*Saturation Flow Rate*

Saturation Flow Rate can be defined with the following scenario: Assume that an
intersection’s approach signal were to stay green for an entire hour, and the traffic
was as dense as could reasonably be expected. The number of vehicles that would pass
through the intersection during that hour is the saturation flow rate.

Obviously, certain aspects of the traffic and the roadway will effect the saturation
flow rate of your approach. If your approach has very narrow lanes, traffic will naturally
provide longer gaps between vehicles, which will reduce your saturation flow rate. If you
have large numbers of turning movements, or large numbers of trucks and busses, your
saturation flow rate will be reduced. Put another way, the saturation flow rate (s) for a
lane group is the maximum number of vehicles from that lane group that can pass through
the intersection during one hour of continuous green under the prevailing traffic and
roadway conditions. The saturation flow rate is normally given in terms of
straight-through passenger cars per hour of green. Most design manuals and textbooks
provide tables that give common values for trucks and turning movements in terms of
passenger car units (pcu).

Determining the saturation flow rate can be a somewhat complicated matter. The
saturation flow rate depends on roadway and traffic conditions, which can vary
substantially from one region to another. It’s possible that someone in the area has
already completed a measurement of the saturation flow rate for an approach similar to
yours. If not, you'll need to measure it in the field. One other possibility, which is
used quite frequently, is to assume an ideal value for the saturation flow rate and adjust
it for the prevailing conditions using adjustment factors. A saturation flow rate of 1900
vehicles/hour/lane, which corresponds to a saturation headway of about 1.9 seconds, is a
fairly common nominal value. Design manuals usually provide adjustment factors that take
parameters such as lane-width, pedestrian traffic, and traffic composition into account.

*Capacity*

Capacity is an adjustment of the saturation flow rate that takes the real signal timing
into account, since most signals are not allowed to permit the continuous movement of one
phase for an hour. If your approach has 30 minutes of green per hour, you could deduce
that the actual capacity of your approach is about half of the saturation flow rate. The
capacity, therefore, is the maximum hourly flow of vehicles that can be discharged through
the intersection from the lane group in question under the prevailing traffic, roadway,
and signalization conditions. The formula for calculating capacity (c) is given below.

c = (g/C) · s

Where:

c = capacity (pcu/hour)

g = Effective green time for the phase in question (sec)

C = Cycle length (sec)

s = Saturation flow rate (pcu/hour)

Capacity can be calculated on several levels, depending on the amount of information
you want to obtain. You could calculate the capacity for each individual lane, or you
could lump the lanes together and find the capacity of an entire approach. You need to
decide what makes sense for your situation.