91101 Flying Scotsman

The Intercity 225, shown here in the livery of the Flying Scotsman, was so named because its design speed is 225 km/h (140 mph).[1]

Metrication of British transport was part of the United Kingdom metrication program that started in 1965. The program was initiated by the engineering industry and received government approval on condition that its implementation was voluntary on an industry-by-industry basis and that costs were absorbed where they fell. By the time the Metrication Board was set up in 1969, much of the background work, especially standardisation, had been done in many sectors including the transport sector, but there was little for the man in the street to see. Once the metrication program ran out of steam, further metrication of transport has been a political football, first on account of cost, and later when it was perceived to be "Meddling by Brussels (EU)".

As of 2012, the transport industry, apart from the sale of petrol, appears to the man in the street to use imperial units, but those parts that are invisible to the man in the street, such as the manufacture of vehicles and design of the transport infrastructure use metric units.


Historically, British industry opposed metrication on grounds that most of British exports went to countries that used the imperial or customary system of units. By the 1960s, changing trade patterns meant that this was no longer the case, and in 1965 the Federation of British Industry (now the Confederation of British Industry) initiated a change to the metric system.[2] The government agreed on condition that the changeover was voluntary on a sector-by-sector basis, that costs would be absorbed where they fell and that there would be minimal legislation.[3]

Until 1969, when the Metrication Board was set up, the Royal Society and the British Standards Institution took the lead. When the Metrication Board was set up much of the groundwork had been done and metrication of many engineering sectors, including transport, was under way. From the early to mid 1970s, the lack of compulsion slowed the process of metrication down, a draft order was prepared to completed the process, but the Government, which had a very small majority, declined to proceed with the order.[4]

Once Britain joined the European Economic Community (1973), she was obliged to adapt local law to accommodate EEC directives that were in place.[5] One of these directives 71/354/EEC required a harmonisation of units of measure "for economic, public health, public safety or administrative purposes". By the late 1970's, the British metrication program had run out of steam and at Britain's request, Directive 71/354/EEC was superseded by Directive 80/181/EEC which amongst other things, permitted the use of miles, yards, feet, inches and fathoms in the United Kingdom and knots until 1989 (subsequently extended to 1994). As from 1 January 1990, the use of miles, yards, feet and inches was restricted to "Road traffic signs, distance and speed measurement[s]" only. The directive did not apply to areas of transport that were subject to governmental international agreements.

Road transport

Road system

Last chance for high siders - - 1373120

A road sign with a distance marked in imperial units only, but with the height warning in dual units.

Road signs in Great Britain are regulated by Traffic Signs Regulations and General Directions (TSRGD) which specifies the design and the units of measure for the signs.[6] Distance signs are specified with miles or yards as the only allowable units. Height limit, width limit and vehicle length limit signs are required to use feet and inches, but with metres allowed as optional supplementary indicators. Weight limits are expressed in tonnes: the 2011 legislation now correctly requires "t" as the symbol to be used for tonnes on road signs ("18 t" for example). Earlier legislation had also allowed the use of "T" to represent "tonnes" ("7.5 T" for example), so older signs using this notation are also in use. Speed limits are in miles per hour with no units shown on the signs. Advance-warning signs display distances in miles often using the character "m" as an abbreviation (clashing with the SI use of "m" as the symbol for metre.[7] When SI units are used (such as metres on height, width and length restriction signs if the optional metric-measurement is given) the SI symbol "m" is correctly used.

Advance-warning signs for road works and other temporary road obstructions are generally positioned at multiples of 100 metres from the feature to which they refer, with the distances indicated in yards – to the nearest 100 yards (which is within the 10% tolerance allowed) to comply with the TSRGD requirement for yards to be used on such road signs.[8]

In TSRGD 1994 the legislation included the allowance of metric units as "supplementary indications" for many (but not all) height limit warning and prohibition signs. Schedules 16.1 and 16.2 of the TSRGD 2002 catalogue the signs that may display metric units in addition to imperial units: maximum headroom warning signs and height, width and length prohibition signs. On 23 February 2006 the Secretary of State for Transport Alistair Darling said on the BBC Question Time programme that the Government had abandoned its previously long-standing plans to convert the UK's 2 million road signs to metric, due to the cost.[9]

In late 2009 and early 2010, the DfT proposed modifying the legislation to make it mandatory to use dual units signs for height and width limit warning and restriction signs,[10] as it was believed that this would reduce bridge strikes. The analysis noted that "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."[11]

In December 2011, some amendments to legislation resulting from that part of the consultation that dealt with metric signs have been put to Parliament in the shape of the TSRGD Amendments 2011. This came into force late January 2012.[12]

