Certified Public Miles (CPM)

Certified Public Miles is the sum of all miles on all publicly owned roads that are passable with standard passenger cars. This also excludes any roads that may be restricted due to gated areas, unpassable roadways, or any other restricted road. You can also think of CPM as the total length of public road centerline miles. It also includes the Bureau of Indian Affairs (BIA) and tribal delineations, in order to get an accurate count of CPM. By June 15 of each year, the state DOT must provide an annual Certification of Public Miles (23 CFR 460) signed by the current Governor or his/her designee to FHWA.

A public road is any road under the jurisdiction of and maintained by a public authority, open to the general public, passable by standard passenger cars and not be a gated, private roadway. Public road mileage also includes all roads on the Indian Reservations in the state. However, much of the time, the distinction between public and private roads is defined at the local level and is based on who is responsible for maintenance and record keeping.

Some other important aspects for what designates public roads and what is included in the Certified Public Mileage (CPM), is cardinality and facility type.  Cardinality  refers to the increasing mileposts, typically beginning is the West or South and increasing mileposts in the Easterly or Northerly direction. The opposite of this is referred to as the non-cardinal direction. For CPM, only the cardinal direction of roadways are included. Facility type corresponds to this as well.  Facility type  is the operational characteristics of a roadway. There are 7 different types of facility types, however only facility type 1 and 2 are used in CPM. Facility type 1 is, one-way roads, where traffic is moving in a single directions, and facility type 2 is, two-way roads, where traffic is moving in both directions. Only the cardinal direction in the facility type is used for undivided roads.

Road ownership plays a key role in CPM. Since CPM uses all public roads, it is important to know the ownership in order to have a accurate number for CPM. There are 3 main components to ownership, state owned roads, non state owned and federally owned roads. State owned roads is exactly how it sounds, they are roads that are owned and maintained by the state DOT. Non state owned roads are those owned by local agencies. these use an estimate of mileage for CPM. Federally owned roads are those owned by the Federal Government. This can by native lands, or federal roadways. Each year the FHWA delivers a report for the state agencies on how much federally owned mileage exists.

CPM is required by the Federal Highway Administration ( FHWA ) as an integral way to determine funding for state departments of transportation. It is also used as a basis for apportionment of a fraction of the funds under 23 U.S.C. 402(c) (safety). FHWA uses this CPM data to populate all highway statistics for publication. Along with CPM data submittal to FHWA for publication purposes, it is also necessary to report for several national level reports, required for states to submit. It is necessary for:

•  State Highway System Log (SHS) 
•  Highway Performance and Monitoring System (HPMS) 
•  National Highway System (NHS) 
• All Roads Network (ARNOLD)

Notice in the map above, there is slight change in CPM each year. This can change for a number of reasons. Ownership can be a big player in CPM change, as ownership changes, i.e. federal roads increasing or decreasing, the mileage will change too. Another reason could be change in facility type, if a road changes to a two way street that was not previous, that will increase the CPM.  Functional class  can also change the CPM, if a road that was not previously functionally classified became a classified road, then the CPM would then represent that. While all these affect the overall CPM year to year, CPM can change for a number of reasons and is not limited to just these.

The All Roads Network of Linear Referencing or ARNOLD, is the requirements for spatially accurate Certified Public Mileage. HPMS requires an annual data submission of an all roads network, which can be used to validate a State's road mileage figure. With the ARNOLD requirements, using actual driven miles that account for elevation and other variabilities in roadways are more accurate than calculated or grouped mileage. Because HPMS requires certified mileage for reporting purposes, it is important to consider these variables for verification. ARNOLD also allows for the incorporation of local roads, this way agencies can calculate the mileage on local roads and federal aid roads. This increase can affect funding from FHWA as including local roads will result in an increase of CPM, which directly affects funding for state DOT's.

The other big part of ARNOLD is that, to meet the ARNOLD requirements, dual carriageways must be utilized to represent divided roadways, ideally used with independent mileage calibration. When looking at a roadway using linear referencing, dual carriageways are split up into mile segments. Dual carriageways must maintain a separate and independent mileage per segment, then provide mileage using a conversion factor. This is used for all Interstate mileposts, and allows for calculating the true mileage on both sides of a dual carriageway, using similar measures on both sides. This also means that measures can be assigned to the non-cardinal sides of the road, resulting in a simplified field inventory.

Typically, state DOT's do not have to include local or private roadways in CPM, however with ARNOLD increasing the GIS networks, states are now responsible for supplying the distinction between public and private roads and store this information in the roadway network. While HPMS Submissions does not require private roads to be reported, doing so is incredible beneficial to emergency response. These can be removed for HPMS submittal, FHWA may accept private roads from state submittal. For more information on ARNOLD, read the  ARNOLD Reference Manual .

Since ARNOLD is increasing the GIS behind CPM, this requires Linear referencing for all roads. This unifies the local roads into the CPM and Linear Referencing System (LRS) using GIS. This change makes it possible to retrieve crash data on local roads that previously was not geographically referenced.

There is a lot of collaboration that goes into collecting CPM. It's a synergetic effort between entities owning public road miles such as, State transportation agencies, other State agencies, county, city, town, and other local governments, Native Nations, toll commissions, airports, Federal agencies (including army etc.) and public/private roads.

Still having trouble understanding what Certified Public Mileage (CPM) is or how it is used? Reach out to the Arizona Department of Transportation, Multi-Modal Planning Division for further explanation.

If you would like more information about different variables required by reporting for HPMS, feel free to visit another story map like this, below:

 Federal Functional Classification (FFC) 

 Lane Miles vs. Maintained Lane Miles 

 Highway Performance Monitoring System (HPMS) Overview 

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