Category: State

CARxR 1.07: Highway Numbering: State Highways and Post MilesCARxR 1.07: Highway Numbering: State Highways and Post Miles

In this episode, we explore numbering systems in state highways. It is the start of a four part miniseries on highway numbers in California. Specifically, this episode explores the rhymes and reason for the assigning of signed route numbers to highways with the state shield. This includes looking at the patterns in those numbers, and how the numbering system stands today after the Great Nenumbering. The subsequent episodes in the miniseries will explore the numbering system of US Highways, the numbering of and the history of California’s Interstates, and the signed county route system.

The episode also explores another numbering system on state highways: Post Miles. As opposed to sequential mileage numbers as is found in other states, California uses a system called post miles that identifies points along a highway using a combination of a county and a mile point from the southern/western county line, possibly with clarifying prefixes or suffixes. We discuss this system is good detail.

Our interview is with Andy Richardson, who retired from Caltrans as a Subject Matter Expert in Geographical Information Systems, Linear Referencing Systems, and Postmiles. Andy worked as a GIS specialist for the State of California since 1988, including Caltrans between 2001 and his retirement in 2021. In his last years at Caltrans, he implemented the Department’s current Linear Referencing System.

Here are some relevant links (I may edit this to add more links in later):

Here’s the link to the show:

And here’s the link on Anchor.FM:

Episode 1.08 will continue the numbering miniseries, and we’ll be looking at numbers on the US Highway system (black and white shields): what are the numbering patterns and algorithms, and how route numbers are approved. We’ll also look at the history of US highways in the state. Looking forward, Episode 1.09 will explore Interstate numbering and all things Interstate; and 1.10, will look at the County Sign Route system. As always, we’re looking for interviews for future episodes. If you have any leads, please contact us:

  • For 1.08: Someone familiar with AASHTO’s role in highway numbering.
  • For 1.10: Someone familiar with the county sign route system, either from the Caltrans office that coordinates the program or the a county Public Works person.

We encourage you to explore our back catalog through this site or our Anchor.FM home.

Episode 0.02 – Route 105 – It Only Takes a Century (Sample Episode)Episode 0.02 – Route 105 – It Only Takes a Century (Sample Episode)

This is a sample episode to show what the Route by Route format will be, as we don’t expect to start exploring routes until the second season. The first season will be mostly background: The history of the California State Highway system, explorations of how highways are numbers, exploring how highways are named, and the entities you’ll be hearing about in the episodes, such as the CTC. But we know you want the routes, so we decided to do a sample one for you. This episode focuses on Route 105.

The episode covers the following topics:

  • Legislative Route 105
  • 1965-1968 Interstate 105
  • Post 1968 Interstate 105 – The Century Freeway
  • Why I-105 doesn’t end at I-5
  • The construction and the consent decree
  • Names on the Highway
  • Significant Problems
  • Major Current Projects

If you want to see the “script” we used, we use a presentation format. Click here for the PDF.

For more information:

Here’s the link to the show:

Here’s a link to the show on

We welcome your comments on the episode. We’re also looking for someone to donate or provide more appropriate theme music.