Navigate the US Interstate System Like a Pro Ever wondered how America’s Interstates are numbered? Here’s the scoop: Main east-west routes are even numbers ending in 0 (like I-10 to I-90), running horizontally. North-south are odd numbers ending in 5 (I-5 to I-95), going vertically. Lower numbers mean farther south/west; higher ones are east/north. For spurs: Three-digit highways (e.g., I-295) match the last two digits of their parent route. Even first digit? It loops back. Odd? It’s a one-way b