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{{about||the South African airport with IATA code KMHA |Johan Pienaar Airport|the Canadian TV Series| km/h (TV series)}}
#redirect[[kilometres per second]]
[[Image:Metric speedometer from a 1992 Euro-spec Passat B3.jpg|thumb|right|A car [[speedometer]] that indicates measured speed in kilometres per hour.]]
[[Image:Ford Mondeo MK3 ST220 - Speedometer.jpg|thumb|right|Automobile [[speedometer]], indicating speed in [[miles per hour]] (MPH) on the outer scale and kilometres per hour on the inner scale. In Canada it is the outer scale that is indicated in km/h and the inner scale in MPH, if the latter appears at all. ]]
The '''kilometre per hour''' ([[American English]]: '''kilometer per hour''') is a [[Units of measurement|unit]] of [[speed]], expressing the number of [[kilometre]]s travelled in one [[hour]].

The [[International System of Units|SI]] unit symbol is '''km/h'''. Worldwide, it is the most commonly used unit of speed on road signs and [[speedometer|car speedometers]].<ref>http://www.us-metric.org/correct-si-metric-usage/</ref><ref>http://ukma.org.uk/docs/ukma-style-guide.pdf</ref>

==History==
Although the metre was formally defined in 1799, the term "kilometres per hour" did not come into immediate use – the myriametre ({{nowrap|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<ref>{{cite book |title=Physique d'Emile: ou, Principes de la science de la nature |volume=1 |first=Emmanuel |last=Develey |year=1802 |location=Paris |url=https://books.google.com/?id=AFsIAAAAIAAJ&pg=PA90&lpg=PA90&dq=vitesse+myriametre#v=onepage&q=myriam%C3%A8tres&f=false}}</ref> and many French maps printed in the first half of the nineteenth century had scales in [[league (unit)|league]]s and [[myriametre]]s, but not in kilometres.<ref>{{cite web |url=http://wn.com/1_myriametre#/images |title=France Pittoresque: Haute Pyrénées |year=1835 |publisher=Languillermie et Rambox |accessdate=13 October 2012}}</ref> The Dutch on the other hand adopted the kilometre in 1817 but gave it the local name of the ''mijl''.<ref>{{cite book |url=https://books.google.com/books?id=XYVbAAAAQAAJ&printsec=frontcover#v=onepage&q&f=false |title=Allereerste Gronden der Cijferkunst |first=Jacob |last=de Gelder |location='s-Gravenhage and Amsterdam |language=Dutch |year=1824
|pages=155–156 |publisher=de Gebroeders van Cleef |trans_title=Introduction to Numeracy |accessdate=2 March 2011}}</ref>

==Notation history==
Several representations of "kilometres per hour" have been used since the term was introduced and many are still in use today; for example, dictionaries list "km/h", "kmph" and "km/hr" as English abbreviations. The SI representations, classified as symbols, are "km/h", "{{nowrap|km h<sup>−1</sup>}}" and "{{nowrap|km·h<sup>−1</sup>}}".

===Kilometres per hour as an abbreviation===

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,<ref>{{cite web |url=http://www.oed.com/view/Entry/103403 |title=The Oxford English Dictionary |accessdate=13 July 2012}}</ref><!--<ref name="OED">{{cite book |title=The Compact Edition of the Oxford English Dictionary |publisher=Oxford University Press |year=1971 |pages=695}}</ref>--> and the compound unit of [[speed]] "kilometers per hour" was in use in the US by 1866.<ref>{{cite book |title=Journal of the Franklin Institute of the State of Pennsylvania for the Promotion of the Mechanic Arts |publisher=Franklin Institute |last=Frazer |first=John F. |date=November 1866 |location=Philadelphia |pages=314 |url=https://books.google.com/books?id=kKIqAQAAIAAJ&pg=PA314&lpg=PA314&dq=%22kilometers+per+hour%22&source=bl&ots=Osx40Oy1Na&sig=RQHVQQeN1oWkgnlX5rgjd6kowpM&hl=en&sa=X&ei=pET-T47dJ6782gWCxPnKDw&ved=0CDkQ6AEwAA#v=onepage&q=%22kilometers%20per%20hour%22&f=false |volume=LII |number=5}}</ref> "Kilometres per hour" did not begin to be abbreviated in print until many years later, with several different abbreviations existing near-contemporaneously.

