Highway Performance Monitoring System

Sample Design

Sampling Frame Construction - The Highway Performance Monitoring System (HPMS) sample is a stratified simple random sample of highway links (small sections of roadway) selected from the State public road system. The 1999 sample consists of about 112,000 sections. Each State maintains an inventory of highway road links for at least those roads that the State is responsible for (in some cases this can be a low percentage of total road miles within the State). Lower jurisdictions (MPO’s, counties, cities, national parks, Indian reservations, etc.) may also maintain inventories of highway links under their jurisdiction. The HPMS sample was originally selected in 1978 based on guidelines provided by the FHWA for sampling highway systems. The sampling frame includes all State-owned and non-State owned public roads, excluding those roads functionally classified rural minor collector or local. Estimates from the expanded sample are used to represent the functional systems in the sample frame for each State. The HPMS sample was designed as a fixed panel sample to minimize data collection costs but adjustments to maintain adequate sample representation of the system are carried out periodically. The HPMS also includes universe reporting (a complete census) of some data for all public roads, and summary reporting of some information. A number of data items (about 30) are reported for the complete universe of public roads; this information contains no sampling error. There are 4 summary tables reported as part of the HPMS.

Stratification - The HPMS sample is stratified by State, type of area (rural, urban, individual urbanized area), highway functional system, and traffic volume level (AADT volume group). Complete information is provided in the HPMS Field Manual.

Weighting - The HPMS sample expansion factors are the ratio of universe mileage to sample mileage in each stratum.

Data Collection

The HPMS data are collected by the 50 States, MPO’s, and other lower jurisdictions. Many of the geometric data items, such as type of median, rarely change; others, such as traffic volume, change yearly. Typically, the States maintain data inventories that are the repositories of a wide variety of data and the HPMS data items are simply extracted from these inventories, although some data are collected just to meet FHWA requirements. The FHWA provides guidelines for data collection, which the States are expected to follow, in the HPMS Field Manual. Adherence to guidelines varies depending on issues such as staff, resources, State perspective, uses of the data, State/MPO/local needs for data, etc.

The State DOT’s report HPMS data annually to the FHWA; in addition to the universe data for each section, the dataset includes about 50 additional sample data items for each sample section. The reporting deadline is June 15. In most cases, all universe and sample data items are reported for each sample section. There is no provision for non-response; if a number is not available for a sample section from the State inventory, the item is left blank or zero filled, depending on the specific coding instructions, to indicate that no data collection has taken place for the specific item (for example, if no pavement roughness value has been collected, the pavement roughness item is to be zero coded.) The HPMS has gone through a major restructuring effort and major data item reductions, modifications, and other changes were implemented with the 1999 data reported in June 2000.

Sampling Error

The sample size is estimated based on traffic volume (AADT) within each stratum. Traffic volume is the most variable data item. Sampling error can be estimated directly based on the sample design for each stratum and aggregated by stratified random sample methods to total values. This exercise was done originally in 1980 for some of the most variable data items including VMT. It has not been repeated since due to the work involved and the limited impact of sampling error as compared to non-sampling error.

Non-Sampling Error

This is a major focus for the HPMS. For some of the most variable and important data items, such as AADT and pavement roughness, guidelines for measurement and data collection have been produced. States are encouraged to use the guidelines, although many use their own procedures. Many data items are difficult and costly to collect and may be reported as estimates or averages not based on direct measurement. The data are collected and reported by many entities and individuals within the responsible organizations. Most do a reasonably good job, but staff turnover, cost, equipment issues, etc., can create difficulties in producing consistent, quality data. Although a response is usually provided for each data item for each roadway link, measurement errors are unknown. However, the difficulty of collecting some of the data items is well known. For highway links not the responsibility of States, the data may be collected by MPO’s and lower jurisdictions using a wide variety of methods. This is a major area of concern and States are encouraged to work with data providers to standardize data collection and quality.

The major HPMS effort is to ensure the collection and reporting of reliable annual data. An annual review of reported data is conducted by the FHWA, both at the headquarters level and in the field offices in each State. Computer software is provided to build the database and conduct logic edits prior to submittal; the software permits easy viewing and editing of section level data. New programs, such as the Intermodal Transportation Database (ITDB) sponsored by the Bureau of Transportation Statistics (BTS), allow viewing HPMS data on a map basis for review and edit. The reported data is subjected to intense editing and comparison with previous reporting and a written annual evaluation is provided to each State to document problems found and to encourage corrective actions. Data resubmittal is requested in cases where major problems are found. While the process involves many people and substantial resources, it provides for extensive quality assurance steps. Complete information on data items, edits, processing, expansion, sample design, definitions, data reporting, etc., is included in the HPMS Field Manual.