Metrication of British transport

In the British railway industry metric units and distances are used on many new systems while most pre-existing systems have retained imperial units, especially for speed and distance. For the roads, fuel is sold by the litre with fuel consumption still generally being given as miles per gallon.
Background
By the late 1960s the British government agreed to facilitate 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.
In some cases it was appropriate to use hard metrication and in other cases soft metrication - soft metrication being defined as "the conversion of ... non-SI measurements to equivalent SI units, within the established measurement tolerances. In general, 'soft metric' products will not differ physically from analogous products.
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, so a draft order was prepared to complete the process. The Government, which had a very small majority, chose not to proceed with the order.
When Britain joined the European Economic Community in 1973, it was obliged to adapt local law to accommodate EEC directives that were in place. By the late 1970s the British metrication programme had run out of steam and, at Britain's request, Directive 71/354/EEC was superseded by Directive 80/181/EEC, which among other things permitted the use of miles, yards, feet, inches and fathoms in the United Kingdom, and knots until 1989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" only. The directive did not apply to areas of transport that were subject to governmental international agreements.
Road transport
Motor fuel has been retailed in litres in the UK since the 1980s and fuel consumption is still commonly quoted in miles per gallon.
Rail transport
The main push towards using the metric system took place during the early 1970s.
Railway operations
Changes in operational procedures have not always been synchronised with changes in technology - for example, in 2010 it was reported that drivers of freight trains operating on single track in the West Highlands had to report the lengths of their trains verbally to the signalling staff using feet even though the computer generated information available to the driver was in metres.
Pan-European signalling systems
The European Rail Traffic Management System (ERTMS) is an initiative backed by the European Union to enhance cross-border interoperability and the procurement of signalling equipment by creating a single Europe-wide standard for train control and command systems. Its main components are European Train Control System (ETCS) and GSM-R communications system. ETCS is a standard for track-train radio communications using balises (Eurobalises) and associated in-cab train control while GSM-R is the GSM mobile communications standard for railway operations. ERTMS can operate at different levels depending on specific local requirements. Under ERTMS speeds are displayed in the driver's cab in km/h and at Level 2, lineside speed indicators are optional. requiring the use of ERTMS on High Speed (TEN-R)and Conventional Trans-European Railway Network (TEN) routes. The response proposed a roll-out plan of ERTMS equipment on existing lines that would be completed by 2044 though the actual timing of the programme will depend on changing circumstances - new trains will be ordered with ERTMS equipment on board and ERTMS would be installed during any electrification programs.
In 2009, the Rail Safety and Standards Board confirmed that km/h would be used on ERTMS lines in the United Kingdom.
With the ongoing introduction of ERTMS, it is anticipated that the metrication of British rail transport will be completed over the next few decades.
A standard feature of the speedometers used by ERTMS/ETCS systems is the use of the metric system.
At a Railway Conference in 2002, it was argued that a changeover to using metric units for speed in advance of the introduction of ERTMS was unlikely to be financially viable unless the decision is taken to adopt Level 2 ERTMS without lineside signalling. The need to handle both mph and km/h would continue though.
A 2010 voluntary standards document published by the Rail Safety and Standards Board addressed this issue when it recommended that the speedometer of a ETCS system be designed so that it switches automatically between mph and km/h depending on the route being traversed. The speedometer would display "mph" when the speedometer was displaying "miles per hour", otherwise would display nothing. Its graduations would be chosen such that the angle of the needle would not change when the system switched from one scale to the other. The conversion between metric and imperial units would be a function of the speedometer, not of any other on-board equipment. In 2012 a technical specification matching this proposal was published.
The Cambrian Line, a low volume rail link between Shrewsbury in the east and Aberystwyth and Pwllheli in the west, was chosen as Britain's trial ERTMS line. This line was selected as its signalling system had reached the end of its useful life, and because it is a low capacity line almost separate from the national network, making it an ideal site on which to gain ERTMS experience. All speeds on the Cambrian Line are shown in km/h in the Western region Sectional Appendix, although distances are still given in miles and chains.
Installation of ERTMS on mainline routes has suffered significant delays. In July 2017 the Department for Transport cancelled the plans for the Great Western Route which included ERTMS as part of the electrification and resignalling work
and was expected to reach Oxford and Newbury by 2016 and Cardiff by 2017.Other mainline conversions to the ERTMS standards were expected to include while the specification for the proposed High Speed 2 (HS2) link from London to Manchester, Leeds and beyond assumes a minimum of Level 2 ERTMS control and signalling for the "day one service". The Crossrail project in London which is also implementing ERTMS technology has been delayed by nine months.

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