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What is the Design Hour Factor (K-Factor)?

January 17, 2024

The Design Hour Factor or K-Factor is the proportion of Annual Average Daily Traffic (AADT) that occurs during the peak hour. For the peak hour volume several options exist. K-30 is the 30th highest hourly volume of the year expressed as a percentage of the AADT. K-50 and K-100 use the 50th and 100th highest hourly volumes of the year, respectively. K-30 represents the reasonable design hour volume (DHV), but not always. Jurisdictions and local DOTs may use K-50 and K-100 for other purposes.

                     Kth highest Volume

K-Factor = -------------------------


Additional notes:
  • K-Factor typically ranges from 7% to 12% depending on whether a facility is in an urban, suburban, or rural area.
  • Either one direction or combined directions can be used to determine the peak volumes and the AADT.
  • K-Factor is best estimated from continuous count stations.
  • As AADT increases, k-factor generally decreases.
  • K-Factor is highest for recreational facilities, followed by rural facilities, and lowest for urban facilities.
  • K-Factor depends on the analysis hour, traffic flow patterns, road geometry, and location.
Further Reading:
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