What is the longest U.S. Interstate highway by mileage?

Interstate 90 is the longest U.S. Interstate highway by mileage.

I-90 stretches about 3,099.7 miles from Seattle, Washington, to Boston, Massachusetts, crossing the northern United States. It passes through 13 states, including Montana, South Dakota, Illinois, Ohio, New York, and Massachusetts.

The route was established in 1956 as part of the Interstate Highway System and largely follows the path of older transcontinental roads. Its eastern sections include several toll-road segments, especially from Illinois toward Massachusetts.

A common mix-up is Interstate 80, which also crosses the country and is longer than many major Interstates. However, I-90 is the mileage leader. Interstate 10 is the southernmost transcontinental Interstate, while I-95 is the nation’s longest north–south Interstate.