Vehicle restriction signs

Vehicle restriction signs

METRICATION: 20% complete
Metric Non-metric
Private sector signs use metric-only Imperial-only signs still in use
Metric units compulsory on all new signs Imperial units compulsory on all new signs
Road sign specification in millimetres In-cab height displays are imperial-only

Vehicle restriction signs indicate the maximum height, width or length of a road vehicle on a route. Any vehicle exceeding the indicated value is prohibited.

On public roads, vehicle restriction signs are important for safety. Incidents where road vehicles strike low bridges present a serious hazard to both rail and road users. A significant number of such incidents occur each year. It takes only a relatively small amount of force to displace bridge girders sufficiently to derail a train. The damage done can also take months to repair and incur significant cost. It is therefore important that the signs and markings on low bridges are correctly installed and maintained to a high standard.

In the private sector, height restriction signs are also important for safety in petrol stations, car parks, delivery bays and other entrances.

Petrol stations

Maximum height limits at petrol stations are indicated in metric only. Generally, such indications are not as prominent as signs on public roads. However, where such locations are likely to encounter a significant number of high vehicles, such as at motorway service stations, it is prudent for the signage to be more prominent.

Car parks and delivery bays

Maximum height limits at many car parks and delivery bays have been indicated in metric-only since the 1980s.

Public roads

The Traffic Signs Regulations and General Directions (TSRGD) governs the use of vehicle restriction signs on public roads.

Since 2016, the TSRGD has required both metric and imperial units to be used on all new height, width and length restriction signs. Prior to 2016, all restriction signs were required to use imperial units, with metric units permitted as supplementary units. However, existing imperial-only signs, installed before 22 April 2016, are allowed to remain in place until they reach the end of their useful life or become damaged.

On UK public roads, there exists a plethora of different signs all of which serve the purpose of indicating either a width, height or length restriction. New signs continue to be added to the list. As recently as 2012, two new signs were introduced; a dual unit height restriction triangular warning sign, and a text-based red panel to indicate reduced headroom.

The gallery above includes diagrams of some signs that are no longer prescribed. However, examples of each of these signs can still be found on public roads in the UK.

The end of imperial-only restriction signs

Since 1970, the Department for Transport has resisted the need to metricate road signs, including vehicle restriction signs. Successive UK governments have also avoided tackling the issue.

Nonetheless, in 1994, the option to show vehicle restrictions in metric units, in addition to mandatory imperial units, was finally introduced into the Traffic Signs Regulations and General Directions. The reason for this may have been more to do with addressing the safety issue of foreign drivers not understanding imperial signs rather than any progressive step towards metrication.

From at least 2004, official advice had been that dual unit signs should be used in preference to imperial-only ones, but only on main roads likely to be used by foreign drivers.

“Metric heights may be shown in addition to imperial heights at any bridge. This is recommended for all bridges on main routes and on roads used frequently by foreign drivers.”

Traffic Signs Manual – Chapter 4, 2004

However, for many highway authorities the recommendations were not persuasive enough for them to put an end to the use of imperial-only restriction signs, even at roadworks on motorways, or at many bridges on A-roads.

Imperial-only width restrictions on the M6 – 2008

In 2007, a report commissioned by the SPARKS Programme estimated that over 3 million foreign registered vehicles were entering the UK each year. The report coincided with a general increase in the number of over-height vehicles striking bridges.

Together with the decreasing familiarity of imperial units among British drivers, there was also the fact that drivers of foreign registered vehicles could not be expected to understand vehicle restriction signs in imperial units, as all other countries on our continent use metric.

Given the widespread successful use of metric-only signs at garage forecourts, car parks and other sites, the obvious solution to the safety issue posed by poorly understood imperial restriction signs on UK roads, should have been to replace all remaining imperial signs with international standard metric-only signs as soon as practicable. However, the Department of Transport chose not to do this, but to continue to move towards full adoption of dual units instead.

However, this approach would not only prove to be less effective than switching to metric-only signs once and for all, but will also be more expensive in the long run – for the obvious reason that all new dual unit signs themselves will eventually have to be replaced again, with metric-only signs.

In September 2009, as part of a three-year review of the Traffic Signs Regulations and General Directions (TSRGD), the Department for Transport stated its intention to require both metric and imperial units to be shown on all width and height restriction signs:

“We are making changes to require both metric and imperial triangular warning signs to be displayed to give warnings of restricted headroom, with the upgrade being complete in four years’ time. Using the imperial sign on its own will no longer be permitted.

We are making similar changes to require both metric and imperial measurements to be displayed on all width and height restriction roundel signs, with the upgrade being complete in four years’ time. The current imperial-only signs shown in diagrams 629 and 629.2 will be withdrawn.”



