This road sign on Wick Lane has been puzzling me lately, because the metric measurements are ridiculously precise.
12 foot 9 inches is fine, but 3.890m is given to the nearest thousandth of a metre, i.e. to the nearest millimetre, and that's not the way low bridge signs normally work.
I could have done some proper research, but instead I speculated wildly like a man in a comments box. Surely this was incorrect? Aren't all low bridge signs supposed to be to one decimal place, or maybe two? I think I saw a sign like this somewhere else once? Maybe it's an old sign? What if all low bridge signs are actually supposed to be like this? Perhaps the EU are to blame? Isn't this typical of local councils nowadays? Who employs idiots like these? Obviously this can't be right?
And then I got tired of speculating without any evidence and did some proper research by tracking down the Department for Transport's Traffic Signs Manual. All the answers I needed were in Chapter 4, a meaty tome, twelve of whose 109 pages are given over to signs for bridges. Here we go...
The standard minimum clearance over every part of the carriageway of a public road is 16’‑6” (5.03 m). Where the clearance over any part is less than this, signs should be provided.
This is why you've never seen a low bridge sign saying 17 foot something or 6 metres something. In fact you won't have seen a sign saying 5 metres something either, but we'll get to that.
The Regulations require heights on new signs to be shown in both imperial and metric units.
This dual marking is because bridge strikes are a very serious hazard, and the government doesn't want anyone versed in only one of the measurement systems to misjudge.
Imperial and metric heights should be calculated separately.
It may not surprise you that there are very specific rules for this. Let's start with the rules for imperial.
The imperial figure shown on signs to indicate the available headroom should be at least 3 inches less than the measured height to allow a safety margin. If the resulting figure is not a multiple of 3 inches, it should be rounded down to the nearest lower multiple of 3 inches.
The DfT provide two examples to help you get your head round this.
Example 1: measured height 15’‑2”
• subtract 3” to create a safety margin: 14’‑11”
• round down to nearest multiple of 3”
• sign as 14’‑9”
Example 2: measured height 14’‑6”
• subtract 3” to create a safety margin: 14’‑3”
• sign as 14’‑3” (rounding down not required as already a multiple of 3”)
This means if you see an imperial height on a low bridge, the real clearance is three, four or five inches higher than that. For example the road sign in this photo shows 10’‑3”, so the real clearance could be 10’‑6”, 10’‑7” or 10’‑8”.
The highest a low bridge can be, you may remember, is 16’‑5”.
Thus, the maximum headroom that will normally appear on a sign is 16’‑0”.
This is the kind of top fact I like to unearth during research.
Next, the rules for metric measures.
To obtain the metric figure shown on signs, the bridge height should be measured to two decimal places, rounding down to the nearest 0.01 m. The following method is then used to calculate the appropriate signed height:
a) if the second decimal digit is 8 or 9, delete it and sign the bridge with the remaining whole number and the first decimal digit;
b) if the second decimal digit is 7 or less, delete it and reduce the first decimal digit by 1. Sign the bridge with the remaining whole number and first decimal digit, as reduced.
Examples are again provided (and arguably even more necessary).
Example 1: measured height 4.19m
• remove the final 9 (subtract 0.09m)
• sign as 4.1 m
Example 2: measured height 4.17m
• remove the final 7 (subtract 0.07m)
• reduce first decimal digit by 1 (subtract 0.1m)
• sign as 4.0 m
Essentially measurements ending in 8, 9 or 0 round down once, and measurements ending 1, 2, 3, 4, 5, 6 or 7 round down twice.
This means if you see a metric height on a low bridge, the real clearance is at least eight centimetres higher than that, and maybe as much as seventeen.
The maximum headroom that will normally appear on a sign is 4.9 m.
This is why you never see a low bridge sign saying 5 metres something. Even though 5.02m officially counts as a low bridge, it'll always be signed 4.9m instead.
And finally, importantly, this.
The height shown on the sign must be to only one decimal place.
Here we have official confirmation that the sign I saw on Wick Lane was incorrect. It shouldn't say 3.890m, it shouldn't even say 3.89m, it should be rounded down to 3.8m. Whoever made the sign broke the rules, because the rules say three decimal places is not allowed.
With my initial query solved, I was now intrigued by the fact that imperial and metric heights are rounded differently. One might be rounded down a lot and the other a little, depending on what its final digit was, and the two heights displayed on the bridge might no longer be equivalent.
A quick bash with a spreadsheet confirmed that the difference between imperial and metric measures is normally small. Three quarters of the time they're no more than two inches apart, but the difference can occasionally be as much as four.
The greatest difference occurs when a bridge has a clearance of 3.57m (11’‑9”), in which case 3.4m is four inches less than 11’‑6”. The maximum difference in the opposite direction occurs when a bridge has a clearance of 2.88m (9’‑2”), in which case 8’‑9” is just over two inches less than 2.8m. But this is all by the by.
I'll end by analysing this classic low bridge on Coppermill Lane on Walthamstow Marshes.
It says 1.5m, which means the actual metric clearance must be somewhere between 1.58m and 1.67m. But 1.58m is 5’‑2”, which rounds down to 4’‑9” and the sign alongside doesn't say that. Similarly 1.67m is 5’‑6”, which rounds down to 5’‑3” and the sign alongside doesn't say that either. This means the actual clearance must be somewhere between 1.59m and 1.66m, which may be some consolation if you rick your back while cycling underneath it.