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did you know?

how the interstate system works

Long before the information superhighway made our lives easier, the interstate superhighways were doing the same thing. In this page, we'll explain the complex interstate system, including how interstates get their numbers and which states have the most interstate mileage.

How interstates are numbered

Odd or even interstates

road

Before GPS and smartphones, there were maps. Sadly, these weren't the easiest documents to manage when you were lost on a busy, unfamiliar road. That's why officials used interstate numbering to make it easier for drivers to quickly decipher their direction. They reserved even numbers for interstates running east-west and odd numbers for ones running north-south.

Low or high interstates

To give wayward motorists even more direction, highways were numbered in graduating order from south to north and west to east. For instance, interstates far south or west typically have low numbers — like I-10, the southernmost transcontinental, or I-5, which skirts the Pacific — while those far north and east are higher — like the northern I-90 or the Atlantic-adjacent I-95.

3-digit numbers

If an interstate has 3 numbers, it's an auxiliary route, an extension of a major interstate built to handle the needs of a particular urban area. The first digit of these roads (or the first 2 if the main interstate has only one number) are prefixes, and the second (or third) number tells you the main highway from which it branches off. For instance, I-405 is an extension of I-5 and I-295 is an extension of I-95. If the prefix is even, the auxiliary road connects to the interstate on both ends — beltways and bypasses. If the prefix is odd, it connects on only one end — a spur.

Auxiliary routes fill one the following roles:

  • Beltway — a continuous loop around an urban area, with the principal interstate running through the middle (most common in the east)
  • Bypass — branches off from the main interstate to wind around (or through) an urban area, then rejoins the main interstate down the line, like California's I-405
  • Spur route — connects a main interstate with another highway or population center

Interstate facts

Interstate myths and mysteries

For whom the road tolls

Dwight Eisenhower, the "father" of the interstate system, actually envisioned toll roads making up the entire system. That's how he was going to pay for it.

Luckily, it didn't work out that way. So why do you have to pay to drive on certain interstates while other stretches of the country are free?

For the most part, today's toll roads were built before Congress started approving federal funding for our interstates. So, essentially, if you're driving on a toll road, that's a stretch of pavement your (or your ancestors') tax dollars did not help build. If you're riding toll-free, on the other hand, you have yourself (and your hard-earned dough) to thank.

Interstates in Hawaii, Alaska, and Puerto Rico

Despite being cut off from the continental 48 states, Alaska, Hawaii, and the U.S. territory of Puerto Rico have interstates, too. Hawaii and Alaska each have 4 interstates, and Puerto Rico has 3.

We know what you're thinking: By definition, doesn't an interstate have to connect multiple states? Not exactly. The name is just a name, and what's in that anyway? While it would make more sense to call the highways in these 3 places intrastates, it's worth noting that there are routes within the contiguous 48 that also receive the "interstate" designation despite never crossing a state line (like the aforementioned I-97).

Roads, as the old adage goes, are named from whence their funding came. And with Hawaii, Alaska, and Puerto Rico's highways being funded by Eisenhower's interstate initiatives, linguistic and geographical inconsistencies were laid aside in favor of inclusion and consistency.

Interstates as landing strips?

According to a popular myth, interstates were built with intermittent straight stretches so planes could land on them if necessary. This is intriguing, but false. While planes have been known to land on interstates in emergencies, the roads were never designed with that purpose in mind.

Related links

History of the interstate
Now that you know how interstates work, find out how (and why) they were built.

Speed limits
Find out who decides whether you can drive 55.