It’s a Numbers Game

Spearfish, South Dakota

We traveled just a short while today from Hill City to Spearfish which will be our base while touring the northern Black Hills area.

There are lots of state highways to choose from we travel, and today was no exception.

I’m finally figuring out how those highway numbers work. Even numbered roads run east and west; odd numbered roads run north and south. Most drivers know that…

But what I recently read taught me lots about how the numbering system works. And yep… I’m a data geek who is fascinated by sh*t like this! 750px-FHWA_Auxiliary_Route_Numbering_Diagram.svg Within the continental US, priamary interstate highways are assigned two digit numbers under 100.

Three digit numbered highways can be decoded by separating their first digit and the last two digits.

The last two digits refer to the primary highway the road is a subroute of. The first digit defines what that subroute does.

If the first digit is even, the route goes through or around the city.

If the first digit is odd, the route is a deadend spur into the city.

In example A above, Route 210 is a subroute of Interstate 10 that goes through the city.

Example B  shows two routes. Route 310 is a spur off of Interstate 10. Route 610 is a subroute of Interstate 10 that circles around the city.

Example C shows a combination of the route in Example A and the spur in Example B.

So now the next time you’re playing Trivial Pursuit, you’ll nail the highway numbering system question.

download (5) *Image from Wikipedia.