The factual accuracy of part of this article is
disputed. The dispute is about the implied role of the BIPM wrt to the regulation of the English language. Please help to ensure that disputed statements are
reliably sourced. See the relevant discussion on the
talk page.(November 2022) (
Learn how and when to remove this template message)
The kilometre per hour (
SI symbol: km/h; non-standard abbreviations: kph, km/hr) is a
unit of
speed, expressing the number of
kilometres travelled in one
hour.
History
Although the metre was formally defined in 1799, the term "kilometres per hour" did not come into immediate use – the myriametre (10,000 metres) and myriametre per hour were preferred to kilometres and kilometres per hour. In 1802 the term "myriamètres par heure" appeared in French literature.[1] The Dutch on the other hand adopted the kilometre in 1817 but gave it the local name of the mijl (
Dutch mile).[2]
Notation history
The SI representations, classified as symbols, are "km/h", "km h−1" and "km·h−1". Several other abbreviations of "kilometres per hour" have been used since the term was introduced and many are still in use today; for example, dictionaries list "kph",[3][4][5] "kmph" and "km/hr"[6] as English abbreviations. While these forms remain widely used, the International Bureau of Weights and Measures uses "km/h" in describing the definition and use of the International System of Units.[7] The entries for "kph" and "kmph" in the Oxford Advanced Learner's Dictionary state that "the correct scientific unit is km/h and this is the generally preferred form".[8]
Abbreviations
Abbreviations for "kilometres per hour" did not appear in the English language until the late nineteenth century.
The
kilometre, a unit of
length, first appeared in English in 1810,[9] and the compound unit of
speed "kilometers per hour" was in use in the US by 1866.[10] "Kilometres per hour" did not begin to be abbreviated in print until many years later, with several different abbreviations existing near-contemporaneously.
With no central authority to dictate the rules for abbreviations (other than the official km/h symbol dictated by the International Bureau of Weights and Measures), various publishing houses have their own rules that dictate whether to use upper-case letters, lower-case letters, periods and so on, reflecting both changes in fashion and the image of the publishing house concerned.[28] For example, news organisations such as
Reuters[29] and The Economist[30]
require "kph".
In informal Australian usage, km/h is more commonly pronounced "kays" or "kays an hour". In military usage, "klicks" is used, though written as km/h.[31]
In 1948, as part of its preparatory work for the
SI, the CGPM adopted symbols for many units of measure that did not have universally agreed symbols, one of which was the symbol "h" for "hours". At the same time the CGPM formalised the rules for combining units – quotients could be written in one of three formats resulting in "km/h", "km h−1" and "km·h−1" being valid representations of "kilometres per hour".[33] The SI standards, which were
MKS-based rather than
CGS-based, were published in 1960 and have since then have been adopted by many authorities around the globe including academic publishers and legal authorities.
The SI explicitly states that unit symbols are not abbreviations and are to be written using a very specific set of rules.[33] M. Danloux-Dumesnils[34] provides the following justification for this distinction:
It has already been stated that, according to Maxwell, when we write down the result of a measurement, the numerical value multiplies the unit. Hence the name of the unit can be replaced by a kind of algebraic symbol, which is shorter and easier to use in formulae. This symbol is not merely an abbreviation but a symbol which, like chemical symbols, must be used in a precise and prescribed manner.
SI, and hence the use of "km/h" (or "km h−1" or "km·h−1") has now been adopted around the world in many areas related to health and safety[35] and in
metrology[36] in addition to the SI unit
metres per second ("m/s", "m s−1" or "m·s−1"). SI is also the preferred system of measure in academia and in education.[37]
Alternative abbreviations in official use
km/j or km/jam (Indonesia and Malaysia)
km/t or km/tim (Norway, Denmark and Sweden; also use km/h)
During the early years of the motor car, each country developed its own system of road signs. In 1968 the
Vienna Convention on Road Signs and Signals was drawn up under the auspices of the
United Nations Economic and Social Council to harmonise road signs across the world. Many countries have since signed the convention and adopted its proposals. Speed limits signs that are either directly authorised by the convention or have been influenced by the convention are shown below:
100 km/h sign following the most common implementation of the Vienna Convention style (Hungary)
Swedish 30 km/h speed limit – the yellow background provides a contrast in case snow covers the background against which one perceives the road sign.[38]
Since the text "km/h" on this Irish speed limit sign is a symbol, not an abbreviation, it represents both "kilometres per hour" (English) and "ciliméadar san uair" (
Irish)[39]
60 km/h speed limit in
Arabic numerals (below) and Arabic script (above) (
UAE)
In 1972 the EU published a directive[40] (overhauled in 1979[41] to take British and Irish interests into account) that required member states to abandon CGS-based units in favour of SI. The use of SI implicitly required that member states use "km/h" as the shorthand for "kilometres per hour" on official[Note 1] documents.
Another EU directive, published in 1975, regulates the layout of speedometers within the European Union, and requires the text "km/h" in all languages,[42] even where that is not the natural abbreviation for the local version of "kilometres per hour". Examples include:
Dutch: "kilometer per uur" ("hour" is "uur" – does not start with "h"),
Portuguese: "quilómetro por hora" ("kilometre" is "quilómetro" – does not start with "k")
Irish: "ciliméadar san uair"
Greek: "χιλιόμετρα ανά ώρα" (a different script).
In 1988 the United States
National Highway Traffic Safety Administration promulgated a rule stating that "MPH and/or km/h" were to be used in speedometer displays. On May 15, 2000, this was clarified to read "MPH, or MPH and km/h".[43] However, the Federal Motor Vehicle Safety Standard number 101 ("Controls and Displays") allows "any combination of upper- and lowercase letters" to represent the units.[44]
^Until 2010, the directive covered "economic, public health, public safety or administrative purposes"; since then
it covers all aspects of the
EU internal market.
^de Gelder, Jacob (1824).
Allereerste Gronden der Cijferkunst [Introduction to Numeracy] (in Dutch). 's-Gravenhage and Amsterdam: de Gebroeders van Cleef. pp.
155–156. Retrieved 2 March 2011.
^Aircraft Year Book. Vol. 15. Aerospace Industries Association of America, Manufacturers Aircraft Association, Aeronautical Chamber of Commerce of America. 1933. pp. 391–393.
^Bulletin. Central Electric Railfans' Association. 1939. p. cxii.
^Danloux-Dumesnils (1969). The Metric System: A Critical Study of its Principles and Practice. The Athlone Press of the University of London. p. 32.
^"RLO: SI Units". School of Nursing and Academic Division of Midwifery; University of Nottingham. 1 December 2006. Archived from
the original on 23 December 2012. Retrieved 21 July 2012.