Analysis of 24-Hour versus 48-Hour Traffic Counts for HPMS Sampling

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 Highway Performance Monitoring System (HPMS) routes, as opposed to the current 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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  • Supplemental Notes:
    • Project title: Impact of HPMS Sample Counting for 24 Hours versus 48 Hours.
  • Corporate Authors:

    University of Illinois, Springfield

    One University Plaza
    Springfield, IL  United States  62703-5407

    Illinois Department of Transportation

    Bureau of Materials and Physical Research
    126 East Ash Street
    Springfield, IL  United States  62704-4766

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Authors:
    • Hall, James P
    • Sung, Chung-Hsien
    • Morgan, William
  • Publication Date: 2014-4


  • English

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Filing Info

  • Accession Number: 01526273
  • Record Type: Publication
  • Report/Paper Numbers: FHWA-ICT-14-007, ICT-14-007, UILU-ENG-2014-2007
  • Contract Numbers: R27-SP22
  • Created Date: May 28 2014 3:26PM