Peak Hour Factor
Traffic engineers focus on the peak-hour traffic
volume in evaluating capacity and other parameters because it represents the most critical
time period. And, as any motorist who travels during the morning or evening rush hours
knows, its the period during which traffic volume is at its highest. The analysis of
level of service is based on peak rates of flow occurring within the peak hour because
substantial short-term fluctuations typically occur during an hour. Common practice is to
use a peak 15-minute rate of flow. Flow rates are usually expressed in vehicles per hour,
not vehicles per 15 minutes. The relationship between the peak 15-minute flow rate and the
full hourly volume is given by the peak-hour factor (PHF) as shown in the following
equation:
If 15-minute periods are used, the PHF is computed as:
Where
V = peak-hour volume (vph)
V15 = volume during the peak 15 minutes of flow (veh/15 minutes)
Typical peak-hour factors for freeways range between 0.80 and 0.95. Lower factors are
more typical for rural freeways or off-peak conditions. Higher factors are typical of
urban and suburban peak-hour conditions.
|