{|
|-
| valign="top" width=200pt |
* 1889: "k. p. h."<ref>{{cite book |title=American meteorological journal: A monthly review of meteorology, medical climatology and geography |publisher=Meteorological Journal Company |last1=Harrington |first1=Mark W. |last2=Rotch |first2=A. Lawrence |last3=Herdman |first3=W. J. |date=May 1889 |pages=226 |volume=6 |url=https://books.google.com/books?id=0MEWAQAAMAAJ&pg=PA226&lpg=PA226&dq=%22km/h%22+%22kilometers+per+hour%22&source=bl&ots=9dJuM0-Tv3&sig=6Obk_ZTGNarHWdyvAFRn-2eNgTs&hl=en&sa=X&ei=bkf-T9CxD8aI2gWA2fjHDw&ved=0CDgQ6AEwAA#v=onepage&q=%22km%2Fh%22%20%22kilometers%20per%20hour%22&f=false}}</ref>
* 1895: "km:h"<ref>{{cite journal |url=https://books.google.com/books?id=I4hNAAAAYAAJ&pg=PA117&lpg=PA117&dq=%22km/h%22+%22kilometers+per+hour%22&source=bl&ots=WOqqVpscAB&sig=kue4ZLNbowkAqYytFDDV3fsx6wg&hl=en&sa=X&ei=00n-T7zXH6WO2QXKy4jqDw&ved=0CDYQ6AEwAA#v=onepage&q=%22km%2Fh%22%20%22kilometers%20per%20hour%22&f=false |title=Power consumed on electric railways |journal=The Street Railway Journal |date=February 1895 |volume=11 |issue=2 |pages=116–117}}</ref>
* 1898: "km/h"<ref>{{cite book |title=Bulletin – United States Geological Survey, Volumes 151–152 |publisher=USGS |year=1898 |pages=ix |url=https://books.google.com/books?id=QgwlAQAAIAAJ&q=%22km/h%22+kilometer&dq=%22km/h%22+kilometer&source=bl&ots=SQVFIN5ctK&sig=Hptqzifz899z3Q2rTwQC6fhX2hE&hl=en&sa=X&ei=jFD-T7i3D8y02AXjy5XMBA&ved=0CEcQ6AEwAg}}</ref>
* 1899: "km./hr." <ref>{{cite journal |title=The Stability of the Motion of a Bicycle |last=Whipple |first=F. J. W. |journal=The Quarterly Journal of Pure and Applied Mathematics |year=1899 |volume=30 |pages=342}}</ref>
* 1900: "kms./hr."<ref>{{cite book |title=The Theory of the Trace: Being a Discussion of the Principles of Location |last=Launhardt |first=Wilhelm |publisher=Lawrence Asylum Press |location=Madras |year=1900 |url=https://books.google.com/books?id=KkaXPI_aekoC}}</ref>
* 1902: "k.m.p.h."<ref>{{cite journal |url=https://books.google.com/books?id=DB3nAAAAMAAJ&pg=PA207 |journal=The Railway Engineer |volume=23 |editor1-last=Saunders |editor1-first=Lawrence |editor2-last=Blundstone |editor2-first=S. R. |page=207 |date=July 1902 |title=The Electric Problem of Railways |last=Swinburne |first=J}}</ref>
| valign="top" width="200pt"|
* 1903: "KMph."<ref>{{cite book |url=https://books.google.com/books?id=i0s1AQAAMAAJ |title=Observations Made at the Royal Magnetical and Meteorological Observatory at Batavia |last=Figee |first=S. |publisher=Government of Netherlands East India |year=1903 |volume=24 |page=196}}</ref>
* 1910: "km ph"<ref>{{cite book |url=https://books.google.com/books?id=CXg5AAAAMAAJ&pg=PR19 |title=Electric Trains |last=Hobart |first=H. M. |publisher=D. Van Nostrand Company |location=New York |year=1910 |page=xix}}</ref>
* 1911: "K.P.H."<ref>{{cite journal |title=Foreign Notes on Aviation |last=Ball |first=Jack |journal=Town & Country |date=August 1911 |pages=26 |url=https://books.google.com/books?id=1xhUAAAAYAAJ&pg=PA26&lpg=PA26&dq=%22kph%22+kilometer&source=bl&ots=qcyTcMQkdk&sig=HK14YIxFmt8UBcobLENErN0glHE&hl=en&sa=X&ei=HFn-T_P8KMqe2wX1gv3lBA&sqi=2&ved=0CF4Q6AEwBg#v=onepage&q=%22kph%22%20kilometer&f=false}}</ref>