“… approximately 10 – 12% of bridge strikes involved foreign lorries. This is disproportionately high in terms of the number of foreign lorries on the road network.

… Furthermore, for several years this Department has recommended, through the Traffic Signs Manual, the use of the dual unit height limit warning and regulatory signing in preference to the imperial only alternative.”

Traffic Signs (Amendment) Regulations and General Directions 2010 – consultation

Evidence was presented in the form of an impact assessment that projected a reduced incidence of bridge strikes following the requirement for metric height indications on all height restriction signs. The impact assessment showed that the costs of replacing all existing signs over a 4 year period would be outweighed, by a factor of more than four, by the savings made from the associated reduction in overheight roof bridge strikes.

Over a ten year period it was estimated that £1.8 million would be saved.

If the changes had gone ahead, this would have led to the mandatory use of dual units on all new width and height restriction signs installed from April 2010 onwards, with a proposed cut off date of 2014 for the replacement of all remaining imperial-only restriction signs. However, in spite of the projected reduction in bridge strikes, and the injuries associated with such incidents, the decision to abandon the use of imperial-only restriction signs was reversed by the Secretary of State for Transport, Philip Hammond, in 2010.

From 30 January 2012, a new dual unit variant of the maximum headroom warning triangle sign was introduced. This was designed as an alternative to the use of two single-unit signs side-by-side.


The de-authorisation of imperial-only restriction signs finally took place with the 2016 edition of the Traffic Signs Regulations and General Directions. No cut off date was set for remaining imperial-only restriction signs, but all new signs installed since April 2016 have been required to show restrictions in both metric and imperial.

In addition to the withdrawal of the imperial width and height restriction roundel signs, the TSRGD 2016 also saw the withdrawal of the two single unit variants of the maximum headroom warning triangle sign – both the imperial and metric variants were withdrawn.


530 metric

In-cab vehicle height indicators

For safety purposes, vehicles over a certain height (measured in metres) are required by law to carry a display, viewable from the driver’s seating position, of the vehicle’s overall travelling height (in feet and inches).

The Road Vehicles (Construction and Use) Regulations 1986 states:

“No person shall use or cause or permit to be used on a road a vehicle to which this regulation applies if the overall travelling height exceeds 3.66 m unless there is carried in the vehicle … a notice clearly indicating in feet and inches and in figures not less than 40 mm tall, the overall travelling height.”


In-cab vehicle height indicators in feet and inches are no longer fit for purpose. They cannot be used at petrol stations, motorway services, warehouses, or the many other sites where height restrictions are in metric. They are also of no value when driving outside the UK.

Since the majority of all height restriction signs on public roads now use dual units, it follows that practically all height restriction signs across all locations now include a metric height indication. There is therefore no good reason why in-cab height indicators should not now be metric-only.

Dual unit signs

Imperial Dual units Metric

The use of dual units in a metrication programme can be useful during a transition period, but experience shows that their use is best kept to the shortest time possible. This is because they reduce clarity by adding clutter, and can cause confusion. Also, contrary to their intended effect, they can actually discourage the switch to metric by allowing the old units to continue to be used.

Restriction signs need to be clear and concise in order for them to be seen and understood quickly without unduly distracting a driver’s attention from the road ahead. It is therefore undesirable for imperial units to be allowed to linger in the form of dual unit signs for a prolonged period. However, this is precisely what has happened in the UK.

The option for dual unit vehicle restriction signs was first introduced into the TSRGD in 1994, and has only been mandatory on all new restriction signs since 2016. This means that the transition period has now lasted for nearly 30 years, with no end in sight. Temporary inconveniences have become long term issues.

Dual unit height restriction signs

Imperial Dual units Metric

The metric and imperial values shown on a dual unit height restriction sign are each calculated separately according to their own rules. One is not a conversion of the other. The Department for Transport’s Traffic Signs Manual – Chapter 4 describes these rules. Both values include safety margins:

Imperial height
To obtain the imperial figure shown on signs, the bridge height is measured in feet and inches, rounding down to the nearest inch.

  • 3 inches is subtracted from the measured value.
  • The remaining value is then rounded down to the nearest multiple of 3 inches.

The bridge is signed with the rounded down value in feet and inches.

Metric height
To obtain the metric figure shown on signs, the bridge height is measured in metres to two decimal places, rounding down to the nearest 0.01 m.

  • If the second decimal digit is 8 or 9, it is deleted.
  • If the second decimal digit is 7 or less, it is deleted and the remaining number is reduced by 0.1.

The bridge is signed with the remaining whole number and the first decimal digit.