Since the late 1960s, British roads have been designed using metric units.[citation needed] Location marker posts are erected at 100-metre intervals [13] on the hard shoulder giving the distance from a notional reference point in kilometres to enable maintenance workers, emergency services and the like to pinpoint specific points on the motorway. The digits on these posts were barely visible to motorists. This number was also encoded into the emergency phones that could be used by stranded motorists. The advent of the mobile phone meant that the location of motorists could no longer be pinpointed by reference to the emergency telephone that they were using. To enable such motorists to communicate with the emergency services, driver location signs were erected at approximately 500-metre intervals in England during the period 2007 to 2010.[14][15] These signs replicate the distances shown on the smaller location marker posts though no units are shown, but as of 2012 don't appear on Welsh motorways.[16]

Motor vehicles

Motor fuel has been retailed in litres since the 1980s. Fuel consumption is still commonly quoted in miles-per-imperial gallon. Legislation requires that the official fuel economy guide from which advertisers may quote must catalogue "fuel consumption … in [either] litres per 100 kilometres (l/100 km) or kilometres per litre (km/l), and quoted to one decimal place, or, to the extent compatible with the provisions of Council Directive 80/181/EEC … in miles per gallon".[17]

Almost all motor vehicles first used on public roads on or after 1 April 1984 are required to have speedometers fitted which can display speeds in both miles per hour and kilometres per hour (simultaneously or separately).[18][19]

Metric units (kW for power, km/h for speed, kg for weight and cc for engine capacity) are used in legislation relating to driving licences.[20]

Metric units are used in legislation relating to vehicle emissions (grams of CO2 per km), which affects vehicle taxation bands, and entry requirements to low emissions zones.[21]

Rail transport

Tyne and Wear Metro and National Rail speed limits

Speed restrictions in km/h for the Tyne and Wear Metro (hexagonal sign) below mainline speed restriction signs in mph (round signs).

An 1845 Act of Parliament fixed British track gauges at 4 ft 8½ in and Irish track gauges at 5 ft 3 in.[22] The 4 ft 8½ in gauge was the basis of 60 % of the world's railways, but is now expressed as 1,435 mm (including in at least one Rail Safety and Standards Board document[23]) – a decrease of 0.1 mm, but well within the engineering tolerances. The Irish 5 ft 3 in gauge is now referred to as a 1,600 mm gauge – the difference between the metric and imperial values being 0.2 mm, again well within engineering tolerances.

Metric units are used throughout for engineering purposes and rolling stock is designed using metric units as it is required to meet the loading gauge requirements[23] (most of which are specific to Britain).[24] Track distances of most of Britain's rail network are shown in miles and chains, with speed limits in miles per hour, although lineside signs and in-cab computer displays are now metric on routes where the latest 'ERTMS' signalling system has been installed and on the Channel Tunnel Rail Link.[25] Metro and light-rail systems such as the London Underground,[26] Tyne and Wear Metro, and Croydon metro also operate using metric units.


Salcombe Harbour 6 - - 1077324

Depth gauge at Salcombe harbour, calibrated in metres).

The units of "sea measure" used by the British Admiralty in their 1995 manual of seamanship are the international nautical mile (1852 metres), the sea mile (the distance equivalent to one minute of arc measured along the meridian at the latitude of measurement) which is used on large scale Admiralty charts and which varies in length from 1843 metres to 1862 metres dependent on the position on the earth's surface, the cable which is one tenth of the now obsolete British standard nautical mile at 608 feet, the knot as a measure of speed at one nautical mile per hour (1.852 kilometres per hour) and the now obsolete fathom of 6 feet (1.8288 metres).[27] The nautical mile was designed to be equal to corresponding to an change of one minute of arc of latitude. In 1929, the Extraordinary Hydrographic Conference, Monaco defined the international nautical mile as being 1852 m exactly, as opposed to the metric equivalent of of the British nautical mile (1853.18 m).[28]

The role of the imperial system in international affairs is shown in text of the 1948 version of the International Convention for the Safety of Life at Sea[29] Although the English and the French texts are given equal prominence, the conversion factors show that the treaty was drawn up using imperial units which were then converted to metric units - for example the metric equivalent of "10 feet" is quoted as being 3.05 metres in both texts.


