Height Restrictions and the Correct Approach

Last month I was invited to provide a presentation to the IHE Traffic Signs Panel once more, and this year I focused on a problem that causes numerous difficulties for both managers of highway networks and the railway system alike.

There are an estimated 1,800 bridge strikes a year on the network which on average is a staggering five a day; and the cost of each one is at least £13,000 provided that the structures in question don’t require massive attention for repairs. Considering a correct system of signs can cost as little as £1,000 the return on investment is high so there truly is no excuse for the poor signing examples that litter the network nationally.

That said, welcome to the tedious world of height restriction signing. First off, let’s run through the basics:

What is a low bridge/structure?

A quick glance at Traffic Signs Manual Ch. 4 – 7.3.2 & 7.3.3 describes one as any bridge or structure over the carriageway where the headroom is less than 16 feet, 6 inches or 5.03 metres.

There is no requirement to sign structures that exceed this minimum height but some abnormal load routes do so anyway as a navigational aid. It is possible to see signs with heights over 17 feet indicated as a result.

Who is responsible for height restriction signs?

The Highway Authority is responsible for provision of signing as per Section 122 of the Road Traffic Regulation Act 1984. However, the bridge or structure owner must make allowances for this (e.g. power supply for illumination where needed) and ensure that maintenance is unimpeded.

The Highway Authority has several ways to tackle height restrictions, from advisory warning signs to mandatory restrictions backed up by Section 36 of the Road Traffic Act. Originally in 1964 (let’s ignore the pre-Worboys era for now), the only option was to use warning signs – it surprisingly took until 1975 before Traffic Regulation Orders could be applied without Secretary of State approval for low bridges. In 1994, the requirement for TROs was dropped and a rapid conversion policy from warning to regulatory signs was recommended so that the police could take advantage of the new Section 36 powers.

1964 and 1975 extracts from TSRGD – note that it was not until 1981 that a separate metric units roundel was permitted, albeit with site specific approval.

How should these restrictions be signed?

I won’t repeat the guidance in Chapters 3 and 4, but I will direct you to the relevant page on the DfT website; it’s very useful (even for a lay person who may be querying poor signage!):

Department for Transport – Traffic Signs Manual

That said, the obvious answer is as clearly as possible, but this seems to cause some difficulties for less capable sign designers. A simple checklist is provided below that I will expand on in more detail further along;

  1. Determine which type of structure to deal with – is it a beam bridge, arch bridge, overhanging building, or low trees? There are different rules for each kind of obstruction in terms of signing – prevention of strikes.
  2. Ensure all signs are present, correct, and well maintained – mitigation against strikes.
  3. Assess the structure and ensure visibility is simple. Place regulatory signs at the last suitable ‘escape’ point so a high vehicle can turn off the road – prevention of and mitigation against strikes.
  4. Assess the route which has the restriction and determine the suitability alternative routes for high vehicles – prevention of strikes.

Arch bridges are a unique case as the height restriction varies dependent on where the vehicle is – but simply put you need to use the specific sign for arch bridges on approaches and standard triangles on the structure itself. Regulatory signs are not permitted. Any other structure, with the exception of trees, should use regulatory signs.

Assessing routes is a relatively simple task but not one that is frequently undertaken. In my local authority days I was tasked twice with sorting out freight routes, one in hilly Calderdale which is notorious for being full of unsuitable quasi-mountain roads and the only major roads often are obstructed by low bridges. The aim here was to ensure HGVs stayed on the nearby M62 until it was no longer practical; especially given the main routes into Brighouse were hemmed in by low bridges. This resulted in a huge replacement of advance direction signs to discourage blind Sat-Nav compliance, which really should be the bane of any network manager. The second was in Blackburn, which again has two railway lines pass through the town but virtually every structure on these lines is substandard in height meaning of the numerous radial routes into the town, only two or three are actually suitable. The recent completion of the A678 and inner ring roads should enable a much better routing of traffic avoiding the A666 which has three of the borough’s most hit structures on it. At the time I was asked to write a report these rotues were still ‘may happens’, so unfortunately most of the work I put into a large report was rendered somewhat obsolete rather quickly.

The best way to create a workable strategy is to get a 1:50,000 scale map of the area you’re interested in, mark the low bridges on it, and then produce a series of lines to avoid it. This then will clearly show you where you need signing improvements as well. Once this is done the task of improving signs at the bridge itself becomes less desperate and you can avoid the need for costly detection systems and turning facilities because you’ve correctly managed the network.

Please avoid the current fashion for decorating bridges with giant signs that do not actually give relevant information.


Despite the low bridge text, this bridge still got hit. Probably because the relevant information – the height limit – was drowned by the needless decorations surrounding it.

This one, on the A666 in Blackburn, is particularly infuriating. The legally relevant information is once again obscured by a needless panel that was needlessly enlarged the same night I was presenting to the IHE about the pointlessness of it all.

The importance of sign cleaning, good illumination, and positioning cannot be overstated. Maintenance is costly so doing things right in the first place should save money – for example making sure tree canopies don’t obscure the signs or cause moss growth, lighting units should ideally be easy to access and not reliant on cables being strung along the bridge structure – if solar units are possible then seriously consider them.

Getting these things right first time is often the key to ensuring no problems further down the line, as rectifying errors is costly especially if the result is a damaged bridge as a result of someone hitting it! As ever if in doubt there are always professionals that can provide advice – the IHE forum on Traffic Signs & Traffic Management is a good place for a formal discussion. Alternatively some of us can be found on Twitter and elsewhere!

5 thoughts on “Height Restrictions and the Correct Approach

  1. Good article, nice one.

    Makes me wonder exactly who at Network Rail thinks these extra decorations are a good idea – and what proof they have that they are effective. Said proof being non-existent of course.

    Get the prescribed signing right in the first place should always be the first thing done. But that is too simple and not being innovative enough – as is the ridiculous fashion these days.

    The prescribed roundels are available in four sizes, 750, 900, 1200 and 1500mm diameter. If NR are concerned about bridge strikes, why aren’t they asking the highway authorities to install 1500s even if TSM says something smaller is appropriate for a given route? The larger text will give approaching drivers a longer reading distance and thus a bit more chance to stop before they hit the bridge.


    1. A lot of evidence of insular thinking and appeals to false authority in this comment and the post body. Five bridge strikes per month, let alone per day, should be sufficient proof that the current regime is seriously broken. It is not possible to ‘get the prescribed signing right in the first place’ or know how much a ‘correct system of signs can cost’ if such a thing is not available in TSRGD.

      I have no idea whether the overwhelming ‘decoration’ is down to Network Rail or not. Presumably highway authorities could order its removal if they were able to show it was making the situation worse? But I can appreciate that after they had wasted a couple of decades begging another department in the same ministry to replace confusing imperial—according to the testimony of motorists who’d crashed into their bridges—with metric and then just getting doubly-confusing signs (additionally larger with different mounting points) instead, there would be an expectation of bad faith. Layering one set of kludges over another might well be the British way, but it’s vanishingly unlikely to fix the underlying problems.

      It seems to me as an amateur, based on no evidence whatsoever, that putting regulatory signs, however large, several metres above ground on a non-motorway where users are not looking is really quite monumentally silly. Likewise overloading already cluttered direction signage with height merely-warnings at the escape point and not physically preventing the wrong path under an arch.


  2. Placement of the decorative nonsense is at the discretion of Network Rail – they own the bridge and they can do what they like as the railway trumps road in the hierarchy.

    However, yes – signs definitely should be at eye level rather than 4 metres above ground where practical. They do this in France and seem to cope.


  3. As a HGV driver, I can tell you that the reason drivers hit bridges, is not because the actual height sign is obscured or drowned out by over the top colourfull markings. It is simply because the driver did not do his job. Check your vehicle height, and check your route for height restrictions (and weight limits etc ). A Shell Truckers Atlas, which has all height restrictions marked, is about £10, and responsible drivers have one with them. It’s the ‘point and go’ drivers, of which we seem to get more and more, that dont take any notice.
    What we drivers do need, is beter advance signage (there is no point siting a sign AFTER the junction/last option to turn around) and if possible, signed diversions suitable for HGVs. I can find the low bridge on the map, but it is not always obvious which is the best, and shortest, way around it.


    1. With the least respect, that is quite possibly the most classically British muddle imaginable 🥴. Such a tightly packed bundle of confusion that it’s difficult to know where to start unpicking it:
      •A typical [nearly vertical] HGV front window has a significant blind-spot caused by the roof, meaning regulatory signs on the bridge parapet must go out-of-sight even sooner than it would in a motor car—to which height limits are rarely pertinent—and renders the decisions to put and keep it up there even more dangerously bizarre. The Tarquins with their masters degrees in humanities strike again; pun intended…
      •Blaming the individual end-user for systemic failings of inept and clueless custody and oversight of signage. Forelock-tugging madness. Does give some insight into why British motorists are so pathologically baffled by the success of principles like Sustainable Safety elsewhere, though.
      •Makes one wonder exactly why we are paying what must amount to tens of millions of pounds every year for the DFT traffic signs office and further tens (hundreds?) of millions through the highway authorities when their combined efforts can’t even reliably impart safety-critical information such as height limits to those who truly need it. Going back to the situation after cyclists had invented the concept of warning signs but before motor-centric highway bodgers had become reliant on the crutch of prohibition signs would save us a fortune.
      •Those operators you [xenophobically?] like to denigrate as ‘point and go’ are the product of your own industry and reflect very badly on its customary practices, economic imperatives and working conditions. Get your own house in order before the matter is taken out of your hands and a solution imposed—you’ll like that even less! Parenthetically, I note that UK railways have genuinely robust procedures—i.e. not sign-make-it-better—to prevent out-of-gauge loads and I’m pondering whether the highways and road haulage industries in other countries engender comparable levels of bridge crashes? Bryn, did your presentation look at this?
      •If an ‘atlas (~£10)’ and adequate preparation allegedly suffices for ‘responsible’ operators (presumably those not crashing into 5 bridges per day?) then they wouldn’t need any advance signage at all—yet you’re calling for not only that but to be nannied with ‘signed diversions suitable for HGVs’ and ‘shortest’ route planning—an effectively impossible ask without knowing your ultimate destination, any intermediate stops and, indeed, vehicle height in each case. Yet more eye-watering expense to be borne by the public but whose benefit accrues almost entirely to a small minority, another motoring subsidy. As you rightly say, your irresponsible colleagues don’t take any notice even of height limit signs so it is laughable to claim they would even register such information signs and we’d still suffer the current rate of bridge crashes.


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