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 fifteenminute volume. For example,
during the peak hour, there will probably be a fifteenminute 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 fifteenminute 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 fifteenminute
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).
