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Frequently Asked Questions

Estimating percent truck data

Question: How should percent truck data be reported for HPMS sample sections where there is not site-specific data available?

Answer: Every sample section needs to have percent truck data coded; zero should only be coded if there is no truck traffic on the section or if the percent trucks is less than one-half of one percent (result of rounding to nearest whole percent). Zero should not be coded if the percent of trucks is unknown; an estimate of the value should be used instead. Coding only sections for which there is an actual measured value results in too many sample sections with zero trucks; since the HPMS uses a single expansion factor for all variables, this practice distorts the information resulting from the expanded sample.

Where States are collecting data that results in rounded percent truck values of zero, a note in the submittal comment file is appropriate. This may be the case on high volume routes, especially in urban areas, where the volume of trucks may be significant but their percent of total AADT is insignificant.

The preferred way to eliminate this problem over time is to upgrade equipment used for short count purposes to counters that also provide vehicle classification information. Used where needed for short counts on HPMS sample sections, they will permit reporting measured values.

When it is necessary to use an estimate, the State should determine the best way to estimate percent trucks based on the information available. The most credible method is to assign known site-specific values to other samples that are located on the same route. Other methods include assigning known site-specific values to other samples that are located on similar facilities with similar traffic characteristics that are located in the same geographical area and are in the same volume group; or, assigning known site-specific values to other samples that are in the same functional class and are located in the same area type (rural, small urban, urbanized) with similar travel characteristics. Average statewide values calculated by functional class should not be used.

Supplemental methods and sources may be particularly useful in urban areas; some of these include turning movement studies, origin and destination studies, license plate surveys, design estimates and projections, and MPO/municipal data obtained for other purposes. Short-term visual observation of truck travel on a sample section can also be of help in developing an estimate. The HPMS analyst should enlist the assistance of the State traffic engineering or traffic operations unit in developing percent trucks estimates.

Percent of average daily trucks should be reported as an annualized value. This is consistent with the new TMG that has as a goal of traffic monitoring programs the ability to adjust short-term truck data to represent truck AADT. Until States are able to estimate truck AADT values, percent truck data that best represents average conditions should be reported.

Percent of peak trucks should be reported as the proportion of trucks in the traffic stream during the hour or period of peak total traffic flow on the sample section.

Reporting K Factors

Question: What is a K-Factor?

Answer: The K-factor is the design hour volume (30th highest hour) as a percentage of the annual average daily traffic. An automatic traffic recorder (ATR) for continuous traffic monitoring every day of the year is needed to identify which hour is the 30th highest hour of travel during the year at a given location.

Question: How should K-factor data be developed for HPMS reporting purposes?

Answer: The most accurate way to do this is to have an ATR station on every HPMS sample section. This is very unlikely because of the amount of money and staff needed to maintain and operate that type of system. However, every sample section needs to have K-factor data coded, and an estimate should be provided for sections without a direct measurement; zero coding is not an option for this data item.

Question: How are States developing estimates of K-factors on sample sections without ATR stations?

Answer: States are using a variety of methods to develop K-factor estimates. In general, we encourage the States to use the same procedures for HPMS sample sections that are used to estimate K-factors for project level engineering and design decisions.

Question: What are some of the common estimating methods used by the States?

Answer: There are a number of estimating methods in use, including:

  1. use of K-factors computed for an ATR site for samples having similar road type and traffic characteristics;
  2. use of the highest hourly volume from 48 hour short counts for the sample section as a percent of the AADT;
  3. use of the peak hour volume from short term counts for the sample section as a percent of AADT. States may use either one direction or combined directions to determine the peak hour;
  4. use of available project level information for the sample section, or for a nearby section with similar physical and traffic characteristics. Information from turning movement, volume, and/or classification counts may be used to estimate a peak hour volume as a percent of AADT;
  5. use of default values that adequately represent typical or average values by functional class and State sub-region or urbanized area. The use of average statewide values by functional class should only be used as an interim procedure until site-specific traffic monitoring data is available.

Question: What other methods are being investigated by the States?

