Taken from Driver’s Ed.com:

Spurs and loops extend from a primary interstate. A spur is a short route that extends to the primary route at only one end. It has a three-digit number that begins with an odd number. A loop is a route that meets the primary route at both ends. It’s three-digit route number begins with an even number. Loops usually bypass cities or congested areas.

In light of that, can someone please explain to me how highways are numbered in the Bay Area?

• Highway 85 is a loop that only has two digits.
• Highway 101 is most definitely a primary interstate, but has three digits.
• Highway 280 runs north south.
• Etc.
• Etc.

See what I mean? It makes no sense.