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Numbering Conventions:
Post Miles


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Post Miles

[Postmile Sign]The postmile is the means by which California tracks highway mileage. The postmile starts at zero at the western or southern end of the route or at the western or southern boundary of the county through which the route is traveling. Mileage increases as you travel north or east and then restarts at zero at the next county line. Postmiles are used as a reference system to aid in the planning, design, construction, and operation of the state highway system. The postmile marker is a black on white marker displaying the route number, county, and mileage to the nearest hundredth of a mile (see the illustration to the right). The mileage displayed on these markers is based on the state highway routing in place on January 1, 1964, with adjustments due to truncation or realignment. The postmile marker was not intended to provide motorist information or guidance.

Postmiles show milage on a per-county basis. There is no one site that shows the overall accumulated statewide mileage on each route. There are highway logs that show the mileage accumulated by district, but those are not available through public websites. However, an alternate source is the Caltrans Traffic Volumes Handbook, which shows postmiles, based on the highway logs. By totaling up the postmile values for each county, you should come up with a rough mileage estimate.

Within any given post mile length, there may actually be more or less than an exact mile of distance. Sometimes, there is a letter before the milage; these have the following meanings:

When a highway is realigned, there will be dual post-mile markers at the point of realignment. They will have the normal information as to highway number, county, but will have "AH" and "BK" designations that accompany the mileage. Something like AH 32.67 on one and BK 34.29 on the other. These designations stand for "ahead" and "back." So, the realignment (in the example) shortened the distance by 1.62 miles.

Post mile are posted in a variety of formats:

In some counties, the designation "MTN" is used instead of the county code. This indicates a mountain road. Only a few counties do this, such at Tulare county.

Mile Markers

[Standard Mile Marker]California does not regularly use mile markers (although a few are installed as either an experiment or for special purposes), as are found in other states. These modern markers, as illustrated to the left, are specified in the MUTCD. California may explore adding this once the current exit numbering project is completed, but even if they are added, they will only be for the convenience of the driving public. For primary milage purposes, the post mile system described above is used.

Here's a history of mile markers in California, prepared by Joe Rouse of Caltrans. At the time of this writing, Joe was in charge of the exit numbering program for Caltrans.

It is unclear when the concept of the milepost was introduced. The 1961 MUTCD encouraged the use of mileposts.

"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."

Special guidance was given for mileposts on Interstate routes:

"On the Interstate system it is prescribed that Mileposts shall be installed...on both sides of the highway, with zero mileage at south or west State lines, or at junctions where routes begin. The mileage numbering is to be continuous within any State. The Milepost panels are green, with 6-inch white reflectorized numerals."

Photo of G61RIn January 1965, the Division of Highways (later to become Caltrans) introduced a mileage marker for use on Interstate freeways, in keeping with the provisions given in the MUTCD. This occurred after publication of a signing policy review in 1964, which considered several design alternatives for mileposts. One of these was an oval "paddle" design, based on one then being used in Michigan, which would have had the (Series D) numerals (whole miles only) ranged horizontally on an oval blank. A good picture of the spread in milepost designs is given by an I.I.H.S. publication, 'Mileposting' (1967), which has pictures of typical mileposts in each state then using them. Caltrans selected the G61R design, a white on green marker similar in size and shape to the mileage marker. It displayed mileage to the nearest tenth of a mile, with 6-inch whole numbers and a 4-inch decimal. The marker gave no other information and had no border. [See the picture to the right, courtesy of Jason Elliot of Oregon Roads and Reno Roads] The policy for the G61R was as follows:

"Use the G61R to indicate the mileage on Interstate routes from the south or west termini. These signs should be placed at 0.2-mile intervals in urban areas, 0.5 in suburban and one mile intervals in rural areas."

The G61R was actually developed in 1960. There were two versions. The G61R-1 had white 6 inch whole numbers and a white 4 inch decimal on a green background and no other information. The G61R-2 was identical to the G61R-1 but it added a white 3 inch county abbreviation at the top, much like what we see on California postmile markers today. If the mileage had more than 3 digits, the county abbreviation was to be removed. The G61R-2 was not to be used on Interstate routes.

