Why does Interstate 880, intersect with 680 and 280? These are the types of questions California drivers are left to ponder, often while sitting in a congested lane on those highways.

But if you’ve never come up with the answer as to why the 405 and 805 were set down so darn close to I-5, the answer might be simpler than remembering how many passengers you need for the carpool lane.

Caltrans directed us to Chapter 20 of the Highway Design Manual. There you’ll learn that our major north to south traffic arteries on the Interstate Highway system are given odd numbers. Our east-west routes typically have even numbers. All of those other – typically shorter – three-digit highways are named because of their links to the primary routes.

That means the 405 and 805 get their names because they branch off from I-5, and the 710 gets its name from its link to I-10.

But those three-digit routes have another naming convention that provides even more information about them, such as whether the route is a direct connector of highways, or a more looping path to service cities.

“Routes with three-digit numbers, the first of which is odd, are interstate spur routes. For example, I-110 is a spur route off of I-10. Routes in three-digit numbers, the first of which is even, are loops through or belt routes around cities.”

That’s why so many California highways around big cities carry even numbers. You can see that in the Bay Area, where 880, 680 and 280 make indirect paths around cities as they branch off from I-80.

Caltrans Map of Interstate Highways in the Bay Area (Credit: California DOT)

Conversely, you can see a more direct spur between Yolo and Solano counties, where I-505 starts with an odd number because it cuts through farmland to connect I-80 and I-5.

Caltrans map of the Interstate Highways (Credit: California DOT)

Further, you may have noticed that I-5 runs down the Washington, Oregon and California, while I-95 is in the east running from Maine to Florida. This too is by design. Generally, the lower interstate numbers are in the west and south, growing larger as you creep across the map.

But, of course, the 765 miles of I-5, and 242 miles of I-10 in California are just small fractions of the massive Federal Interstate Highway system. Those two roadways run from Mexico to Canada and Santa Monica to Jacksonville respectively.

The total network crosses 46,876 miles, according to the Department of Transportation – that’s enough to circle the globe twice.

Now that you know how the highways got their numbers, you can go back to arguing with your friends from the other end of the state whether it should be called “the 101” or just “101.”