Answer: There are a number of estimating methods being used by States that are not common practice and which may require further investigation for applicability to other States. These include:

  1. use of default values, such as functional class versus AADT, determined from a regression analysis of computed K-factors at ATR stations. This may be useful for rural States and for low traffic volume locations;
  2. use of an average K-factor developed from ATR data on a specific route or functional class for an individual urbanized area or a group of urbanized areas (grouped only for analyzing traffic data). Travel characteristics of the HPMS sample location and the ATRs averaged should be similar in terms of number of lanes, percent trucks, and peak direction. This may be more accurate than short term counts because the daily variability is eliminated;
  3. use of average highest hour volume from short term counts as a percent of the AADT at locations grouped by number of lanes actually monitored and by one or both directions (ie: 8 lanes monitored in two directions are grouped to develop an average K-factor for that group). All travel lanes may not be monitored in multi-lane high volume locations with traffic surveillance strategies in place. This may be appropriate for urban, high volume, or multi-lane locations.

Question: Does FHWA endorse any particular estimation method?

Answer: No. States should use K-factors from site-specific traffic monitoring data to the greatest extent possible. Any estimating procedures should make best use of the available information and sound traffic engineering judgment; in addition, they should be validated through the execution of a periodic test program that assesses the quality of the relationship between the estimate and factors computed from measured values. FHWA does not support the use of statewide values by functional class.

HPMS Software

Question: What is the latest version of the HPMS software?

Answer: HPMS Version 7.0 is the most current application for submitting and analyzing HPMS data. This version is a server side application that maybe accessed on the internet. Access is restricted to authorized users. A UPACS account must be established and permission must be granted through the FHWA Field Division in the States.

HPMS Version 6.1 is the most recent stand alone tool. Although it is no longer supported by FHWA, the output from Version 6.1 is still valid as import to Version 7.0 and provides many tools to support the structure of HPMS.

Reporting Pavement Roughness Data

Question: How should IRI data (Item 35) older than two years be reported?

Answer: IRI should be remeasured on a two-year cycle; however, retain existing IRI values for sections until they are replaced by new measured values.

Question: What should be coded on a sample section if IRI is not available?

Answer: FHWA needs either an IRI or a PSR value coded for every sample section. On sample sections where IRI is required but not available, a PSR value must be coded instead. If you have coded an IRI, do not also code a PSR; do not report both an IRI and PSR value for any sample section.

Reporting Structural Number (SN) Data

Question: What SN data is required for HPMS?

Answer: SN data is required for each HPMS standard sample section having a flexible pavement surface type. SN data for a section should reflect the value calculated at the time of the last section improvement, and should remain constant in the HPMS until the section is once again improved. A new SN should be calculated only when the section is improved with something more substantial than a maintenance overlay (>1.0").

Question: That means that if a pavement has not been improved since its first inclusion in HPMS, it should still have the same SN as originally reported?

Answer: Yes.

Question: How does the HPMS account for the deterioration that occurs over time if it uses a fixed SN?

Answer: The pavement models using the HPMS data look at the year of last improvement and the starting value for SN for each sample section and deteriorate the pavement based upon the truck loadings accumulated since the year of last improvement. This is done using the AADT and % average daily truck data that are also reported for each HPMS standard sample section.

Question: When a pavement is improved, how should a new SN be estimated?

Answer: The SN should reflect a value for the pavement and base material layers of the improved section. The general form of the SN equation is:

SN = (LC1 x D1) + (LC2 x D2) + (LC3 x D3) + .... + (LCN x DN)

where

D = depth of layer (in inches),
LC = layer coefficient (a measure of strength per inch), and
N = layer number (1 through N)

When a section has been totally reconstructed, new layer data for D and LC should be available from the pavement design and the materials and design sections. When a pavement has been overlayed, old values of D and LC for the existing layers should be used, while new values should be used for the added new layer of surface; while not a perfect solution, this avoids the necessity of coring existing sample sections and retesting the materials to determine new values for LC.

Question: What if section specific information on existing pavements is no longer available, or SN is no longer used to design new, reconstructed, or rehabilitated pavements?

Answer: In this case, the HPMS Field Manual suggests that the HPMS coordinators consult with the State pavement, design, and materials sections to develop a matrix of default typical SN values by pavement type and functional system. Many States use, and have used, fairly standard flexible pavement designs for various types of highways in the State; this should allow, for instance, the development of a typical SN value for rural arterial roads in your State, etc.

As a last resort, using information on the roadway cross-section from as-built or construction plans, the following table can be used to estimate an SN for pavements similar to those shown. To use the table, it will be necessary to interpolate an SN value for the pavement cross-section actually used on the sample section.

