What’s the significance of interstate highway route numbers?

Dear Cecil: I’m looking at the road map. The main interstate route down the Atlantic coast is 95. In the New York City area there are spurs off it numbered 495, 295, and 895. Near Washington, D.C., it intersects with the beltway, route 495. The bypass around Philadelphia is 295. This has got to be a pattern, right? Eddie, Hoboken, New Jersey

Cecil replies:

You got it, Sherlock. Let me run through the whole numbering system, which was devised by the American Association of State Highway Officials (and I’ll bet that’s an acronym you don’t hear pronounced too often). Even-numbered routes run mostly east-west, odd ones mostly north-south. Major routes have one- or two-digit numbers, and the really important routes, which form a more or less evenly spaced grid across the country, end in 5 or 0. The lowest route numbers are in the west and south to avoid local duplication of the older U.S. route numbers, which are lowest in the northeast and midwest.

Now, getting down to your question: three-digit numbers are reserved for adjuncts to the major routes. Circumferential roads and beltways have the main route number with an even-numbered prefix; radial and spur routes have an odd-number prefix (usually). It’s simple. Just don’t ever ask me to explain zip codes.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.