* 1914 "km. hr."<ref>{{cite journal |url=https://books.google.com/books?id=x9pQAAAAYAAJ&pg=PA1141&lpg=PA1141&dq=%22km/hr%22&source=bl&ots=4eohj9q92V&sig=phERDBIQUn5Mtf2UFGR4INs80Pk&hl=en&sa=X&ei=Cbr_T4S_OofI2gW6tMWxBA&ved=0CDsQ6AEwAQ#v=onepage&q=%22km%2Fhr%22&f=false |title=A Review of Some European Electric Locomotive Designs |last=Dodd |first=S. T. |journal=General Electric Review |date=January 1914 |volume=17 |issue=1 |pages=1141}}</ref>
* 1915: "km/hour"<ref name="Automobile1915">{{cite journal |url=https://books.google.com/books?id=oxVaAAAAYAAJ&pg=PA710&lpg=PA710&dq=kph+%22kilometers+per+hour%22&source=bl&ots=oSV9840is5&sig=232C8QxgvKlEgItPZ3_fkfLywHQ&hl=en&sa=X&ei=5pz_T-7WD8y8qAGkhPSdBw&ved=0CEwQ6AEwBA#v=onepage&q=kph%20%22kilometers%20per%20hour%22&f=false |title=Data on Mixed Motor Fuels of Interest for American Export Trade |journal=The Automobile |date=October 1915 |volume=33 |issue=15 |pages=709}}</ref>
* 1915: "km.-hr."<ref name="Automobile1915" />
| valign="top" width="200pt" |
* 1916: "km. per hour"<ref>{{cite journal |url=https://books.google.com/books?id=O-MfAQAAMAAJ&pg=PA560&lpg=PA560&dq=kph+%22kilometers+per+hour%22&source=bl&ots=K_KMl6h4LJ&sig=3OPTvJsQMzVNZRNHfSL0AahG6KE&hl=en&sa=X&ei=5pz_T-7WD8y8qAGkhPSdBw&ved=0CFcQ6AEwBg#v=snippet&q=%22km.%20per%20hour%22&f=false |title=Tractive resistance tests with an electric motor truck |journal=Engineering and Contracting |date=December 1916 |volume=46 |issue=25 |pages=560}}</ref>
* 1921: "kms/hr."<ref>{{cite book |title=Meteorological Report for the Year [1916?] |publisher=Ministry of Public Works, Egypt |page=xvii |url=https://books.google.com/books?ei=9iOmUv2oNcqiyAHCoIHYBA |year=1921 |first=Maṣlaḥat al-Arṣād |last=al-Jawwīyah}}</ref>
* 1922: "Kmph"<ref>{{cite journal |url=https://books.google.com/books?id=hRU6AQAAMAAJ |journal=Railway Electrical Engineer |volume=13 |page=392 |title=French Railway Begins Electrification Program |last1=Candee |first1=A. H. |last2=Lynde |first2=L. E. |publisher=Simmons Boardman |year=1922}}</ref>
* 1927: "kmph."<ref>{{cite book |url=https://books.google.com/books?id=y5lTAAAAMAAJ |title=Pressure Airships |last=Blakemore |first=Thos. L. |year=1927 |page=230 |publisher=Ronald Press}}</ref>
* 1933: "KPH"<ref>{{cite book |title=Aircraft Year Book |publisher=Aerospace Industries Association of America, Manufacturers Aircraft Association, Aeronautical Chamber of Commerce of America |year=1933 |pages=391–393 |url=https://books.google.com/books?id=39cEAAAAMAAJ&q=%22kph%22+kilometer&dq=%22kph%22+kilometer&source=bl&ots=PyQ0_6UZCq&sig=8O5rmhK8Yjut3p0RsSoU3WItHPo&hl=en&sa=X&ei=tln-T9WMNsi-2gWfk6zcBA&ved=0CDwQ6AEwAQ |volume=15}}</ref>
* 1939: "kmph"<ref>{{cite book |url=https://books.google.com/books?id=CWYjAQAAMAAJ |title=Bulletin |publisher=Central Electric Railfans' Association |year=1939 |page=cxii}}</ref>
|}
<!-- to be added
* ????: "km/hr"
* ????: "KMHR"
* 1940: "KMPH"<ref>{{cite book |url=https://books.google.com/books?id=uA5LAAAAMAAJ |title=Electric Transportation |last=Thompson |first=Francis R. |year=1940 |page=165 }}</ref>
-->

