Questions
Frequently asked.
The questions that come up most about how the Interstate system is numbered and built.
Why do some Interstate numbers repeat in different states?
Three-digit auxiliary numbers are tied to a two-digit parent that passes through many cities, so the same number is reused near each city. There are also a handful of duplicate two-digit routes, such as two separate I-76 and I-84 routes, that exist for historical reasons.
What is the longest Interstate?
I-90 is the longest, running about 3,020 miles from Seattle to Boston. I-80 is the second longest at roughly 2,899 miles.
What is the shortest two-digit Interstate?
I-97 in Maryland, at about 18 miles between Annapolis and Baltimore, is usually given this title, and it is the only two-digit Interstate contained entirely within a single county. Newer routes that are still being built, such as the signed segment of I-87 in North Carolina, are shorter for now.
How do I read a three-digit number?
The last two digits name the parent route. The first digit tells you the job: an even first digit usually means a loop or bypass, while an odd first digit usually means a spur or connector.
Why does I-99 sit in the wrong place?
Interstate numbers usually rise from west to east, but I-99 in Pennsylvania sits far east of many lower-numbered routes. Its number was written directly into federal law rather than assigned through the normal process.
Are Hawaii and Alaska really on the Interstate system?
Yes. Hawaii's H-1, H-2, and H-3 are signed Interstates funded by the federal program. Alaska and Puerto Rico also have federally recognized Interstate routes, though Alaska's are not signed with shields.
Why is I-19 signed in kilometers?
I-19 in Arizona runs from the Mexican border to Tucson and was signed in metric units during a period when the United States considered a wider switch to the metric system. The signs were kept.