G61RAn example of a G61R (illustrated at the right, if you couldn't see the one above) may be seen along I-80 travelling east towards Reno from Truckee, at about 11 miles from the California-Nevada border on the California side, there is one of the original mile marker signs. This sign has a dark-green background with darkened text and reads 2080. The numbers on the side are rotated the same direction and way as modern postmiles. There is/was another along I-80 eastbound in Placer County, between PM 36.0 and 37.0: it reads 1430. According to Eric Buchanan's Highway Photo Page, there is another one around mile 155 (probably around PM 48.0 Placer or so) as well as one on BR 80 "just past 99 south."

In 1970, FHWA mandated the use of milepost exit numbering on Interstate freeways. The 1971 MUTCD required the use of milepost markers and milepost exit numbering on all freeways and introduced a standardized milepost marker. That form of G61R was removed from the state Traffic Manual in 1971. There was no explanation given for its removal, although it is most likely due to the fact that it was not in compliance with the MUTCD. A review of the sign specs showed that the G61R was actually revised twice in 1971. The first revision, dated March 10, 1971, changed it into a 36 inch by 28 inch panel with white letters and border and green background. It had the text MILE at the top, with the mileage placed horizontally, not vertically. This version did not last long. There is a handwritten note on the spec sheet that says "Obsolete." A new version of the G61 was developed in December of that year. This version is the same as the white on green mileposts we are familiar with today. There was no revision date for this version of the G61, and no note stating it was obsolete. This implies that it was available for use if the state decided to implement mileposts and exit numbering. It may still remain available for use.

In 1971, the Division of Highways conducted a pilot project on 5 different routes in the Los Angeles area to evaluate the benefits of exit numbering. No milepost markers were installed in this project. 313 signs were installed for 141 exits. A few of these signs remain in place on these freeways, but the numbers are slightly different from the current exit number determinations. The study recommended that exit numbering be installed on all freeways in urban areas in the state. However, there were concerns about the costs of adding and maintaining thousands of new signs on the highway system. There was also the question of whether to convert from the postmile system to a milepost system, which would have required changes to numerous databases. For these reasons, no programs to install additional exit numbering and mileposts were conducted. The study recommended that mileposts be established only on rural freeways, but not urban freeways. This was due to concerns about the accurate placement of the mileposts in urban areas.

In 1999, the California State Senate requested that Caltrans conduct another study regarding the cost of implementing mileposts and exit numbering. In 2001, Caltrans decided to implement an exit numbering program but not place milepost markers. The statewide mileposts and the exit numbering would be determined by calculating the cumulative postmiles of the route. The existing postmile system would remain in place and would continue to be the basis for highway data. The exit numbering system was solely for motorists' benefit. Exit number installation began in early 2002 and continues. The initial completion date for this project was set as November 2004. However, budget constraints have slowed the project, and the deadline has been extended to November 2008, which matches a compliance date for exit numbering in the 2000 MUTCD.

At the time the exit numbering program was implemented, Caltrans was in the process of adopting the 2000 MUTCD to replace the state Traffic Manual. A special California supplement would also be developed for the MUTCD as part of this process. Caltrans eventually adopted the 2003 version of the MUTCD in May 2004. Section 2D.46 of the 2003 MUTCD discusses milepost markers (now known as reference location signs). Section 2E.54 discusses enhanced reference location signs, which in addition to mileage, also show the route shield and cardinal direction. This section also states that milepost markers must be used on all freeways (the enhanced marker is optional). The California Supplement to the 2003 MUTCD changes the mandatory milepost requirement in Section 2E.54 to an option. FHWA did not agree with this change, but approved it as part of the Supplement.

The department's policy is to complete the freeway exit numbering project and then consider the installation of milepost markers. Cost remains the primary issue. If they are installed, it is highly likely that they would be for motorists' convenience only, much like the exit numbering. The existing postmile system will probably remain in place and be the primary reference system for Caltrans usage. Note that California would not be the only state that uses dual mileage systems; Nevada also does this.

Interestingly, there is one recently completed segment of freeway in California that has milepost markers on it. These markers may be found on the new Mojave Bypass, in Kern County on Route 58.


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1996-2004 Daniel P. Faigin.
Maintained by: Daniel P. Faigin <webmaster@cahighways.org>.