page_title: Metrication in the United States
section_title: Transportation
hierarchical_section_title: Metrication in the United States / Current use / Transportation
caption_attribution_description: A road sign on Interstate 19 just south of Tucson, AZ, USA; note the Metric Units used. Photo taken on 18 April 2005.
context_page_description: Metrication is the process of introducing the International System of Units, also known as SI units or the metric system, to replace a jurisdiction's traditional measuring units. Although U.S. customary units have been defined in terms of metric units since the 19th century, as of 2019 the United States is one of only three countries that have not officially adopted the metric system as the primary means of weights and measures.
  The United States has official legislation for metrication; however, conversion was not mandatory and many industries chose not to convert, and unlike other countries, there is no governmental or major social desire to implement further metrication.
  Although customary units are used more often than metric units in the U.S., the SI system is used extensively in some fields such as science, medicine, the military, automobile production and repair, and international affairs. Post-1994 federal law also mandates most packaged consumer goods be labeled in both customary and metric units.
context_section_description: Highway speed limits are posted in miles per hour and distances are largely displayed in miles, yards, or feet, although a few dual mile/km signs can be found, mostly left over from demonstration projects that are no longer supported. One exception is Interstate 19 in Arizona, which is almost completely signed in metric, except for speed limit signs. Signage on this road is, however, being converted back to customary units as it is replaced. Another exception is the turnpike section of Delaware Route 1, which uses a kilometer-based system, in anticipation of the mid-1990s conversion in the U.S. to the metric system, which did not happen. Distances were originally signed in kilometers, but have since been replaced with miles in 1995 and standard mile-markers in 2003. The exit numbers have remained metric starting at the Dover Air Force Base northward to Interstate 95.
  The 2000, 2003, and 2004 editions of the Manual on Uniform Traffic Control Devices were published using both metric and American Customary Units. Metric signs, including speed signs in kilometers per hour, were displayed in the manual even though they were not normally used on the roads and there was no definite plan to go metric. For visual distinction, regulatory and advisory metric speed signs have black circles around the numbers followed by "km/h" and other regulatory metric signs have yellow plates reading "METRIC" in black on top of the main signs. Metric signs and metric measurements were removed for the 2009 edition and replaced with an appendix of metric conversion tables. This is in marked contrast to the position in the (largely metric) United Kingdom, where metric road signs are prohibited by law (except for those denoting widths and height restrictions, which include both metric and imperial units).
  Newer signs on state highways on the island of Kaua‘i in Hawaii also include distances in both miles and kilometers. Mile markers along highways on the Island of Hawaii will indicate both units whenever the whole kilometer corresponds approximately with the whole mile (e.g., 5 mi/8 km). In Houston, Texas, many speed limits have km/h signs underneath the MPH signs. These are located near both airports (Bush Intercontinental and Hobby) and in the Texas Medical Center. The U.S. territory of Puerto Rico generally uses American-style (MUTCD) signs with metric measurement on highways, displaying kilometer posts and distances in km, yet speed limit signs are in miles per hour. The Maine Turnpike has metric measurements on its highway signs in addition to customary measurements, partly to assist the large numbers of Canadian tourists who visit Maine's beaches.
  The first signs in metric in the United States were posted on February 12, 1973, on Interstate 71 in Ohio.
  Gasoline and diesel fuel are sold by the U.S. gallon, and fuel economy is rated in miles per gallon (MPG). In most other countries, using the metric system, fuel consumption is measured in liters per 100 kilometers or kilometers per liter. The European Union official standard is liters per 100 km.  Stations in Point Roberts, Washington, a short, noncontiguous peninsula that is accessible from the rest of the state of Washington only by crossing into Canada, sells its gasoline from pumps calibrated to dispense in liters. Gasoline is dispensed in liters in Puerto Rico. Automobile crankcase oil is sold by the quart, antifreeze by the gallon, brake fluid by the ounce (fluid), and air conditioning refrigerant by the ounce (mass). Windshield wiper replacement blades are measured in inches.
  Domestic airline flights are assigned altitudes in feet and measure speed in knots. Nautical charts show depth in fathoms and use the nautical mile for distance. (One minute of arc of latitude at the radius of the earth at sea level was the standard for one nautical mile until about 1929. The nautical mile is now defined as exactly 1,852 meters.) Railroads use the standard gauge of 4 feet ​8 ¹⁄₂ inches, as does most of Europe (where it is expressed instead as 1,435 mm). 
  U.S. Federal Motor Vehicle Safety Standard 101
caption: "Valencia Road, Tucson Airport: 500 m" on Interstate 19
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