Speed limits

Speed limits throughout most of the world are set in kilometres per hour (km∕h). The UK remains the only country in Europe, and the Commonwealth, that still defines speed limits in miles per hour (mph).

mph km/h comparison

Speed limits in kilometres per hour (km∕h) would provide Traffic Authorities with a more versatile range of speed restrictions and enable speed limits to be more finely tuned to the circumstances of individual roads.

To set a speed limit in the range from 30 to 70 km∕h (19 to 43 mph), a Traffic Authority would be presented with a choice of five km∕h options, whereas there are only three options using current mph speed limits.

Again, in the range from 70 to 120 km∕h (43 to 74 mph), there would be a choice of six km∕h options, whereas there are only three mph options.


All vehicles registered in the UK since 1977 have been required to have a speedometer capable of displaying speeds in kilometres per hour (km∕h) as well as miles per hour (mph).

km/h speedometer display

Foreign registered vehicles are generally not required to have speedometers capable of displaying miles per hour. This means that the majority of drivers visiting the UK have vehicles fitted with speedometers that display speeds in kilometres per hour only.

It has been estimated that more than 3 million foreign registered vehicles enter the UK each year and 140 000 of them are present at any given time. Across the whole of the UK, foreign registered vehicles travel a total of almost 5 billion kilometres each year. The vast majority of these vehicles have speedometers incapable of showing speeds in miles per hour.

Changing all UK speed limits to kilometres per hour will mean that practically every vehicle on Britain’s roads will be able to show speeds in the relevant units. UKMA believes that this can only improve the safety of all drivers in the UK.

Digital tachographs

Digital tachographs are instruments that are used to record driver and vehicle activity to ensure compliance with drivers’ hours rules.

Digital tachograph
Digital tachographs use kilometres and km∕h only

Distances and speeds are recorded and displayed exclusively in kilometres and kilometres per hour. Many goods vehicles exceeding 3.5 tonnes, and some coaches, are legally required to be fitted with a digital tachograph.

Speed limiters

Speed limiters are devices that are fitted in order to physically restrict vehicles from exceeding a pre-defined maximum road speed.

To comply with UK legislation, by 1 January 2008, speed limiters will have been fitted to many older, and all recently registered, buses, coaches and goods vehicles over 3.5 tonnes. All speed limiter settings are specified in kilometres per hour.

56 mpg sign on lorry
This vehicle is fitted with a 90 km∕h speed limiter

For certain classes of vehicle, the speed limit on motorways, as shown in the Highway Code, is now significantly greater than the maximum legal speed permitted by these vehicles’ speed limiters.

This anomaly is acknowledged by VOSA (Vehicle and Operator Services Agency) in their leaflet “New Speed Limiter Legislation“, which states, “It is likely, once all the changes to vehicles requiring road speed limiters have taken place (after 1 January 2008), the national motorway speed limit for goods vehicles over 3.5 tonnes and buses will be lowered”.

Currently, 70 mph is the maximum speed limit on motorways for goods vehicles not exceeding 7.5 tonnes maximum laden weight, and 70 mph is also the limit for buses and coaches.

Will these speed limits be brought into line with speed limiter settings? i.e. 90 km∕h (approx. 56 mph) and 100 km∕h (approx. 62 mph).

UKMA believes that using two incompatible systems for the legal definition of speed limits makes no sense. UKMA contends that the best way to resolve such anomalies is to set all national speed limits in kilometres per hour, as part of a comprehensive programme of conversion to metric units for all of Britain’s road signs and regulations.