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|Highway Statistics 2004||OHPI > Highway Statistics > 2004 > Roadway Extent > Discussion of Travel Growth|
Table VM-2 “Functional System Travel” is based on travel reported by the various States to the Federal Highway Administration’s Highway Performance Monitoring System (HPMS). The HPMS uses State provided road section Annual Average Daily Traffic (AADT) estimates to calculate travel (AADT x section length). AADT values are generated from continuous and short-term traffic counts using long-established procedures documented in publications such as the FHWA Traffic Monitoring Guide and the AASHTO Guidelines for Traffic Data Programs.
Under most State programs, actual traffic counts are taken only periodically such as once every three years. Some States count high volume roadways more frequently, while some count low volume roadways less frequently than average. In non-count years, AADT values are generally estimated from the last actual count, using a short-term historic growth factor.
When a count is actually taken, it is usually over a short time period of a few days. This short count is then adjusted based on seasonal and daily patterns at those locations in the State were traffic is counted continuously. It is typical that these factors reflect the patterns of prior years rather than the patterns of the year in which the traffic is actually counted.
Travel estimates derived from the HPMS are best viewed as three year rolling averages; they lag change because adjustments used to develop the estimate are based upon historic trends. As a result, they are insensitive to rapid changes in actual travel, as might be measured in real time systems, such as those supporting Intelligent Transportation Systems or as reflected in the monthly reports provided by FHWA’s Traffic Volume Trends (TVT) program.
HPMS travel estimates are best used for their originally intended purposes including long term supply/demand planning and management, investment needs assessment, and tracking changes in travel patterns over time. These uses are best served by a stable time series that is relatively unaffected by short-term aberrations in what have been stable, multi-decade trends. HPMS travel estimates are not well suited to measurement of change over short time periods, or for operational planning because of the insensitivity to micro changes in short term trends.
The TVT program makes monthly estimates of the change in the Nation’s highway travel using data from continuous counters operated by the States.
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