Post Mile Designations
Many California historical landmarks and their plaques can be located by referring to their post mile designations. Such designations are easily visible to highway users on those little white, paddle-shaped signs that line state highways.
The California Department of Transportation many years ago for highway planning, management, and maintenance purposes set up post mile designations. They are still in use today and are also used by law enforcement personnel to record accident information and assist disabled vehicles requiring towing assistance.
Post mile signs include mileage within a particular county, running from south to north and usually from west to east. For example, post mile designations along U.S. Highway 395 in Inyo County start with 0.0 at the southern county line and increase to the north.
Post miles are denoted in tenths of a mile with signs posted at strategic spots along the road such as intersections and bridges, and at the even post mile locations such as P.M. 5.0, 10.0, 20.0, etc. There may actually be more or less than an exact mile of distance within any given post mile length. When this is true, an "R" designation is used (R 11.3, for example. This indicates that some kind of realignment has taken place within the original post mile limit established for the highway. This allows flexibility when highways undergo minor changes and precludes redesignating the entire route within the county.