Aside from congestion, cost, and urban blight, there are many things to like about the US interstate freeway system. For example, the naming scheme. Many people apparently don't realize this (to my surprise), but freeways are named according to conventions that can tell you, the driver, something about the road you're driving on.
Here are some general rules. (They don't apply in every case. There are other subtleties as well.)
Two-digit (primary) routes
- Numbers are intended to be unique.
- Even numbers run east-west.
- Odd numbers run north-south.
- Freeways divisible by 10 (I-10, I-90, etc.) represent major E/W freeways. The lower the number, the further south the road. I-10 runs from Los Angeles to Jacksonville, FL. I-90 runs from Seattle to Boston.
- Freeways divisible by 5 (I-5, I-95, etc.) are major N/S freeways. The lower the number, the further west. I-5 runs from Canada to Mexico through Seattle and Los Angeles. I-95 runs from Maine to Miami.
Other two-digit routes
Two-digit freeways not divisible by 5 or 10 (I-84, I-88, I-76) are often spurs that link other interstates, often older roads renamed. Different numbers exist in order to avoid duplicated numbers when possible.
Three-digit (auxiliary) routes
These are routes that take off from and sometimes return to a primary route.
This excellent graphic from kurumi.com summarizes these conventions:
- Numbers are intended to be unique within a state. For example, there is theoretically one (each) I-405 in CA, OR, and WA.
- An even starting number (I-405, I-225) means that the route meets an interstate at both ends.
- An odd starting number (I-195) meets an interstate at only one end.
- I-4xx numbers are often bypasses that touch the same freeway on both ends.
Here are some examples:
I-405 in Seattle connects to I-5 at Burien and Lynnwood.
Denver has I-225 and I-270, which connect to I-25 and I-70. Denver also has I-76, which connects I-70 and I-80.
Makes sense? Now you know why I-90 and I-5 meet in Seattle. Need more? Here ya go: