United States Department of Transportation
i
Analysis of 24-hour versus 48-hour traffic counts for HPMS sampling.
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2014-04-01
[PDF-1.18 MB]
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Abstract:The Illinois Department of Transportation (IDOT) has requested a waiver from the Federal Highway Administration (FHWA) to
allow IDOT to implement a 24-hour traffic-count program on the non-state HPMS routes, as opposed to the current Highway
Performance Monitoring System (HPMS) count cycle and duration requirement for a 48-hour minimum counting cycle. IDOT
proposes to count these routes twice in a 5-year cycle. IDOT’s concern is that the collection of 24-hour counts, as opposed to
48-hour counts, would enable more efficient use of agency resources, yet still maintain the statistical integrity of the annual
average daily traffic (AADT) estimation process for HPMS reporting. IDOT had conducted research in the 1980s comparing
the two count durations and has been following the conclusions from that study in conducting 24-hour counts. IDOT had been
granted an FHWA waiver for the 24-hour counts on state routes since 1992. Because traffic patterns have changed over the
years, IDOT wanted to review the relative differences between the two count durations.
The objective of this study was to perform a statistical analysis on IDOT’s automated traffic recorder (ATR)
continuous-count traffic data, collected from 103 statewide ATR locations, to compare the relative differences between 24-
hour count periods and 48-hour count periods that are factored to compute AADT. Statistical analyses were performed for
statewide ATR data across various roadway functional classification categories and also split by District 1 and downstate
(Districts 2–9). In general, the analyses found that, with the application of appropriate daily traffic-count adjustment factors,
the 24-hour counts were statistically comparable to 48-hour traffic counts.
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