With no central authority to dictate the rules for abbreviations, 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.<ref>{{cite book |title=Eats Shoots and Leaves |first=Lynne |last=Truss |publisher=Profile Books |year=2003 |pages=188–189 |isbn=1 86197 6127}}</ref> For example, news organisations such as [[Reuters]]<ref>{{cite book |url=http://handbook.reuters.com/extensions/docs/pdf/handbookofjournalism.pdf |page=278 |title=Reuters Handbook of Journalism |date=April 2008 |publisher=[[Reuters]]}}</ref> and [[The Economist]]<ref>{{cite news |url=http://www.economist.com/style-guide/abbreviations |accessdate=9 December 2013 |title=Style Guide |publisher=[[The Economist]] |date=27 September 2011}}</ref>
require "kph".

In Australian unofficial usage, km/h is sometimes pronounced and written as ''klicks'' or ''clicks''.<ref>{{cite web |date=June 2012 |url=http://www.oed.com/view/Entry/241046 |title=klick |work=[[Oxford English Dictionary]] |accessdate=9 July 2012}}</ref>

===Kilometres per hour as a symbol===
The use of symbols to replace words dates back to at least the late medieval era when [[Johannes Widman]], writing in German in 1486, used the symbols "+" and "−" to represent "addition" and "subtraction".<ref>{{MacTutor|id=Widman|title=Johannes Widman}}</ref> In the early 1800s [[Berzelius]] introduced a symbolic notation for the [[chemical element]]s derived from the elements' [[Latin]] names.<ref>{{cite web |url=http://www.bjupress.com/resources/pdfs/science/chemistry/c02-history-of-chemical-symbols.pdf |title=The History of Chemical Symbols |publisher=BJU Press |location=Greenville, South Carolina |accessdate=18 July 2012}}</ref> Typically, "Na" was used for the element [[sodium]] (Latin: ''natrium'') and H<sub>2</sub>O for water.

In 1879, four years after the signing of the [[Metre Convention|Treaty of the Metre]], the [[CIPM]] proposed a range of symbols for the various metric units then under the auspices of the [[CGPM]]. Among these were the use of the symbol "km" for "kilometre".<ref>{{cite book |url=https://books.google.com/?id=jlM907kFhcgC&pg=PA127&lpg=PA127&dq=cipm+1879+metre#v=onepage&q=cipm%201879%20metre&f=false |title=From Artefacts to Atoms: The BIPM and the Search for Ultimate Measurement Standards |first=Terry |last=Quinn |year=2012 |isbn=978-0-19-530786-3 |page=132 |location=Oxford |publisher=[[Oxford University Press]]}}</ref>

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 {{nowrap|"km/h"}}, {{nowrap|"km h<sup>−1</sup>"}} and {{nowrap|"km·h<sup>−1</sup>"}} being valid representations of "kilometres per hour".<ref name=SI>{{SIbrochure8th|page=124}}</ref> The SI standards, which were [[MKS system of units|MKS-based]] rather than [[CGS|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.<ref name=SI/> M. Danloux-Dumesnils<ref>{{cite book |title=The Metric System: A Critical Study of its Principles and Practice |publisher=The Athlone Press of the University of London |author=Danloux-Dumesnils |year=1969 |pages=32}}</ref> provides the following justification for this distinction:
{{quote|text=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 {{nowrap|"km/h"}} (or {{nowrap|"km h<sup>−1</sup>"}} or {{nowrap|"km·h<sup>−1</sup>"}}) has now been adopted around the world in many areas related to health and safety<ref>{{cite web |url=http://intralibrary.rlo-cetl.ac.uk:8080/intralibrary/open_virtual_file_path/i1405n24399t/2.html |title=RLO: SI Units |accessdate=21 July 2012 |publisher=School of Nursing and Academic Division of Midwifery; University of Nottingham |date=1 December 2006}}</ref> and in [[metrology]].<ref>{{cite web |title=Information and Harmonization |url=http://www.metrologyinfo.org/oiml-2.html |publisher=[[International Bureau of Weights and Measures]] and [[International Organization of Legal Metrology]] |accessdate=20 July 2012}}</ref> It is also the preferred system of measure in academia and in education.<ref>{{cite journal |url=https://www.tru.ca/library/pdf/tru-ol_editorial_style.pdf |title=OLA Editorial Style Guide |publisher=[[Open Learning Agency]] (OLA), Government of British Columbia |year=2000 |location=Burnaby, British Columbia |accessdate=26 July 2012}}</ref>

