Abstract

Peak-Hour Factor (PHF) is used to address the effect of traffic flow fluctuations on the highway system. Conventionally, PHF is defined as the ratio of one-hour volume and the equivalent one-hour volume based on the highest 15-minute volume count. While this definition is conceptually clear, there is no guideline on when to obtain the hourly volume and its highest 15-minute volume. To be sure, it is not clear at what time point in the peak period the volume should be measured since on many highways the duration of the peak flow exceeds one hour. Similarly, the selected highest 15-minute volume count may not represent the true maximum if data were not continuously recorded through using some kind of automated system. In practice, as the above problems are not resolved, many engineers simply pick a PHF value between 0.9 and 1.0, a common range used for traffic analysis.

This research examined the effect of PHF on the capacity of the highway system. It demonstrated that an arbitrarily selected PHF, even in a small value range of 0.1, can lead to very different service volume (and therefore, difference level of service) estimates both in freeways and at intersections. Such differences can substantially affect the operational efficiency of traffic control schemes. Field data collected in this study suggest that it is not always advantageous to use the highest 15-minute volume to determine PHF, as traffic dynamics today can experience dramatic changes in a much shorter time interval. In addition, the data also showed that the impact of PHF on traffic control is adversely compounded by a longer duration of the peak flow and a higher Degree of Saturation prior to the onset of the peak flow. Statistical tests were performed to investigate the stability of PHF when different sizes of time intervals other than 15 minutes are used to derive PHF and findings from the tests are discussed.