Variation of Volume Counts and Peak Hour Factors

Variation of volume counts can be further sub-divided into daily, weekly and seasonal variation. For studying the daily variation, the flow in each hour has been expressed as percentage of daily flow. Weekdays, Saturdays and Sundays usually show different patterns. That's why comparing day with day is much more useful. Peak Hour Volume is very important factor in the design of roads and control of traffic, and is usually 2 - 2.5 times the average hourly volume. Apart from this there is one additional feature of this variation: two dominant peaks (morning and evening peak), especially in urban areas. These mainly include work trips and are not dependent on weather and other travel conditions.

Similar to daily variation, weekly variation gives volumes expressed as a percentage of total flow for the week. Weekdays flows are approximately constant but the weekend flows vary a lot depending upon the season, weather and socio-economic factors. Seasonal variation is the most consistent of all variation patterns and represents the economic and social condition of the area served.

Peak hour factors should be applied in most capacity analyses in accordance with the Highway Capacity Manual, which selected 15 minute flow rates as the basis for most of its procedures. The peak-hour factor (PHF) is descriptive of trip generation patterns and may apply to an area or portion of a street and highway system. The PHF is typically calculated from traffic counts. It is the average volume during the peak 60 minute period $V_{av}^{60}$ divided by four times the average volume during the peak 15 minute's period $V_{av}^{15}$ .

$\displaystyle PHF=\frac{V_{av}^{60}}{4\times V_{av}^{15}}$

(1)

One can also use 5, 10, or 20 minutes instead of 15 minutes interval for the calculation of PHF. But in that case we have to change the multiplying factor in the denominator from 4. Generalizing,

$\displaystyle PHF=\frac{V_{av}^{60}}{\frac{60}{n}\times V_{av}^{n}}$

(2)

where $V_{av}^{n}$ is the peak

minute flow. The Highway Capacity Manual advises that in absence of field measurements reasonable approximations for peak hour factor can be made as follows:

0.95 for congested condition
0.92 for urban areas
0.88 for rural areas

General guidelines for finding future PHF can be found in the Development Review Guidelines, which are as follows:

0.85 for minor street inflows and outflows
0.90 for minor arterial
0.95 for major streets