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Signal Timing Design: Theory and Concepts

Peak Hour Volume, Peak Hour Factor, Design Flow Rate

Peak Hour Volume

The peak hour volume is the volume of traffic that uses the approach, lane, or lane group in question during the hour of the day that observes the highest traffic volumes for that intersection. For example, rush hour might be the peak hour for certain interstate acceleration ramps. The peak hour volume would be the volume of passenger car units that used the ramps during rush hour. Notice the conversion to passenger car units. The peak hour volume is normally given in terms of passenger car units, since changing turning all vehicles into passenger car units makes these volume calculations more representative of what is actually going on.  

The peak hour flow rate is also given in passenger car units/hour. Sometimes these two terms are used interchangeably because they are identical numerically.

Peak Hour Factor

The peak hour factor (PHF) is derived from the peak hour volume. It is simply the ratio of the peak hour volume to four times the peak fifteen-minute volume. For example, during the peak hour, there will probably be a fifteen-minute period in which the traffic volume is more dense than during the remainder of the hour. That is the peak fifteen minutes, and the volume of traffic that uses the approach, lane, or lane group during those fifteen minutes is the peak fifteen-minute volume. The peak hour factor is given below.

Peak Hour Factor:

PHF = Peak hour volume/(4*Peak fifteen minute volume)

Design Flow Rate

The design flow rate or the actual flow rate, for an approach, lane, or lane group is the peak hour volume (flow rate) for that entity divided by the peak hour factor. A simpler way to arrive at the design flow rate is to multiply the peak fifteen-minute volume by 4. However you derive the figure, most calculations, such as those that measure the current use of intersection capacity, require the actual flow rate (design flow rate).