There's a fun bit of trivia to that; it's actually I-84 that's the exception. Back when the Interstate Highway System was created, it was designed with intenti

There's a fun bit of trivia to that; it's actually I-84 that's the exception. Back when the Interstate Highway System was created, it was designed with intentional forks in the road that will both go to the same start and end points but get there in different ways. These forks shared the same number but had directional indicator suffixes. One example of this was I-80S and I-80N. Eventually, AASHTO decided that system was confusing, so they decided to eliminate most directional suffixes and assign unique numbers to each highway. As such, I-80S became I-80, and I-80N became I-84. They would've used I-82, but that highway already existed, and they decided against resigning an existing non-suffixed interstate. Some directional suffixes continue to exist, but only for interstates that fork for the duration of a single metropolitan area with two principal cities. The best-known example of this is I-35, which forks into I-35E and I-35W twice, once in Dallas-Fort Worth and again in Minneapolis-St. Paul.