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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