In its early days, most of British aviation used imperial units. With the advent of metrication, the industry started using metric units and as of 2012 a significant proportion of the information supplied by the Civil Aviation Authority is in metric units, though a few measurements are still given in imperial units - runway lengths, visibilty, aircraft weights, fuel quantities, pressures and airfield reference temperatures are all quoted in metric units while altitudes are quoted in feet. Distances are quoted in a mixture of nautical miles and kilometres.[30][not in citation given] A publication by the Liaison Group of UK Airport Consultative Committees follows the units of measure used by the CAA apart from runway lengths which are given in feet.[31]


  1. Barnett, Roger (June 1992). British Rail’s InterCity 125 and 225 (Report). Berkeley, California: The University of California Transportation Center. p. 34. UCTC No. 114. Retrieved 19 October 2011. </noinclude>
  2. White Paper on Metrication (1972) - Summary and Conclusions - §42. London: Department of Trade and Industry Consumer and Competition Policy Directorate. 
  3. Hemenway, David (April 1979). Standards systems in Canada, the UK, West Germany and Denmark: An overview. National Bureau of Standards.
  4. Historical Perspectives on Metrication by Jim Humble who was the last Director of the UK Metrication Board.. Retrieved on 23 March 2012.
  5. Error on call to Template:cite web: Parameters url and title must be specified. Library Services. University of Hull (30 September 2011). Retrieved on 22 March 2012.
  6. Statutory Instrument 2002 No. 3113; The Traffic Signs Regulations and General Directions 2002. Controller of HMSO (16 December 2002).
  7. International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), pp. 116, 118, ISBN 92-822-2213-6, 
  8. Traffic Signs Manual. Chapter 8; Traffic Safety Measures and Signs for Road Works and Temporary Situations; Part 1: Design (2nd ed.). London: Design Department for Transport/Highways Agency, Department for Regional Development (Northern Ireland), Transport Scotland and Welsh Assembly Government. 2009 [2006]. pp. 253–255. ISBN 978-0-11-553051-7. Retrieved 26 August 2011. 
  9. "– Question Time, Milton Keynes, 23 February 2006". BBC News. 23 February 2006. Retrieved 2 August 2011. 
  10. Traffic Signs (Amendment) Regulations and General Directions (TSRGD) 2010. Department for Transport. Retrieved on 4 October 2010.[dead link]
  11. Impact Assessment of the Traffic Signs (Amendment) Regulations and General Directions 2010 and of the Traffic Signs (Temporary Obstructions) (Amendment) Regulations 2010.. Department for Transport. Retrieved on 8 December 2009. (Paragraph 55 of the Impact Analysis)
  12. The Traffic Signs (Amendment) (No. 2) Regulations and General Directions 2011. Department for Transport (31 January 2012). Retrieved on 1 March 2012.
  13. Hansard. 21 Oct 2009 : Column 1446W. Retrieved on 4 November 2009.
  14. Highway Agency. Driver Location Signs (Driver Location Signs). Retrieved on 7 June 2009.
  15. Project Support Officer (Name blacked out) (1 February 2010). Freedom of Information Response reference HAIL 8870045. Highways Agency. Retrieved on 6 June 2011.
  16. Interim Advice Notes Status in Wales. Welsh Government (19 September 2011). Retrieved on 8 May 2012.
  17. The Passenger Car (Fuel Consumption and CO2 Emissions Information) Regulations 2001. Vehicle Certification Agency (30 October 2001). Retrieved on 22 October 2011.
  18. The Road Vehicles (Construction and Use) Regulations 1986: Section 35. UK Crown. Retrieved on 6 June 2011.
  19. Council Directive 75/443/EEC of 26 June 1975 on the approximation of the laws of the Member States relating to the reverse and speedometer equipment of motor vehicles. Council of the European Communities. Retrieved on 6 June 2011.
  20. Motorcycle licence requirements. DirectGov – Public services all in one place. Retrieved on 25 August 2011.
  21. How to Tax your Vehicle. Owning a vehicle. DVLA. Retrieved on 20 February 2012.
  22. An Act for regulating the Gauge of Railways (18 October 1846). Retrieved on 26 April 2010.
  23. 23.0 23.1 Guidance on Gauging. Rail Safety and Standards Board Limited (3 October 2009). Retrieved on 26 April 2010.
  24. The exception is HS1, which uses the continental gauge – as will HS2
  26. Line facts. Transport for London. Retrieved on 26 April 2010.
  27. Admiralty Manual of Seamanship. HMSO. 1995. ISBN 0-11-772696-6. 
  28. International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 128, ISBN 92-822-2213-6, 
  29. International Convention for the Safety of Life at Sea, 1948. Government of the United Kingdom of Great Britain and Northern Ireland. Retrieved on 23 March 2012.
  30. "AIRAC 5/2012". United Kingdom Aeronautical Information Publication. Effective date: 3 May 2012 (Civil Aviation Authority/NATS). 22 March 2012. Retrieved 6 May 2012. 
  31. Airports Data. Liaison Group of UK Airport Consultative Committees (2 May 2012). Retrieved on 6 May 2012.
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