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What Is Traffic Signal Coordination?
TRAFFIC SIGNALS
Traffic signals assign the right-of-way to various traffic movements for
different time intervals depending on traffic flow levels.
In the Truckee Meadows region, most of the traffic signals are actuated. Actuated
traffic signals use detectors watching for the presence (or absence) of vehicles
on each approach to a traffic signal to monitor and assign the time intervals for
right-of-way based on traffic demand. As soon as a significant ‘lull’ in traffic
is noted on one street, the signal will change and provide service to the other
street.
The signal must be effective and functional for pedestrians, bicyclists, automobiles,
buses, and large trucks. By figuring out the heaviest usage of each intersection,
we are able to figure out the best possible timing scenario at each intersection.
TRAFFIC SIGNAL COORDINATION
The goal of traffic signal coordination is to get the greatest number of
vehicles through the system with the fewest stops in a comfortable manner. It would
be ideal if every vehicle could proceed without stopping. This is not possible,
even in well-spaced, well-designed systems. Therefore, in traffic signal coordination,
the green times for the heaviest traffic movements are given more weight than the
smaller traffic movements. If you are waiting for a green light to cross over or
turn left onto the "major" street where there is heavy traffic on the main street
and very light traffic on the side street, you will undoubtedly wait for a longer
time, on average, under a coordinated signal system than under a fully-actuated
system. However, once you are on the major street and a part of the larger flow,
you are more likely to travel through several intersections without stopping. The
overall result is lowered total traffic delay to the driving public. Also, a number
of other benefits result.
Below are brief definitions of some of the key elements of traffic signal coordination:
- Cycle length:
The cycle length is the total time to complete one sequence of signalization around
an intersection. Typically, this value is between 80 and 140 seconds.
- Split times:
Based on the latest traffic volume counts, green times are determined for each movement.
Pedestrian clearance time intervals are also used to develop split times. In determining
optimum split times, it is important to ensure that all movements are assigned sufficient
green time to serve the majority of the vehicles while keeping coordination along
the major corridor(s).
- Offset:
Appropriate signal timing offsets between intersections allows for a consistent
flow of traffic between intersections with minimal delays. Coordination emphasis
is typically given to the corridor with the higher through traffic volumes.
- Phasing order:
Changes to the order in which left turn green movement are served. At some intersections,
the left turn phasing was changed from [lead-lead] both directions starting the
left green arrow first (i.e., before the through movements) to [lead-lag] one direction
going first and the other going after.
BENEFITS OF SIGNAL COORDINATION
Some of the benefits of coordination are:
- Reduction in overall stops and travel delays
- Reduction in the number of stops reduces vehicle emissions and thus improves air
quality. Most of the vehicle emissions occur during acceleration (stop and go traffic)
- Allowing large groups of vehicles to efficiently flow through a series of traffic
signals without stopping
- Reduction in the number of severe accidents.
- Reduction in aggressive driving behaviors.
DISADVANTAGES OF SIGNAL COORDINATION
Practically the only noticeable disadvantage of signal coordination is that side
street drivers typically experience a longer wait time when crossing or turning
left onto the major arterial roadway that is being progressively timed.
In the development of signal coordination, we have to manage the competing interests
of providing continuous flow of traffic on the main street, providing adequate time
for pedestrians to cross the street, and minimizing the wait time for side street
traffic.
LIMITATIONS OF SIGNAL COORDINATION
While traffic signal coordination can reduce stops and travel delays along a particular
corridor, travel along a particular street may not completely experience non-stop
free-flow conditions due to the following conditions:
- Capacity issues as a result of increased traffic caused by growth
- Complexity of the street system
- Equipment malfunction
- Street construction or some other temporary taking away of a travel lane
- Traffic accident
- Pedestrian crossings
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