Abbreviation development
The use of abbreviations dates back to antiquity, but abbreviations for "kilometres per hour" did not appear in the English language until the late nineteenth century.
Although the unit of length kilometre first made its appearance in English in 1810, the compound unit of speed "kilometers per hour" was in use in the US by 1866. "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."
1895: "km:h"
1898: "km/h"
1899: "km./hr."
1911: "K.P.H."
1914 "km. hr."
1915: "km/hour"
1915: "km.-hr."
1916: "km. per hour"
1933: "KPH"
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, for example style guides of news organisations such as Reuters and The Guardian tend to use "kph" (along with "C" or "F" instead of "°C" or "°F" for temperature).
The SI explicitly states that unit symbols are not abbreviations and are to be written using a very specific set of rules. M. Danloux-Dumesnils 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 and in legal metrology. It is also the preferred system of measure in academia and in education.