== Regulatory use ==
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:

<gallery perrow="6" widths="120px">
File:Hungary road sign C-033-100.svg|100&nbsp;km/h sign following the most common implementation of the Vienna Convention style (Hungary)
File:Sweden road sign C31-3.svg|Swedish 30&nbsp;km/h speed limit – the yellow background provides a contrast in case the sign is covered by snow.<ref>{{cite web|url=http://www.grsproadsafety.org/our-knowledge/conspicuity-and-signs|title=Conspicuity and Signs: Road signing|publisher=[[International Red Cross and Red Crescent Movement#The International Federation of Red Cross and Red Crescent Societies|International Federation of Red Cross and Red Crescent Societies]]|year=2012|accessdate=19 September 2012}}</ref>
File:Ireland road sign RUS 043.svg|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 language|Irish]])<ref>{{cite journal|url=http://www.transport.ie/upload/general/12971-TSM_CHAPTER_1-1.PDF|title=Department of Transport / ''An Roinn Iompair''|date=November 2010|page=1|accessdate=16 July 2012|deadurl=yes|archiveurl=https://web.archive.org/web/20120428064628/http://www.transport.ie/upload/general/12971-TSM_CHAPTER_1-1.PDF|archivedate=28 April 2012|df= }} </ref>
File:UAE Speed Limit - 60 kmh.svg|60&nbsp;km/h speed limit in Arabic and Latin scripts ([[United Arab Emirates|UAE]])
File:Vesiliikennemerkki 11.svg|Waterways speed limit of 9&nbsp;km/h ([[Finland]])
Image:Samoa - Speed Limit.svg|[[Samoa]] uses both [[miles per hour]] and kilometres per hour
File:Mexico road sign SR-09.svg|50&nbsp;km/h sign in [[Speed limits in Mexico|Mexico]]
</gallery>

[[File:Speedo angle.jpg|thumb|right|Automobile [[speedometer]], measuring speed in [[miles per hour]] on the outer track, and kilometres per hour on the inner track. In Canada "km/h" is shown on the outer track and "MPH" on the inner track.<ref>{{cite journal|url=https://techinfo.honda.com/rjanisis/pubs/om/CV0505/CV0505O00060A.pdf |title=Maintenance Required Indicator |accessdate=4 August 2011 |deadurl=yes |archiveurl=https://web.archive.org/web/20160304100125/https://techinfo.honda.com/rjanisis/pubs/om/CV0505/CV0505O00060A.pdf |archivedate= 4 March 2016 |df= }}</ref>]]
In 1972 the EU published a directive<ref>{{cite EU directive |serial=71/354/EEC |year=1971 |date=18 October 1971 |description=on the approximation of laws of Member States relating to units of measurement |eurlextag=DD:I:1971_III:31971L0354:EN:PDF}}</ref> (overhauled in 1979<ref>{{cite web |url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31980L0181:EN:NOT |author=The Council of the European Communities |title=Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC |accessdate=7 February 2009}}</ref> 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<ref group=Note>Until 2010, the directive covered "economic, public health, public safety or administrative purposes"; since then [http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32009L0003:EN:NOT it covers all aspects] of the EU [[internal market]]</ref> 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,<ref>{{cite EU directive |serial=75/443/EEC |year=1975 |date=26 June 1975 |description=on the approximation of the laws of the Member States relating to the reverse and speedometer equipment of motor vehicles |eurlextag=CONSLEG:1975L0443:19970725:en:PDF}}</ref> even where that is not the natural abbreviation for the local version of "kilometres per hour". Examples include:
*Dutch: "''kilometer per uur''" ("hour" is spelt "''uur''" – does not start with "h"),
*Portuguese: "''quilómetro por hora''" ("kilometre" is spelt "''quilómetro''" – does not start with "k")
*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".<ref>{{cite journal |url=https://www.federalregister.gov/articles/2000/05/15/00-11493/federal-motor-vehicle-safety-standards-fmvss-101-technical-correction-speedometer-display |title=Federal Motor Vehicle Safety Standards; FMVSS 101--Technical Correction--Speedometer Display |author=National Highway Traffic Safety Administration |journal=Federal Register |date=May 2000 |volume=64 |issue=94 |pages=30915–30918}}</ref> However, the Federal Motor Vehicle Safety Standard number 101 ("Controls and Displays") allows "any combination of upper- and lowercase letters" to represent the units.<ref>{{cite book |title=Federal Motor Vehicle Safety Standard (101: Controls and Displays) |author=National Highway Traffic Safety Administration |pages=237 |url=https://docs.google.com/gview?url=http://lemon.onecle.com/wp-content/uploads/fmvss/49cfr571.101.pdf&chrome=true}}</ref>

