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05:24 PM
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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.
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Three-digit (auxiliary) routes
These are routes that take off from and sometimes return to a primary route.
- 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.
This excellent graphic from kurumi.com summarizes these conventions:
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:
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