If dual units are used for trade, the metric unit is the legal unit, and the imperial unit acts only as an optional supplementary indication. However, the TSRGD makes no such distinction when it comes to dual unit restriction signs. It must therefore be assumed that the imperial restriction is as legally valid as the metric restriction.

Having two different, but equally valid, restrictions on one sign creates legal ambiguity. It is not clear whether a vehicle must comply with both values, or whether compliance with one value is sufficient.

This issue is not insignificant. The difference between the metric and imperial values on the same sign can be greater than 8 cm. A vehicle might appear to be compliant with a dual unit sign when a driver uses an imperial tape measure, but not when a metric tape measure is used.

For example, in the case of a bridge with a measured height of 3.97 m, the two values on the resultant sign differ by 8.6 cm.

  • A measured height of 3.97 m would be signed as 3.8 m in metric.
  • The same height measured in imperial would be 13′ 0 3⁄10″, which would be signed as 12′-9″ (equal to 3.886 m).
Example of the legal ambiguity of dual unit height restrictions
Vehicle height
measured in imperial
Compliant with
metric restriction
Compliant with
imperial restriction
12′ 10″
3.91 m
12′ 9″
3.89 m
12′ 8″
3.86 m
12′ 7″
3.84 m
12′ 6″
3.81 m
12′ 5″
3.78 m
Any vehicle with a measured imperial height of 12′ 6″, 12′ 7″, 12′ 8″ or 12′ 9″ would have a height greater than the 3.8 m maximum height limit, but would still not exceed the imperial maximum height of 12′ 9″.

This 3.8 m 12′ 9″ sign would prohibit a vehicle with a measured imperial height of 12′ 6″

From a logical point of view, if one value on a dual unit sign prohibits a vehicle, then surely the vehicle is prohibited.

Therefore, to be completely sure that their vehicle complies with a dual unit restriction sign, a driver needs to know which is the lowest of the two values on the sign. This can only be done by converting one of the values into the same units as the other value, and then comparing the lowest of the two values with the vehicle’s measured height. Of course, this is a ludicrous and impractical thing to expect a driver to do while driving.

The potential for confusion will always be present when dual units are used. Situations resulting in legal ambiguity can be tolerated for a short time during a transition to metric, but in the case of restriction signs, the transition phase has already lasted for decades.

The end of dual unit restriction signs

All of the issues associated with dual unit signs can be ended simply by switching to the use of a single measurement unit as soon as practicable.

Reverting to imperial-only would make no sense, as that would worsen the two-systems muddle that already exists, where the majority of locations not on public roads use metric. Switching to metric-only on the other hand would ensure the use of a single system for all purposes and locations. Metric also has the obvious advantage that it is the standard system used internationally, and would be universally understood by all drivers on the UK’s roads.

Vienna Convention on Road Signs and Signals 1968

The UK is a signatory country to the Vienna Convention on Road Signs and Signals 1968. The contracting parties to this treaty agreed upon its provisions:

“… recognizing that international uniformity of road signs, signals and symbols and of road markings is necessary in order to facilitate international road traffic and to increase road safety”

For height, width and length restrictions, the protocol prescribes 3 signs only. Each sign is a standard red roundel. The only measurement unit prescribed is the metre. Imperial units are not authorised.

Vehicle restriction signs compliant with the Vienna Convention

Road signs compliant with the Vienna Convention protocol can still be seen in some places on public roads. Their design dates back to the 1980s. However, apart from the metric length restriction sign (which can only be used with an accompanying imperial sign), they are currently prohibited from use on new signs.

Text-based restriction signs

Despite the plethora of prohibitory roundels and triangular warning signs available to choose from, highway authorities sometimes install signs for width, height or length restrictions using a sign that is purely text-based.

In September 2008, the then Transport Minister, Rosie Winterton, announced “the biggest review of British road signs for 40 years”, which she claimed would “consider all aspects of road signing”, including safety, efficiency, cost, the environment and the “street clutter” caused by unnecessarily large and complex signs.

This presented a golden opportunity for a major overhaul of the UK’s road signs, including the long overdue switch to the exclusive use of metric units. Many of the review’s goals could only be fully achieved by the metrication of all road signs. Dual unit restriction signs were an obvious target for reducing clutter, by upgrading to standard metric-only signs.

The review lasted for three years, but opportunities were squandered, and little came of it. Despite lobbying by advocates of metrication, the switch to metric units was ruled out at an early stage.

One easy win should have been to end the use of all language-dependent text-based signs, where standard roundels, or triangular warning signs, could be used instead. This would have improved the signing of vehicle restrictions.

