Peak Hour Volume, Design Flow Rate, PHF

1. Peak Hour 4

A car fire on the Sydney Harbour Bridge has caused traffic chaos in the middle of peak hour this morning. Emergency services were called to the bridge around 8.30am after smoke was spotted. Peak-hour traffic banks up near Hyperdome after two-car collision. Judith Kerr, Quest Newspapers. June 8, 2020 4:00pm. Subscriber only. Share this on Facebook. Share this on Twitter.

The following excerpts were taken from the 1994 Highway Capacity Manual, published by the Transportation Research Board.

Peak Hour and Design Hour

Capacity and other traffic analyses focus on the peak hour of traffic volume, because it represents the most critical period for operations and has the highest capacity requirements. The peak hour volume, however, is not a constant value from day to day or from season to season.

If the highest hourly volumes for a given location were listed in descending order, a large variation in the data would be observed, depending on the type of route and facility under study.

Rural and recreational routes often show a wide variation in peak-hour volumes. Several extremely high volumes occur on a few selected weekends or other peak periods, and traffic during the rest of the year is at much lower volumes, even during the peak hour. This occurs because the traffic stream consists of few daily or frequent users; the major component of traffic is generated by seasonal recreational activities and special events.

Urban routes, on the other hand, show little variation in peak-hour. . . .

The relationship between the 15-min flow rate and the full hourly volume is given by the peak hour factor, defined in Part A of this chapter (see below).

Whether the design hour was measured, established from the analysis of peaking patterns, or based on modeled demand, the peak-hour factor (PHF) is applied to determine design hour flow rates.

Peak-hour factors in urban areas generally range between 0.80 and 0.98. Lower values signify greater variability of flow within the subject hour, and higher values signify little flow variation. Peak-hour factors over 0.95 are often indicative of high traffic volumes, sometimes with capacity constraints on flow during the peak hour.

(Description of PHF from Part A, as referred to above.)

Peak rates of flow are related to hourly volumes through the use of the peak-hour factor. This factor is defined as the ratio of total hourly volume to the peak rate of flow within the hour:

PHF = Hourly volume/Peak rate of flow (within the hour)

If 15-min periods are used, the PHF may be computed as

PHF = V/(4 x V15)

Where
PHF = peak-hour factor,
V = hourly volume (vph), and
V15 = volume during the peak 15 min of the peak hour (veh/15 min).

Where the peak-hour factor is known, it may be used to convert a peak-hour volume to a peak rate of flow, as follows:

v = V/PHF (2-3)

Where
v = rate of flow for a peak 15-min period (vph),
V = peak-hour volume (vph), and
PHF = peak-hour factor.

Equation 2-3 need not be used to estimate peak flow rates where traffic counts are available. The chosen count interval must allow the identification of the maximum 15-min flow period. The rate may then be directly computed as 4 times the maximum 15-min count.

Many of the procedures use this conversion to allow computations to focus on the peak flow period within the peak hour.

Peak Hour Volume, Design Flow Rate, PHF

It is commonly known in your area that the heaviest traffic flow rates occur between 4:00 PM and 6:30 PM. Your assignment for the day is to find the peak hour volume, peak hour factor (PHF), and the actual or design flow rate for an existing one-lane approach. To do this, you obtain a click-counter and position yourself at the intersection. For each fifteen-minute interval, you record the numbers of right-turns, left-turns, straight-through trucks, and straight-through passenger cars. Your tabulated values are as shown below.

If a truck is equal to 1.5 passenger cars and a right-turn is as well, and if a left-turn is equal to 2.5 passenger cars, then calculate the peak hour volume, peak hour factor (PHF), and the actual (design) flow rate for this approach.

[Solution Shown Below]

Solution

The first step in this solution is to find the total traffic volume for each 15 minute period in terms of passenger car units. This is done by multiplying the number of trucks by 1.5, the number of right turns by 1.5, and the number of left turns by 2.5. We then add these three numbers and the volume of straight-through cars together to get the total volume of traffic serviced in each interval. Once we have this, we can locate the hour with the highest volume and the 15 minute interval with the highest volume. The peak hour is shown in blue below with the peak 15 minute period shown in a darker shade of blue.

The peak hour volume is just the sum of the volumes of the four 15 minute intervals within the peak hour (464 pcu). The peak 15 minute volume is 135 pcu in this case. The peak hour factor (PHF) is found by dividing the peak hour volume by four times the peak 15 minute volume.

PHF = 464 /(4 * 135) = 0.86

Peak Hour 4

The actual (design) flow rate can be calculated by dividing the peak hour volume by the PHF, 464/0.86 = 540 pcu/hr, or by multiplying the peak 15 minute volume by four, 4 * 135 = 540 pcu/hr.