The US uses miles because the British colonies adopted them in the 1600s and Congress never made the metric switch mandatory. Multiple conversion acts (1866, 1975) made metric legal but voluntary. Industry inertia, infrastructure cost, and lack of political demand kept the US on imperial units even as the rest of the world standardized on kilometers in the 1800s and 1900s.
The mile, from Roman origins
The word "mile" comes from the Latin "mille passus", a thousand paces. A Roman pace was two steps (left foot, then right foot, back to left), about 5 modern feet. So a Roman mile was about 5,000 feet, somewhat shorter than the modern English mile of 5,280 feet. The Roman mile traveled with the Roman legions across Europe and was the basis for medieval European distance units.
Each European region developed slightly different mile lengths over centuries. England's mile drifted until 1593, when Queen Elizabeth I's Weights and Measures Act standardized it at 5,280 feet (1,760 yards). This is the statute mile that the British colonies, and later the US, inherited.
The metric system arrives in France
The French Revolution produced the metric system in the 1790s as an Enlightenment-era project: replace all the arbitrary feudal-era units with a rational, decimal system based on natural references. The meter was defined as one ten-millionth of the distance from the equator to the North Pole along the Paris meridian. The kilometer (1,000 meters) became the natural unit for road and travel distances.
France adopted the metric system officially in 1799. Napoleon's armies spread it across Europe during the early 1800s. The newly independent Latin American countries adopted metric in the mid-1800s. The British Empire (including India) eventually adopted metric in the 20th century. By 1900, most of the world used kilometers for road distances.
The US considered conversion. Repeatedly.
Thomas Jefferson, while Secretary of State in the 1790s, proposed a decimal system of weights and measures inspired by the new French metric system. The proposal went nowhere in Congress. A ship sent to France to bring back metric standards was famously captured by pirates, delaying the question further.
Congress passed the Metric Act of 1866, which made metric legal in the US but did not require it. The act protected merchants who chose to use metric but allowed everyone else to continue with imperial. The result: industry largely ignored it.
A century later, President Gerald Ford signed the Metric Conversion Act of 1975. The act created the US Metric Board to coordinate voluntary conversion. Highway signs were experimentally posted with both miles and kilometers in some states. Nutrition labels added gram measurements. School curricula taught metric alongside imperial. But the board had no enforcement power, and without mandatory rules, US industry had little reason to absorb the conversion cost.
President Ronald Reagan abolished the US Metric Board in 1982 when no measurable conversion was happening. The legal status of metric stayed (you can sell goods in metric), but the cultural switch never followed.
Why switching is so expensive
Switching the US to kilometers on roads alone would cost estimated tens of billions of dollars. Every interstate, highway, state route, and city street sign would need replacement. GPS units, vehicle speedometers, driver education materials, and traffic enforcement systems would all need conversion. The state-by-state nature of road infrastructure in the US (each state runs its own DOT) makes a coordinated national switch even harder than a unitary government would face.
Beyond infrastructure: every map, every printed atlas, every restaurant menu listing "located 3 miles from the airport", every real estate listing using "10-minute drive" measurements assumed in miles, would need either replacement or dual labeling. The cultural transition would take a generation.
The 1959 international yard agreement
One thing did change in 1959: the international yard and pound agreement standardized the inch as exactly 25.4 mm worldwide. As a consequence, the mile became exactly 1,609.344 meters everywhere. Before 1959, US and UK miles differed by tiny amounts (parts per million); engineers working at high precision had to specify which mile they meant. Since 1959, an international mile is universal.
Where the US already uses kilometers
US science, medicine, military, automotive engineering, and aerospace all use metric internally. The US military expresses ranges and distances in kilometers in nearly all contexts (NATO interoperability). The auto industry shifted to metric bolts and fasteners in the 1970s and 80s. Scientific publications use SI units. Medical doses are in milliliters and milligrams.
What stays imperial: road distances (and speed limits), maps for consumers, real estate (square feet), aviation altitude (worldwide), body height and weight, weather temperatures, recipe measurements, and consumer product packaging.
The likely future
The US will probably not formally convert in any foreseeable timeframe. The political cost is high, the economic cost is in tens of billions, and there is no public demand. What is happening gradually:
- Younger Americans are more comfortable with km from international travel and the internet.
- Dual-display speedometers showing both mph and km/h are standard in most modern cars.
- Race events use 5K and 10K formats even in the US.
- Engineering and science contexts have already converted.
- Recipes and product packaging increasingly dual-label in both units.
The pragmatic end state: the US becomes culturally bilingual, with miles still on road signs and consumer goods, but km available in every context where a global audience cares. Other countries have done the same thing in reverse (the UK uses metric officially but pints and miles survive in everyday life). Coexistence beats either pure outcome.