The hidden logic behind U.S. highways: Mind-blowing secrets about how interstates are numbered

Families in the United States will soon be jumping in their cars for the annual summer tradition of the great American road trip. While these days most people likely plug their destination into their onboard GPS mapping system or Google Maps on their phone, in the old days we used to break out the large-scale Rand McNally Road Atlas to chart the route.

Pouring over those paper maps a pattern quickly jumps out when looking at the major highways that crisscross the nation, the majority have just two numbers. However, many don’t really give that a second thought, but a group of people from across the nation spent a number of months over a hundred years ago hammering out a simple numbering system so that travelers would always know they were on the right road.

The hidden logic behind U.S. highways: “It stares one in the face”

Originally, the federal government took little interest in a nationwide road system to connect the country from end to end. Railroads were the main way to transport goods and people over long distances in the early days of the automobile. So, the job was left up to automobile associations and enthusiasts.

Local groups had established over 250 named routes by the mid-1920s. However, these could be confusing for drivers, not least because several of them overlapped, and some would take travelers well out of the way as the sponsors had a vested interest in having them pass through specific locations along the route.

The idea of numbering roads actually dates back to 1918 and started in Wisconsin. But it didn’t spread widely and people, especially their boosters, often preferred names instead. When the federal government finally decided to act, it was realized that a more organized system was needed.

The numbering system was the brainchild of Edwin Warley James, which he presented to his colleagues in August 1925. “As you know, the U.S. is about twice as wide as it is from North to South, and with this I saw a complete pattern of just what I wished,” he explained years later. “It stares one in the face, it is so simple and so adjustable.”

His numbering system used two-digits for main roads in the federal U.S. highway system, with north-south roads ending with an odd number and east-west roads ending with an even number. The first digit in this system increases from north to south for east-west roads and east to west for north-south roads. Zero was reserved for principal coast to coast routes, while one and five were reserved for major roads that go from border to border.

Furthermore, three-digit numbers were assigned to alternate routes and branches with the first digit, or prefix, rising sequentially along the route.

The Interstate Highway System, begun in 1956 and completed in 1992, uses even and odds in the same manner as the federal highway system, but only zero and five are used for principal coast to coast and border to border routes, respectively. Another difference is that the first digit in this system increases from south to north for east-west roads and west to east for north-south roads.

The prefix for a three-digit route in the Interstate Highway System has a different function than the federal highway system. Odd numbers are used spurs or radial roads from the main interstate route while even numbers are used for circumferential beltways around or within urban areas.

In some cases, the main interstate route splits into two when it traverses a major metropolitan area like Interstate 35 through Dallas–Fort Worth, Texas and Minneapolis-St. Paul, Minnesota. In these cases, a letter suffix is added, E (east) and W (west) in the case of I-35 in both metro areas.

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