California uses a postmile system on all of its state highways, including U.S. Routes and Interstate Highways. The postmile markers indicate the distance a route travels through individual counties, as opposed to mile markers that indicate the distance traveled through a state.
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A postmile marker is a metal paddle placed along the state highway. Each marker is stenciled with the route, county, and postmile at that location.[1]
According to the Manual on Uniform Traffic Control Devices, they, however, encouraged the use of mileposts like all the other states and stated that:
| “ | It is recommended that [mileposts] be simple panels without border, having 4 to 6 inch numerals and no other legend. The numerals should be black on a white background or white on a green or black background with suitable reflectorization.[1] | ” |
One of the common formats for postmiles are located on a freeway on bridges over cross streets. According to Caltrans, it displays the name of the bridge, the county and route number, and the postmile.[2] The postmile is often painted onto the piers and/or abutments of bridges and overpasses.
These are the white markers placed at one-mile (1.6 km) intervals, with additional markers placed at significant features along the highway such as bridges and overpasses, junctions, or culverts. The markers are roughly the same size as a standard milepost used elsewhere, but they are white with black text.
Postmiles are also shown on callboxes. A blue placard is mounted on each of the state's callboxes, the top of which shows what county one would be in, and on the bottom, it shows the 2-letter county abbreviation, along with the route number and the location's postmile (in tenths of miles).[2] Postmiles on callboxes are approximate, because of a convention that all callboxes on one side of the road are assigned even numbers, while all those on the other side are assigned odd numbers.
Alphabetic prefixes on postmile markers and bridges differ from callbox prefixes because the callbox system is maintained by each county, while Caltrans maintains postmile markers and bridge signs.[3] The following table lists postmile and callbox prefixes by county.
| County | Bridge | Callbox[4] | County | Bridge | Callbox[4] |
|---|---|---|---|---|---|
| Alameda | ALA | AL | Orange | ORA | OR |
| Alpine | ALP | AP | Placer | PLA | PL |
| Amador | AMA | AM | Plumas | PLU | PM |
| Butte | BUT | BU | Riverside | RIV | RV |
| Calaveras | CAL | CV | Sacramento | SAC | SA |
| Colusa | COL | CO | San Benito | SBT | ST |
| Contra Costa | CC | CC | San Bernardino | SBD | SB |
| Del Norte | DN | DN | San Diego | SD | SD |
| El Dorado | ED | EL | San Francisco | SF | SF |
| Fresno | FRE | FR | San Joaquin | SJ | SJ |
| Glenn | GLE | GL | San Luis Obispo | SLO | SL |
| Humboldt | HUM | HU | San Mateo | SM | SM |
| Imperial | IMP | IM | Santa Barbara | SB | SR |
| Inyo | INY | IN | Santa Clara | SCL | SC |
| Kern | KER | KR | Santa Cruz | SCR | SZ |
| Kings | KIN | KN | Shasta | SHA | SH |
| Lake | LAK | LK | Sierra | SIE | SI |
| Lassen | LAS | LS | Siskiyou | SIS | SK |
| Los Angeles | LA | LA | Solano | SOL | SO |
| Madera | MAD | MD | Sonoma | SON | SN |
| Marin | MRN | MR | Stanislaus | STA | SS |
| Mariposa | MPA | MP | Sutter | SUT | SU |
| Mendocino | MEN | MC | Tehama | TEH | TE |
| Merced | MER | ME | Trinity | TRI | TR |
| Modoc | MOD | MO | Tulare | TUL | TU |
| Mono | MNO | MN | Tuolumne | TUO | TM |
| Monterey | MON | MY | Ventura | VEN | VE |
| Napa | NAP | NP | Yolo | YOL | YO |
| Nevada | NEV | NV | Yuba | YUB | YB |
Listed in miles, postmile values usually increase from south to north or west to east depending upon the general direction the route follows within the state. The postmile values increase from the beginning of a route within a county to the next county line. The postmile values start over again at each county line.
Enforcement officers, maintenance forces and others use the postmile markers in the field to locate specific incidents or features with reference to the postmile system.
On some stretches of road, the following prefixes may precede the mileage on a postmile marker:[1][2]
| Prefix | Description |
|---|---|
| R | Realignment |
| M | Realignment of a prior realignment |
| N | Realignment of M mileage |
| C | Commercial lanes paralleling main highway |
| D | Duplication (due to meandering county line) |
| G | Reposting duplicate PM at end of route |
| H | Realignment of duplication |
| T | Temporary connection |
| S | Spur |
| L | Overlap due to correction or change |
The postmile system is the only route reference system used by Caltrans. In 2002, the State started the Cal-NExUS program, which would create a uniform exit numbering system for freeways.[5] Included was a pilot program for the placing of mile markers along rural freeways. Currently, three freeway segments are apart of the experimental program: the Route 14 Freeway, the Route 58 Freeway in Kern County, and State Route 180 in Fresno. Caltrans has not decided if the program will be expanded to all rural freeways.[6]
The states of Nevada and Ohio use reference markers very similar to California's postmile markers. Like California, these two states record mileages through individual counties in their respective route logs. Ohio's system is nearly identical to California's with its reference markers listing the route number, 3-letter county abbreviation, and mileage through the county. The Nevada system is also similar, utilizing 2-letter county abbreviations. However, Ohio uses standard mileposts in addition to reference markers on freeways, while Nevada uses mileposts in conjunction with postmile panels on major Interstate highways only. Also, all non-Interstates in Illinois have markers showing mileage from the western or southern border of the county.