The Red Cliff Arch Bridge -- known to CDOT as F-11-T -- is one of the most picturesque spans in the state. Photo from Wikipedia Commons by Hustvedt, computer scientist with a camera.

Confessions of a Road Geek.

I’ve told you before that I am kind of a geek when it comes to transportation. I knew even before the state rebuilt the Evans Avenue overpass at Santa Fe Drive that a SPUI was not an Italian ice cream, but a type of traffic intersection called a Single-Point Urban Interchange.

I know a diamond interchange from a parclo, a turbine from a trumpet. Having grown up in New Jersey, I even know jughandles and circles.

We’ll get back to the serious transportation news in a moment, especially with the legislature starting up this week.

CO 145 over the Dolores River in Montezuma County is designated N-03-F.

You’ve seen them too.

They carry letter-number-letter combinations like E-16-HT or H-02-CA or F-16-DS.

I’ve pondered what could possibly be the system for assigning these letters and numbers, and could not think it through. I am pretty intuitive at seeing patterns and all, but I was stumped.

So finally, I called CDOT.

Thanks to CDOT spokeswoman Mindy Crane, I have the explanation. And from now on I won’t be able to drive around the state without picturing myself on a giant grid.

The first two segments of the three-part designation refer to the grid point on the official Colorado state road map. Seriously. Get the official CDOT foldout highway map. Don’t have one handy? Click this link and you’ll get one.

Along the top and bottom is a series of numbers from 1 through 28. Along the sides are letters running alphabetically downward from A through P. Anybody who’s ever looked up a location on a map knows what comes next. You locate places on the map by referring to the blocks located at the intersections of the appropriate letters and numbers.

Bridges are named for the block on the map in which they are located. Gilpin County, for instance, falls within blocks E-14, E-15, F-14 and F-15. All the state bridges in Gilpin County would begin with one of those combinations.

In metro Denver, the state highway bridges all begin with E or F, between 16 and 18.

Then all of the bridges that are located within each block on the map get the third part of their designation from the chronological order in which they were built. It starts with “A” and goes from there, picking up with “AA” once “Z” is reached.

US 160 bridge over the San Juan River near Four Corners is P-01-G -- grid P-1 being in the southwest corner of the state map.

Using this system, I found the CDOT bridge located farthest to the northwest to be B-01-B, on CO 318 over Vermillion Creek near Dinosaur National Monument. The farthest northeast is a culvert under Interstate 76 just south of the Nebraska state line near Julesburg, labeled A-28-W. To the southeast, it’s bridge number P-26-H, a seven-span bridge on U.S. 287 over Sand Arroyo about 10 miles north of Campo, and the bridge closest to Colorado’s southwest corner is P-01-A, a steel span over the San Juan River just one-tenth of a mile north of the New Mexico line near Four Corners.

The oldest state highway bridge in Denver is the Alameda Avenue bridge over the South Platte River – F-16-BI, built in 1911.