The interstate system in the US, and to a similar extent, the US numbered highways, follow a pattern. Once you understand how the system works, finding your way across the country becomes much easier.
Even-numbered interstates run east-west, with lower interstate numbers in the south (e.g., I-10) and higher numbers in the north (e.g., I-94). Odd-numbered interstates run north-south, with lower numbers in the west and higher numbers in the east.
All main interstates are 2 or 3 digit numbers from 4 to 99. An interstate ending with a “0” or a “5,” such as I-5 or I-90, designates an interstate that runs coast to coast or from Canada to Mexico. A 3-digit interstate (e.g., I-290, I-355, I-275) is called a spur. If the first number is even, it is a spur around or through a major metropolitan area and will reconnect with the parent interstate. If the first number is odd, it’s a spur into the metropolitan area and will not reconnect to the interstate.
Of course, there are a few exceptions.
In Virginia, I-77 south and I-81 north run together for a few miles. I-99 is broken up into different sections that are nowhere near each other, as is I-66. I-69 is a north-south interstate from Lansing, MI, to the Mexican border, with some sections still under construction south of Indianapolis. However, from Lansing to Port Huron, it is an east-west interstate and is labeled as such.
Since its beginning in 1955, the interstate system has been continually expanding. Work is ongoing along the future I-69 corridor to connect Sarnia, ON, to Hidalgo, TX, to become a major Canada-Mexico trade corridor in an attempt to relieve some of the traffic from Detroit-Windsor.
With the US highways, even numbers run north-south, and odd numbers run east-west. However, the lower numbers are to the north and east.