==Conversions==
* 3.6&nbsp;km/h ≡ 1&nbsp;m/s, the [[SI unit]] of speed, [[metre per second]]
* 1&nbsp;km/h ≈ 0.277 78&nbsp;m/s
* 1&nbsp;km/h ≈ 0.621 37&nbsp;[[miles per hour|mph]] ≈ 0.911 34&nbsp;[[feet per second]]
* 1&nbsp;[[Knot (unit)|knot]] ≡ 1.852&nbsp;km/h (exactly)
* 1&nbsp;mile per hour ≡ 1.609344&nbsp;km/h (~1.61&nbsp;km/h)<ref>1&nbsp;yard ≡ 0.9144&nbsp;m and<br>1&nbsp;mile = 1760&nbsp;yards thus<br>1&nbsp;mile = 1760 × 0.9144 ÷ 1000&nbsp;km</ref>

{{Speed conversions}}

== See also ==
* [[Knot (unit)|Knot]]
* [[Meters per second]]
* [[Miles per hour]]
* [[Orders of magnitude (speed)]]
* [[Speed limits in Canada#Signage|Speed limits in Canada]]

==Notes==
{{Reflist| group = Note}}

==References==
{{Reflist|30em}}

{{North America topic|Speed limits in}}

{{DEFAULTSORT:Kilometres Per Hour}}
[[Category:Units of velocity]]
[[Category:Non-SI metric units]]

Revision as of 17:00, 3 October 2017

For the South African airport with IATA code KMHA, see Johan Pienaar Airport. For the Canadian TV Series, see km/h (TV series).
A car speedometer that indicates measured speed in kilometres per hour.
Automobile speedometer, indicating speed in miles per hour (MPH) on the outer scale and kilometres per hour on the inner scale. In Canada it is the outer scale that is indicated in km/h and the inner scale in MPH, if the latter appears at all.

The kilometre per hour (American English: kilometer per hour) is a unit of speed, expressing the number of kilometres travelled in one hour.

The SI unit symbol is km/h. Worldwide, it is the most commonly used unit of speed on road signs and car speedometers.[1][2]

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[3] and many French maps printed in the first half of the nineteenth century had scales in leagues and myriametres, but not in kilometres.[4] The Dutch on the other hand adopted the kilometre in 1817 but gave it the local name of the mijl.[5]

Notation history

Several representations of "kilometres per hour" have been used since the term was introduced and many are still in use today; for example, dictionaries list "km/h", "kmph" and "km/hr" as English abbreviations. The SI representations, classified as symbols, are "km/h", "km h−1" and "km·h−1".

Kilometres per hour as an abbreviation

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,[6] and the compound unit of speed "kilometers per hour" was in use in the US by 1866.[7] "Kilometres per hour" did not begin to be abbreviated in print until many years later, with several different abbreviations existing near-contemporaneously.

  • 1889: "k. p. h."[8]
  • 1895: "km:h"[9]
  • 1898: "km/h"[10]
  • 1899: "km./hr." [11]
  • 1900: "kms./hr."[12]
  • 1902: "k.m.p.h."[13]

With no central authority to dictate the rules for abbreviations, 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.[25] For example, news organisations such as Reuters[26] and The Economist[27] require "kph".

