1. DESIGN OF SIGNALIZED INTERSECTION
One of the important steps in designing a fixed time signal
system is to determine the cycle time.
• Main consideration in selecting the cycle time should be
that the least delay is caused to the traffic passing
through the intersection.
• If the cycle time is small, the proportion of the time
lost to the cycle time will be high, resulting in an
inefficient signal operation and lengthy delays.
• If the cycle time is too large, there is a danger that a
good portion of the green time will be used by
unsaturated flow of traffic, which again leads to
inefficiency.
Dr. Rizwan Memon 1
2. The optimum cycle time can be obtained by the following
equation:
seconds
Where Co = optimum cycle time
L = total lost time per cycle (in seconds)
Y = y1 + y2 + ………….. + yn
and y1, y2, … yn are the maximum ratios of flow to saturation
flow for phase 1, 2,….n
(i.e. (q / s) where q is the flow and s is the saturation flow).
Y
L
Co
1
5
5
.
1
Dr. Rizwan Memon 2
3. The above figure shows that as soon as the green signal is given, the rate of
discharge begins to pick up and some time is lost before the flow reaches the
maximum value (saturation flow).
Dr. Rizwan Memon 3
4. Saturation Flow:
In determining the y values the saturation flow should be
measured at existing sites, wherever possible. In absence of
such data the following simple formula devised by the Road
Research Laboratory (RRL), U.K can be used.
s = 525 w (PCU/hour)
where s = saturation flow
w = width of approach road in meters
The above formula is valid for widths of from 5.5 to 18 m. For
lesser widths the values may be obtained from following
table.
Width w in
(m)
3.0 3.5 4.0 4.5 5.0 5.5
s (PCU /
hour)
1850 1875 1975 2175 2250 2900
Dr. Rizwan Memon 4
