Answer Man

What’s in a number? The way we name interstate highways

How do they number those interstates?

The Answer Man navigates the complexities of the U.S. interstate highway numbering system, including why there is no Interstate 50. The answer to that one runs right through the heart of the metro-east area in Illinois.
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The Answer Man navigates the complexities of the U.S. interstate highway numbering system, including why there is no Interstate 50. The answer to that one runs right through the heart of the metro-east area in Illinois.

Q: While looking at a U.S. road map the other day, it struck me as odd that there is an Interstate 10, I-20 and so on up to I-90, but there’s no I-50. Why?

L.E., of Lenzburg

A: If you’ve never studied it before, the interstate highway system has an ingenious numbering plan. It’s the same one they used to number the older U.S. highways — but a mirror image. So, while U.S. 1 is on the East Coast, I-5 is on the West Coast. Moreover, interstates with odd numbers generally run north-south and those with higher odd numbers are increasingly farther east — I-95, for example, runs down the East Coast while U.S. 95 runs from Idaho to Arizona.

On the other hand, even-numbered interstates run east-west and increase in number from south to north, so I-10 runs from the Pacific Ocean to Jacksonville, Fla., while I-90 runs from Boston to Seattle. (U.S. 90 runs from Jacksonville Beach, Fla., to Van Horn, Texas.)

So why is there no I-50? When formulating the numbering system, planners suggested that no state should have a U.S. highway and interstate with the same number. The idea was to avoid confusion if someone was told to take, say, Route 20. As it turns out, I-50 would have to be built in the middle of the country, the same area where U.S. 50 runs from Ocean City, Md., to West Sacramento, Calif — including the metro-east. Since some states likely would wind up having both, 50 has yet to be used for an interstate route.

Before anyone points out that there is an I-74 and U.S. 74 in North Carolina, let me point out that the two are actually one and the same at some points and, thus, officials thought they were less subject to be confused, I suppose.

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