In Australian unofficial usage, km/h is sometimes pronounced and written as klicks or clicks.[28]

Kilometres per hour as a symbol

The use of symbols to replace words dates back to at least the late medieval era when Johannes Widman, writing in German in 1486, used the symbols "+" and "−" to represent "addition" and "subtraction".[29] In the early 1800s Berzelius introduced a symbolic notation for the chemical elements derived from the elements' Latin names.[30] Typically, "Na" was used for the element sodium (Latin: natrium) and H2O for water.

In 1879, four years after the signing of the Treaty of the Metre, the CIPM proposed a range of symbols for the various metric units then under the auspices of the CGPM. Among these were the use of the symbol "km" for "kilometre".[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".[32] 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.[32] M. Danloux-Dumesnils[33] 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[34] and in metrology.[35] It is also the preferred system of measure in academia and in education.[36]

Regulatory use

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:

Automobile speedometer, measuring speed in miles per hour on the outer track, and kilometres per hour on the inner track. In Canada "km/h" is shown on the outer track and "MPH" on the inner track.[39]

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 spelt "uur" – does not start with "h"),
  • Portuguese: "quilómetro por hora" ("kilometre" is spelt "quilómetro" – does not start with "k")
  • 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]

Conversions

Conversions between common units of speed
m/s km/h mph (mi/h) knot fps (ft/s)
1 m/s = 1 3.600000 2.236936* 1.943844* 3.280840*
1 km/h = 0.277778* 1 0.621371* 0.539957* 0.911344*
1 mph (mi/h) = 0.44704 1.609344 1 0.868976* 1.466667*
1 knot = 0.514444* 1.852 1.150779* 1 1.687810*
1 fps (ft/s) = 0.3048 1.09728 0.681818* 0.592484* 1

(* = approximate values)

See also

Notes

  1. ^ Until 2010, the directive covered "economic, public health, public safety or administrative purposes"; since then it covers all aspects of the EU internal market

