Sub-problem 1a: Traffic Flow Patterns

Trends in the Traffic Volumes
Exhibit 4-10 shows the distribution of weekday, 7-8:00 AM peak hour volumes for the eastbound direction. On the one hand, it is apparent that the volumes can be quite low. On the other, the values range up to 3,483 veh/hr. The mean value is 2,916 veh/hr and the 50^th-percentile is 3,096 veh/hr. The 90^th percentile is 3,340 veh/hr and the 95^th-percentile is 3,385. The question is, which value is the right one to select, one of these or some other?

Most traffic engineers would think about the mean value when they talk about a typical peak hour. But if you look at Exhibit 4-10, that seems like a pretty low value to use in assessing the facility’s performance. For illustrative purposes, in the next sub-problem we’re going to use the 90^th-percentile volumes instead of the average volumes. This means that the analysis results will represent conditions that will occur 90% of the year during the AM peak hour. (We’ll come back and look at the facility’s performance during the average peak hour conditions right after that.)