Q: Interstates That Don’t Travel Between States?

Federal funding is behind "interstate" designation


From I-45 to 635 and 820, North Texans are wondering why some of our most notable freeways are called interstates -- when they don't leave Texas at all.

At least a dozen viewers asked the question, “Why is I-45 an interstate, when it doesn't go out of the state?” The confusion isn’t isolated to just our Dallas to Houston route, others, like I-27 in Lubbock, don’t leave the state either. In reality no interstate has to actually travel between states.

Started in 1956 the Dwight D. Eisenhower National System of Interstate and Defense Highways, or Interstate Highway System for short, was designed to aid travel throughout the nation, be useful in emergency situations for evacuations or troop deployment, and to assist transportation of goods. Though the system built major roadways that run through multiple states, the “interstate” term refers to the fact that the highways are funded federally with money shared between the states.

Hawaii’s H-1, for instance, was built with federal money from multiple states, making it an interstate highway, even though it runs only on the island of Oahu.  Even short routes can be interstates, with the shortest being the 1.06 mile long I-375 in Detroit, Michigan.

If you’ve seen the red and blue signs for 635 and 820 and wondered, why are they interstates, we have the answer.

Both freeways are auxiliary interstate highways. 635, for example, is one of 6 auxiliary branches of I-35. While some branches actually do travel between states, like I-435, which travels between Kansas and Missouri around the Kansas City area, what we know as 635 is Texas only. Interestingly enough, an I-635 also exists in Kansas City.

Auxiliary interstate highways, sometimes called “three-digit interstates,” are numbered based on their connection with a major interstate they spur from. For instance, our other three-digit interstate, 820, spurs off of I-20.

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