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One US Highway Uses Kilometers Instead of Miles

Mile markers line our highways. Like many other things, they were invented in ancient Greece. Originally, they were called “milestones” because they were stone posts engraved with the number of miles to and from the next town. They were made from granite, marble and other materials; they can look a lot like modern day tombstones and originals still exist in many places.

If you’re not familiar with mile markers, they are small signs next to the highway that indicate the distance from the beginning of the highway. If you’ve ever seen small signs that only contain a number, it’s likely a mile-marker.

In the United States, most even-numbered interstates have mile markers that start at zero on the west coast and increase as the highway runs east. Along odd-numbered interstates, which run north/south, the mile-markers start at zero in the south and increase as the highway goes north. In most States, exit numbers are determined by the nearest mile marker number.

In Canada, the system is similar, although kilometers are used for the mile markers and exit numbers. In India, the markers also indicate the total distance between the two cities where the marker is located. Systems similar to these exist in most other countries.

There are some inconsistencies in the United States where exits are numbered sequentially rather than by mile markers. States that number in this way are Connecticut, Delaware, Massachusetts, New Hampshire, New York, Rhode Island, and Vermont.

There is also one interstate where the mile markers are actually labeled in kilometers. Interstate 19 in Arizona, between Tucson and the Mexican border, uses kilometers for mile markers and destination signs. Speeds are still posted in miles-per-hour.

The metric signs were installed in the 1980s when it looked like the United States was converting to the metric system. Complete conversion to metric never happened, but most of the signs along this interstate still stand. Going forward, metric signs will be replaced with US standard unit signs (“miles”) as construction projects along the highway are complete. As of 2009, one such project has been completed.