References

  1. ^ http://www.us-metric.org/correct-si-metric-usage/
  2. ^ http://ukma.org.uk/docs/ukma-style-guide.pdf
  3. ^ Develey, Emmanuel (1802). Physique d'Emile: ou, Principes de la science de la nature. Vol. 1. Paris.
  4. ^ "France Pittoresque: Haute Pyrénées". Languillermie et Rambox. 1835. Retrieved 13 October 2012.
  5. ^ de Gelder, Jacob (1824). Allereerste Gronden der Cijferkunst (in Dutch). 's-Gravenhage and Amsterdam: de Gebroeders van Cleef. pp. 155–156. Retrieved 2 March 2011. {{cite book}}: Unknown parameter |trans_title= ignored (|trans-title= suggested) (help)
  6. ^ "The Oxford English Dictionary". Retrieved 13 July 2012.
  7. ^ Frazer, John F. (November 1866). Journal of the Franklin Institute of the State of Pennsylvania for the Promotion of the Mechanic Arts. Vol. LII. Philadelphia: Franklin Institute. p. 314.
  8. ^ Harrington, Mark W.; Rotch, A. Lawrence; Herdman, W. J. (May 1889). American meteorological journal: A monthly review of meteorology, medical climatology and geography. Vol. 6. Meteorological Journal Company. p. 226.
  9. ^ "Power consumed on electric railways". The Street Railway Journal. 11 (2): 116–117. February 1895.
  10. ^ Bulletin – United States Geological Survey, Volumes 151–152. USGS. 1898. pp. ix.
  11. ^ Whipple, F. J. W. (1899). "The Stability of the Motion of a Bicycle". The Quarterly Journal of Pure and Applied Mathematics. 30: 342.
  12. ^ Launhardt, Wilhelm (1900). The Theory of the Trace: Being a Discussion of the Principles of Location. Madras: Lawrence Asylum Press.
  13. ^ Swinburne, J (July 1902). Saunders, Lawrence; Blundstone, S. R. (eds.). "The Electric Problem of Railways". The Railway Engineer. 23: 207.
  14. ^ Figee, S. (1903). Observations Made at the Royal Magnetical and Meteorological Observatory at Batavia. Vol. 24. Government of Netherlands East India. p. 196.
  15. ^ Hobart, H. M. (1910). Electric Trains. New York: D. Van Nostrand Company. p. xix.
  16. ^ Ball, Jack (August 1911). "Foreign Notes on Aviation". Town & Country: 26.
  17. ^ Dodd, S. T. (January 1914). "A Review of Some European Electric Locomotive Designs". General Electric Review. 17 (1): 1141.
  18. ^ a b "Data on Mixed Motor Fuels of Interest for American Export Trade". The Automobile. 33 (15): 709. October 1915.
  19. ^ "Tractive resistance tests with an electric motor truck". Engineering and Contracting. 46 (25): 560. December 1916.
  20. ^ al-Jawwīyah, Maṣlaḥat al-Arṣād (1921). Meteorological Report for the Year [1916?]. Ministry of Public Works, Egypt. p. xvii.
  21. ^ Candee, A. H.; Lynde, L. E. (1922). "French Railway Begins Electrification Program". Railway Electrical Engineer. 13. Simmons Boardman: 392.
  22. ^ Blakemore, Thos. L. (1927). Pressure Airships. Ronald Press. p. 230.
  23. ^ Aircraft Year Book. Vol. 15. Aerospace Industries Association of America, Manufacturers Aircraft Association, Aeronautical Chamber of Commerce of America. 1933. pp. 391–393.
  24. ^ Bulletin. Central Electric Railfans' Association. 1939. p. cxii.
  25. ^ Truss, Lynne (2003). Eats Shoots and Leaves. Profile Books. pp. 188–189. ISBN 1 86197 6127.
  26. ^ Reuters Handbook of Journalism (PDF). Reuters. April 2008. p. 278.
  27. ^ "Style Guide". The Economist. 27 September 2011. Retrieved 9 December 2013.
  28. ^ "klick". Oxford English Dictionary. June 2012. Retrieved 9 July 2012.
  29. ^ O'Connor, John J.; Robertson, Edmund F., "Johannes Widman", MacTutor History of Mathematics Archive, University of St Andrews
  30. ^ "The History of Chemical Symbols" (PDF). Greenville, South Carolina: BJU Press. Retrieved 18 July 2012.
  31. ^ Quinn, Terry (2012). From Artefacts to Atoms: The BIPM and the Search for Ultimate Measurement Standards. Oxford: Oxford University Press. p. 132. ISBN 978-0-19-530786-3.
  32. ^ a b International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 124, ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16
  33. ^ Danloux-Dumesnils (1969). The Metric System: A Critical Study of its Principles and Practice. The Athlone Press of the University of London. p. 32.
  34. ^ "RLO: SI Units". School of Nursing and Academic Division of Midwifery; University of Nottingham. 1 December 2006. Retrieved 21 July 2012.
  35. ^ "Information and Harmonization". International Bureau of Weights and Measures and International Organization of Legal Metrology. Retrieved 20 July 2012.
  36. ^ "OLA Editorial Style Guide" (PDF). Burnaby, British Columbia: Open Learning Agency (OLA), Government of British Columbia. 2000. Retrieved 26 July 2012. {{cite journal}}: Cite journal requires |journal= (help)
  37. ^ "Conspicuity and Signs: Road signing". International Federation of Red Cross and Red Crescent Societies. 2012. Retrieved 19 September 2012.
  38. ^ "Department of Transport / An Roinn Iompair" (PDF). November 2010: 1. Archived from the original (PDF) on 28 April 2012. Retrieved 16 July 2012. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  39. ^ "Maintenance Required Indicator" (PDF). Archived from the original (PDF) on 4 March 2016. Retrieved 4 August 2011. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  40. ^ Directive 71/354/EEC of 18 October 1971 of the European Parliament and of the Council on the approximation of laws of Member States relating to units of measurement
  41. ^ The Council of the European Communities. "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". Retrieved 7 February 2009.
  42. ^ Directive 75/443/EEC of 26 June 1975 of the European Parliament and of the Council on the approximation of the laws of the Member States relating to the reverse and speedometer equipment of motor vehicles
  43. ^ National Highway Traffic Safety Administration (May 2000). "Federal Motor Vehicle Safety Standards; FMVSS 101--Technical Correction--Speedometer Display". Federal Register. 64 (94): 30915–30918.
  44. ^ National Highway Traffic Safety Administration. Federal Motor Vehicle Safety Standard (101: Controls and Displays). p. 237.
  45. ^ 1 yard ≡ 0.9144 m and
    1 mile = 1760 yards thus
    1 mile = 1760 × 0.9144 ÷ 1000 km
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