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Impact of Day-to-Day Variability of Peak-Hour Volumes on Signalized Intersection Performance

Traffic signal timing plans are typically developed on the basis of turning movement traffic and pedestrian volume counts aggregated to 15-minute intervals and obtained over a 4 or 8 hour period on a single day. These data are used to compute the peak hour factor (PHF) and the peak hour turning movement traffic volumes which are then typically used as input to the analysis methods defined in the Highway Capacity Manual or within popular signal timing optimization software. However, all of these methods are deterministic in that they ignore the day-to-day variability that exists within key input parameters such as the PHF and peak hour traffic volume. The lack of consideration of this variability may be because (a) it is assumed that the impact of the variability is small; and/or (b) methods have not been developed by which the variability can be considered. This paper presents findings of a study that quantify the impact of day-to-day variability of intersection peak hour approach volumes and demonstrate that this impact is not insignificant and therefore should not be ignored. Finally, the study examined the number of days for which approach volumes should be counted in order to establish intersection delay within a desired level of confidence. The results indicate that for intersections operating near capacity a minimum of 3 days of peak hour volume observations are required to estimate the average intersection delay with an estimation error of 40% of the true mean.