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202
Traffic Signal Systems Operations and Design: Isolated Intersections
In Activity #34, you observed the normal variation in headways in a departing queue, and based on
these observations, you selected a MAH that represents the longest headway in a departing queue
that you are willing to tolerate without terminating the green indication. In this activity, you will
relate this headway to its equivalent unoccupancy time. You will then select a vehicle extension time
based on this unoccupancy time that, in combination with the detection zone length, ensures both
efficient operation and good service quality. In this activity, the detection zone length is 22 feet and
the minimum green time is set to 7 seconds. The vehicle extension time is set to 5 seconds.
Open the movie file: “A35.wmv.”
Collect data.
•
The minimum green time is set to 7 seconds and the vehicle extension time is set to 5 seconds
•
Move the simulation time to 66.0
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Run the simulation and observe the operation of the southbound approach
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At
t
= 66.1 seconds, advance the simulation. Record the following values in Table 19 for phase 4
(serving the SB through movement)
•
Record the simulation clock time that the display changes to green (“Start of green” in the table)
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Record the clock time that the front of each vehicle enters the zone and the rear of each vehicle
exits the zone. The entry time for the first vehicle is noted in the table (“14.0”).
•
Record the clock time that the display changes to yellow (“Start of yellow” in the table)
•
Compute the unoccupancy time for each vehicle pair and record the value in the “Unoccupancy
time” column. The unoccupancy time is the difference in the clock time that the front of the vehicle
enters the zone and the clock time that the rear of the previous vehicle exits the zone. If the value
is negative, a zero should be entered.
A
ctivity
35: R
elating
H
eadway
to
U
noccupancy
T
ime
and
V
ehicle
E
xtension
T
ime