Investigation of Peak-Hour Factor at Freeway Segments: Urban and Suburban Vicinities

The peak-hour factor (PHF) adjusts for traffic volume variability during the peak hour so that traffic demand can be converted to represent the busiest 15 min of the peak hour. Operational and planning analyses of freeways start by adjusting traffic demand by the PHF. Although the importance of the PHF is widely recognized, few efforts have been dedicated to a comprehensive understanding of this factor. The Highway Capacity Manual 2010 (HCM 2010) recommends the use of a single default freeway PHF value equal to 0.94 when local data are not available. The main aim of this study was to provide a quantitative analysis of the PHF values at freeways and the potential sources of its variability including the peak period (a.m., p.m., and weekend periods), day-to-day variation, traffic volume and level of service, early or late peak-hour onset, day of the week, month, season, and weather. Traffic data were collected at 54 sites along 10 freeway corridors in Milwaukee County, Wisconsin, during 2014. A massive data sample including 28,394 PHF records covering different daily, seasonal, and operational conditions was compiled. A wide range of PHF values was observed; they fluctuated considerably from day to day, even at the same site. Traffic volume, level of service, and peak period were found to be major sources of PHF variability; they deserve more attention in practice. Specific conclusions and recommendations for future editions of the HCM are summarized.