and a bypass bridge that you have to take quite a sharp detour down a dip for. It's posted at 7 foot.

If your passenger doesn't know about it, flooring it through the bridge will have them ducking into the footwell of the car ...
The signs for the right hand Dartford tunnel say 5.0m. Following the rules strictly, it should be unsigned. However, common sense has been used, since the left hand tunnel is lower.
It's the same font, but a condensed version, which is necessary in a language notable for its long words. [photo]

Yes, signs for road vehicles are larger, but in the U.K. often over large in city settings. I've nothing against larger on Motorways where traffic is faster.

Cornish Cockney, the circular sign is a regulatory sign, and the triangular sign is a warning sign. So the former says you will comply by law, the latter warns you of a hazard. With regards a height restriction, you can have just warning signs but it is not recommended. Ideally you should have both together (Chapter 4 paragraph 7.12).
The size of sign is dictated by the speed limit.
No point having a tiny sign at 70mph, and unnecessary having a mammoth one at 30mph.

Triangle is a warning. Circle is mandatory, so near me double deck buses go under a 14'0" bridge with a triangle sign. If it was a circle they would have to find another route.
British cycle direction signs are a mess. Some routes are given in miles, some in minutes - sometimes even on the same post. [streetview]

Assuming the 12'9" was calculated according to The Rules, the actual height is somewhere between 13' and 13'2" (3.96 to 4.01m) which would require the metric figure to read either 3.8m or 3.9m. At least nobody has simply converted 12'9" to metres because, if they were insisting on three decimal places, it would be 3.886 metres.
its the official clearance height, its usually rounded down too!
Brilliant article, appreciated all the more given the travel constraints DG is operating under.
I’m with e17Ian about loving Coppermill bridge. Not just for its own sake - it’s also my best route to see the cormorant colony about half a mile up the lane on the 2 islands on reservoir 5, in Walthamstow Wetlands.
I seem to remember that inside the cabs of big lorries they have their official heights on a sign above the windscreen as a reminder of whether to risk it or not.
Perfect article.
Shared this with a colleague and we agreed it reminded us of the formulae for calculating degree classifications.
High vehicles are supposed to have such a sign in their cabs. In my experience most do, but their size, position and prominence varies quite a lot. (I used to be an agency driver, so saw many). In the case of an artic it has to be an adjustable sign, as different trailers may be used
Just up the slope to the right of this underpass was an unlocked entrance to the line. It was a favourite place for children to place copper coins on the rails to see how flattened out they would get by passing trains.
Last year I was on the top deck of a double deck bus when the road ahead was blocked and the driver turned into Chapel Lane. The sign looked too low for the bus so I ran downstairs. An alarm went off in the bus saying “Low bridge”.
Oxford has 2 low bridges, the bus company had to have special low height buses on those routes.
There was warning sign at the bottom of the staircase where the conductor stood, so he could warn a driver by sounding the emergency stop by bell. The same warning sign was in the cab and supplemented by a black or red steering wheel. Despite these measures a number of over height buses had their roofs peeled back like a sardine tin lid, thankfully with no serious injury to passengers.
And you haven't even touched on the Traffic Signs Regulations and General Directions 2016! That is a really meaty tome to get your teeth into, the signs manuals follow on from this.
DG at his best. A classic for the museum of blog posts. An interesting article on a subject I would never have considered
The question still remains in my mind is -how do they measure the height if the bridge and how accurate is it - never mind the rounding down ?
High vehicle drivers should not read this post. Enough bridges are hit already without them thinking there is just a bit more clearance. Even if head height is just sufficient, it is human nature to still duck a little. For some schadenfreude amusement, have a look at or search YouTube for 11 foot 8 bridge. Another, search for Montague Street Bridge South Melbourne.

TridentScan | Privacy Policy