The United States has an on and off history of attempting to convert to a nationwide use of weights and measures in the country from the United States customary units (i.e. the use of feet, inches, and pounds) to what is known as the International System of Units which uses units such as centimeters, meters, and kilos.
The last such attempt was initiated by the signing of the Metric Conversion Act in 1975 by President Ford. The voluntarily nature of the Act contributed to an overall inertia in embracing the metric system and the Metric Board was disbanded in 1982 by President Reagan.
One lone complete adoption of the metric system in the country’s vast network of highways still exists today. A 102 kilometer (63-mile) stretch of highway in from Tucson, Arizona to Nogales near the Mexican border is the only metric highway in the United States. Known as Interstate 19, sign distances are provided in meters and kilometers instead of in miles.
Why is Interstate 19 the only complete highway in the U.S. to have signs using the metric system? Interstate 19, which was first open to traffic in 1962 but not fully completed until 1975, was chosen by the Metric Board to be the first highway with only metric signage as a pilot project for converting all of the U.S. interstate system to the metric system.
There have been some recent efforts to convert the signage back to US customary units but there has been resistance by local retailers and restaurants that advertise their locations based on the kilometer numbered highway exits.
Metric Signage in Louisville, Kentucky on I-295
Outside of Interstate 19, only a few segments along Interstate 295 in Louisville, Kentucky contain signs that use the metric system. This signage was in reaction to attempts in the 1990s by agencies like the Department of Transportation to require signage in the metric system by 2000.
With the signs in this section of Louisville due for replacement, the Kentucky Transportation Cabinet opted to have the new signage include the metric system. The effort to have more widespread metric system signage fizzled when the 1998 Transportation Equity Act struck down any efforts to convert to a metric system.
What Countries Have Not Adopted the Metric System?
Currently, only three countries in the world have not adopted the metric system as the standard: the United States, Myanmar, and Liberia.
LaBarbera, J. (2020, September 29). Kilometers in Arizona? You bet! ADOT. https://azdot.gov/adot-blog/kilometers-arizona-you-bet
Stevens, A. (2019, June 14). Curious Louisville: Why is a stretch of Louisville highway measured in kilometers? 89.3 WFPL News Louisville. https://wfpl.org/curious-louisville-why-is-a-stretch-of-louisville-highway-measured-in-kilometers/
This article was originally published on November 7, 2015 and has since been updated with additional information.