Flexible Pavements
"SN" range Surface type and minimum
thickness
Base type and minimum
thickness
Subbase type and minimum
thickness
4.6 – 6.0 6" asphaltic concrete 12" aggregate 13" aggregate
4.6 – 6.0 4" asphaltic concrete 8" asphaltic concrete 8" aggregate
3.1 – 4.5 4" asphaltic concrete 8" aggregate 8" aggregate
3.1 – 4.5 3" asphaltic concrete 6" asphaltic concrete  
1.0 – 3.0 Surface treatment 4" aggregate 4" aggregate
1.0 – 3.0 2" asphaltic concrete 6" aggregate  

Question: What is FHWA doing to provide a long term solution to the reporting of SN data to the HPMS given that SN is no longer being used in pavement design and will not be available in the future?

Answer: While we realize that the estimation methods described above have a limited utility, they provide a reasonable estimation of the SN values needed for existing pavement models. FHWA will be looking at replacing the pavement models it uses to estimate pavement deterioration with newer models that are consistent with current AASHTO pavement design practices and model recommendations. The objective will be to require the reporting of a minimum number of pavement variables from those already collected by the States for their pavement management systems in the HPMS at some future date.

Reporting Nationally Designated Truck Routes

Question: How should nationally designated truck routes be reported in HPMS?

Answer: Data Item 28 requires the identification of nationally designated truck routes. These are defined as the routes designated for use by dimensioned commercial vehicles under the Surface Transportation Assistance Act (STAA) of 1982. Nationally designated truck routes include the Interstate System (a few sections are exempted in Minnesota and Virginia), non-Interstate routes specifically listed in 23CFR658, Appendix A, as amended, and, in the States of Arkansas, Colorado, Indiana, Iowa, Kansas, Louisiana, Mississippi, Montana, Nebraska, Nevada, Ohio, Oklahoma, South Dakota, Texas, Utah, Washington, and Wyoming, the other existing Federal-aid Primary (FAP) routes as defined in 1991. These routes are to be coded as "1" in HPMS Item 28.

Some States have extended the rules to allow STAA dimensioned commercial vehicles to operate on other State routes. These and other non-national truck network roads used between the STAA national network and terminals and facilities for food, fuel, repairs, and rest under the reasonable access rule are not nationally designated truck routes. These routes are to be coded as "0" in HPMS Item 28.

HPMS Samples on Realigned Road Sections

Question: If a route is realigned (such as by adding a bypass) and the functional class is moved to the new route, should a sample stay on the old roadway or should it move to a section or segment of the new roadway alignment?

Answer: If a route is realigned, the sample remains with the original route; a sample is never just moved to a new alignment. If the original route is reclassified as a rural minor collector or a local road, then the sample is dropped. If a new sample is needed to meet the sampling requirements for the original functional system and volume group, a new sample must be randomly selected from all the available mileage for that functional system and volume group.

Ferryboats and HPMS

Question: Should public ferryboat routes be included in HPMS and in the Certified Public Road Mileage?

Answer: No. Segments of routes carried on ferryboats should not be included in the HPMS database. They are neither roads nor structures, and are inconsistent with the data and data definitions of the HPMS. Also, segments of routes carried on ferryboats do not meet the legal definition of a "public road" and their length should not be included in the Certified Public Road Mileage reported annually to FHWA.

Metric Reporting

Question: Does HPMS data have to be collected in metric units?

Answer: No. However, the HPMS Submittal Software will convert English data to metric units prior to submittal to FHWA. Software users must be careful to specify which unit is being used, and not mix English with metric.

Coding STRAHNET Routes and Connectors

Question: Where is more information posted about the location of the STRAHNET routes?

Answer: FHWA has developed a mappable database of the current STRAHNET. States are asked to review this database and bring any errors to our attention; with your help, these issues can be quickly resolved. Questions about the STRAHNET should be directed to Mr. Michael Neathery at 202-366-1257; STRAHNET coding issues should be directed to Mr. Thomas Roff at 202-366-5035.

Question: What routes should be reported in HPMS as being STRAHNET routes?

Answer: Report all open to traffic Interstate routes, U.S. Department of Defense (DOD) designated non-Interstate routes, and primary connector routes to Priority 1 and 2 installations and ports. These are the STRAHNET and connector routes that are on the NHS. If only one connecting route is designated for a facility, that is the primary connector route. Secondary or additional connector routes that are not on the NHS should not be reported in HPMS as STRAHNET. Unbuilt NHS routes must not be reported as STRAHNET routes; all STRAHNET routing must be on open-to-traffic facilities.