Why Do Some Speed Limit Signs In The US Use Kilometers As Well As Miles?

A speed limit sign anywhere in the United States serves the same basic function, but there are more differences between signs across the country than you might imagine. In Oklahoma, you can find signs confirming that a speed limit has "no tolerance," while in Texas, some speed limit signs have a red border to boost their visibility to drivers.

In very rare cases, you might even see a speed limit sign that uses a completely different system of measurement than usual. There are a few speed limit signs around the country that display the limit in kilometers per hour (km/h) as well as in miles per hour (mph). According to the U.S. Metric Association, examples of these signs have been documented in many different states, from Maryland and Florida on the East Coast to Washington on the West Coast. As well as speed limit signs, there are a number of highway signs with distances posted in kilometers still dotted around the country.

These signs are often a remnant of a push to get the U.S. to move to the metric system, with the Metric Conversion Act being signed into law in 1975. The new law encouraged businesses and organizations to switch to the metric system, but it didn't mandate them to do so. After the bill was passed, campaigners tried to introduce further bills to guarantee a switchover period or speed up the transition, but government enthusiasm for the idea had already fizzled out. The board that was originally set up to oversee the transition was shut down in 1982.

Signs that were installed during various efforts to encourage the metric switch sometimes still carry their metric markings today. However, virtually all of them also have limits posted in mph as well as in km/h. Some of the remaining signs are primarily kept for their historical value rather than for a practical purpose, although in some instances, metric signs are used near the northern border of the U.S. to aid Canadian drivers.

Today, the U.S. is one of only a few countries around the world that still uses mph to indicate speed limits. The only other major country to use mph rather than km/h is the U.K., although, confusingly, many commercial vehicles in the U.K. have speed limiters and tachographs that operate in km/h. Although many industries and authorities in the U.S. did not make the switch to metric, the nation's auto industry did, and as a result, engine displacements have been measured in liters for many decades now.