Appendix C
Data Source and Accuracy Statements
Box 1-1. Highway Performance Monitoring System
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 state
inventory files. The 1997 sample consists of about 120,000 samples. Each state
maintains an independent inventory of highway road links for 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 excluding those
roads functionally classified as local. The sampling frame for the state systems
were the state inventories. The estimates represent the highway systems of each
state. The HPMS sample was designed as a fixed sample to minimize data collection
costs but adjustments to maintain representativeness are carried out periodically.
The HPMS also consists of universe reporting (a complete census) for the Interstate
and the National Highway System, and tabular summary reporting of limited information.
A small number of data items (about 30) are reported for the complete universe.
The universe information contains no sampling error. There are 4 tables reported
as part of the summary.
Stratification
The HPMS sample (and universe) is stratified by state, type of area (rural,
urban, and individual urbanized areas), highway functional classification, and
traffic (annual average daily traffic (AADT) volume groups).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 strata.
Data Collection
Data are collected independently by the 50 states, metropolitan planning organizations
(MPOs), and lower jurisdictions. Many of the geometric data items rarely change,
such as number of lanes. Others change frequently, such as traffic. Typically,
the states maintain data inventories that are the repositories of a wide variety
of data items. The HPMS data items are extracted from these inventories. For example,
each State has a traffic volume counting program. Typically, equipment is installed
or placed on the roads to measure traffic. The counts are then converted to annual
average daily traffic (AADT) and stored in the state databases. AADT is one of
the sample and universe items extracted from the inventories and reported to the
HPMS. The FHWA provides guidelines for data collection in the HPMS Field Manual,
which the states follow to varying extents depending on issues such as staff,
resources, state perspective, uses of the data, state/MPO/local needs for data,
etc. Traffic data collection, for example, is an expensive and dangerous undertaking,
particularly in high volume urban areas.
State departments of transportation report HPMS data annually to the FHWA.
There are about 80 data items reported for the sample component. The reporting
deadline is June 15. Except for special cases where major problems occur, data
items are reported for each sample. There is no provision for nonresponse since
a number is available for each section in the state inventories; however, states
do leave items blank to indicate that no data collection has taken place for a
specific item (e.g., if no system to measure pavement has been implemented in
the state, the pavement condition item may be left blank.) The HPMS has gone through
a major restructuring effort, and major data item reductions, modifications, and
other changes will begin to be implemented with the 1999 data reported by June
15, 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 vehicle-miles traveled. It
has not been repeated since due to the work involved and the limited impact of
sampling error as compared to nonsampling error.
Nonsampling Error
This is a major item of concern for the HPMS. For some of the most variable
and important data items, such as AADT, guidelines for measurement and data collection
have been produced. States have the option of using the guidelines or using their
own procedures. Many data items are difficult and costly to collect and are reported
as estimates 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 identifying data problems. As mentioned before, a response is usually
provided for each link as included in state inventories. Measurement errors are
unknown, but the difficulty of collecting some of the data items is well known.
For highway links not the responsibility of states, metropolitan planning organizations
and lower jurisdictions using a wide variety of methods may collect the data.
This is a major area of concern and efforts are underway within States to standardize
data collection. The major effort with the HPMS is to insure the collection and
reporting of reliable annual data. The FHWA field offices in each state conduct
annual verification of the data reported. Computer software is provided to build
the database and conduct logic edits prior to submittal. The reported data are
subjected to intense editing and comparison with previous reporting and a written
annual report is provided to each state to document problems found and encourage
correction. Data resubmittal is requested in cases where major problems are found.
The process involves many people and substantial resources, but it provides extensive
quality assurance. Complete information on data items, edits, processing, expansion,
sample design, definitions, data reporting, etc., is included in the HPMS Field
Manual.
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