What we actually know

The three words

These are not marketing states. They are a closed vocabulary in the navigation app, pinned by tests, and the rule written above them in the source is that provenance and urgency are never crossed: the sound carries urgency and only the words carry provenance, because a confident chime on an unmeasured guess is the exact lie the product exists not to tell.

Only the not-surveyed call slows down, and nothing about the chime changes with any of the three. The measured line is the only one that carries a date, because a measurement without one is an opinion.

Measured, in the United Kingdom: none

We have not measured the overhead clearance of a single structure in the United Kingdom. Our clearance product reads surveyed point clouds and reports a headroom as a difference between two heights in the same survey on the same day. We hold no such survey of any British bridge, so on every British route that product answers no verdict, and the app says not surveyed. The word measured is in the vocabulary and we have not yet earned the right to use it here.

What we have surveyed ourselves in Britain is road, not height: 54.9 km of Brighton and Hove, driven in one day on 14 August 2026, which is how the roadside capture got started. Great Britain has 580,165 km of road centreline in Ordnance Survey Open Roads. One is not a rounding error in the other. It is roughly one ten-thousandth of it, and saying so is cheaper than being caught implying otherwise.

The way that number stops being zero is knowing where to send a vehicle. We have since run one county through open LiDAR to find out: the Sussex bridge atlas is 676 places a road passes under a structure, of which 104 are confident enough to stand behind. None of them is a measurement, and it does not move the figure above.

Signed: what the map records

Everything else our router knows about a height restriction, it knows because somebody wrote it on the map, usually from a sign. So the honest question is how much of that there is. We measured it on one OpenStreetMap extract of the United Kingdom, including Northern Ireland, snapshot 19 September 2026, using our own open audit tool.

The 11.0% figure is a lower bound, and deliberately so. Its denominator is every bridge way, and a great many bridges cross water, a railway or open ground with no road beneath them to carry a limit at all. A number built to flatter itself would have filtered those out. This one does not, because we would rather quote a floor than a figure we would have to defend.

Where the gaps are

One national percentage hides more than it shows. The same audit grades every tenth of a degree of the country on its own, which is a box about eleven kilometres tall and six or seven wide, and the grades are about the tagging, never about the road. Of the 4,025 cells the extract saw a road in, 8 are well tagged.

Read the pale shading as the answer to the question. 1,932 cells of 4,025 carry no truck-relevant restriction tag at all, which is very nearly half the populated country. A further 2,467 cells hold at least one bridge way and not one of those bridges has a recorded height limit beneath it, and 1,258 of those hold twenty bridge ways or more.

The dark cells are worth as much as the pale ones, and you will have to hunt for them: there are 8 in the whole United Kingdom, single specks at this size. A well tagged cell is a statement about the map, not a promise about the road.

The number moved, and it moved the wrong way

We ran the same tool over the extract behind the United Kingdom map we had built in August, and over the September one. Between the two snapshots the map gained 739 bridge ways and lost 59 recorded height limits beneath them, so the share fell.

Forty-six days and six hundredths of a percentage point. It is a small move and it may well be noise in how the map is drawn rather than anything happening on the ground. It is here because the alternative is a page that only ever reports improvements.

What we cannot tell you

How many low bridges Britain has. Nobody publishes it. There is a list of rail structures that have been struck, there are local authority lists of varying ages, and there are commercial datasets whose counts are their own claim. None of them reconcile against each other, and none of them is an inventory. So this page contains no figure of the form a share of Britain’s low bridges, because we would have to invent the denominator to write one.

Whether a recorded limit is right. Every count above measures whether a tag is present and parseable. It does not measure the bridge. A structure with a height on it may still be wrong, and a structure without one may be perfectly passable.

And absence of a recorded restriction is not evidence that a road was checked. A route returned over an untagged bridge is not a claim that the bridge is high enough. It is a claim that the map records no limit there, which on this data is the ordinary case rather than the exception. Where the graph carries no limit, nothing is surfaced, rather than a plausible guess that would be indistinguishable from a real one.

Check it yourself

Every figure above came from one file and one command. The file is public and the tool is in our repository, so the measurement can be repeated by anyone who disagrees with it.

The audit was run on 20 September 2026. Both the coverage grid and the headline figures are committed to this site’s repository as the tool wrote them, so the picture above is the file rather than a drawing of it.

For comparison, the routing graph in production is cut from an OpenStreetMap snapshot dated 16 September 2026, 20:21 UTC, read from the platform itself when this page was last built, at most ten minutes before you loaded it, from https://api.mapmap.ai/v1/platform. The measured extract above and the graph we route you on are two different files, and this page will not pretend otherwise. The build dates behind the graph are on the status page.