But, rather than end the use of text-based signs for vehicle restrictions, the review actually led to the introduction of another one. On 30 January 2012, a new red text-based sign for reduced headroom became available to highway authorities, in addition to the other options already in the TSRGD.

TSRGD – Diagram 7014.1 – Temporary or permanent reduction in bridge headroom ahead

Metrication will be an opportunity to revise the traffic sign regulations to ensure that every vehicle restriction sign makes use of a standard roundel that is compliant with the Vienna Convention protocol.

Sign specifications

In common with other road signs, the dimensions of vehicle restriction signs are specified in millimetres.

TSRGD – Diagram 629.2A – Vehicles exceeding height indicated prohibited


Clearly, the issues resulting from their use mean that dual unit restriction signs should not be considered to be a permanent state of affairs. To resolve the situation on public roads, a short programme of legislation is required to complete the process of metrication that has already begun.

Meanwhile, private sector locations can avoid the issues arising from dual units by continuing to use metric-only restriction signs.

The following procedure can be used to switch all vehicle restriction signs on public roads to metric-only, at little cost to the tax payer:

  1. Over a time-limited period of one or two years, all remaining imperial-only restrictions would be replaced with dual unit signs.
  2. At the end of this period, the Traffic Signs Regulations and General Directions (TSRGD) would be amended to require all new restriction signs to be metric-only.
  3. The Road Vehicles (Construction and Use) Regulations would also be amended to require in-cab height indications to be in metric-only. To increase safety, the qualifying vehicle height could also be reduced from 3.66 m to 3.5 m.


Metrication will present an ideal opportunity to rationalise vehicle restriction signs in general:

  • Vehicle restrictions signs that rely on text, rather than standard pictograms, can only be understood in the language that they are written. Signs, such as those describing a route as “Unsuitable for long vehicles”, are both vague and unnecessary given the existence of standard internationally recognised vehicle restriction roundels. All text-based vehicle restriction signs should be discontinued.
  • The current distinction that is made between triangular signs warning of “maximum headroom”, and roundel signs that prohibit vehicles exceeding a “maximum height”, is confusing and not easily understood. Internationally, the use of one sign type is not only accepted as sufficient, but also considered to be desirable. The use of triangular warning signs for height restrictions should be ended.
  • Currently, on signs giving an advance warning of a vehicle restriction, distances can be shown on optional plates in two different units; fractions of a mile, or yards. 50 years after education went metric, drivers cannot be expected to know if 800 yards is greater or less than ½ mile. Showing all short exclusively in metres will bring clarity, increase understanding, and further reduce the current plethora of restriction sign variants encountered by drivers.

All of the above improvements will help ensure that a single, easy-to-understand, standard format is used for all vehicle restriction signs.

5 measurement units – inches, feet, yards, miles, metres
1 measurement unit

Cost savings

There is no need to wait for every remaining imperial-only restriction sign to be replaced with a dual unit sign before beginning the nationwide switch to metric-only restriction signs. On the contrary, the total eventual cost of the metrication programme can be reduced by eliminating the costs that would be incurred if some signs were to be replaced twice.

The amount saved will be greatest if full metrication begins without delay.

Costs can also be reduced if the switch to metric restriction signs takes place at the same time as a national programme to switch all distance signs to metric units. This is because vehicle restriction signs are used not only as standalone signs at the sites of hazards like bridges, but also in the form of advance warning signs with plates giving distance information.

It will improve the efficiency of the overall metrication process if all vehicle restriction signs can be upgraded at the same time as their distance plates.

Road works

The full metrication of road signs will present an opportunity to rationalise and de-clutter road signs in general.

3 measurement units 4 measurement units 1 measurement unit

For example, on the temporary restriction sign above, metrication will enable all of the following improvements:

  • A single measurement unit for all dimensional information will improve understanding.
  • The use of the same symbol, m, for both the restriction dimension and the distance to the restriction will improve clarity.
  • The use of the “m” symbol, instead of a language-dependent unit name, for distance will improve understanding.
  • Improving the layout of the direction arrows will clarify which traffic lane passes through the restriction.
  • Removing superfluous and language-dependent text, such as “ANY VEH”, will improve clarity and de-clutter the sign.


Action required by : Government

  • Vehicle restriction signs – Legislation is required to upgrade all current dual-unit vehicle restriction signs to metric-only signs. This could be carried out without any further delay. However, if restriction signs are upgraded as part of a general metrication programme for all official road signs, savings could be made by replacing all associated distance plates at the same time.



A bridge with a measured height of 3.97 m would have a dual unit maximum height restriction of 3.8 m 12′-9″.
This sign would prohibit a vehicle with a measured imperial height of 12′-6″.