Civil Engineering Reference
In-Depth Information
c speed that can be achieved through a series of
signalized intersections depends also on the spacing of the signals. Uniform, or near
uniform, signal spacing is essential in maximizing traf
The maximum average traf
c speed. Where this is the
case, the through band, which indicates the amount of traf
c that can pass through a
series of signals during the green phase, equals the green time provided by the
signal. Thus signals placed at quarter mile intervals can permit a through band in
each direction, at the designated progression speed (e.g., 30 mph using a 60 s
cycle). If a new signal is introduced, say one quarter mile from the next signal, the
through band of progression is signi
cantly reduced.
16.4.5 Operating Strategies [
5
]
Cycle lengths, phase splits, offsets, and progression speeds can vary throughout the
day. In some cases, where traf
c volumes are unbalanced (e.g., inbound in the
morning and outbound in the evening) the signal progression should favor the
heavier traf
c
fl
ow.
16.4.6 Equipment
Signals can be pre-timed or traf
c-responsive. Computer control and coordination
of signals is a common practice. Fully traf
c-actuated signals work best at isolated
intersections. Semi-actuated signals are appropriate at minor streets and should be
integrated with the coordination pattern.
Traf
c adaptive signals represent the state of practice and are increasingly
popular in large metropolitan areas. These traf
c adaptive controls, used for
example in Los Angeles and Manhattan, adjust the signal splits of each cycle based
on traf
c volume, but can retain the speci
ed background cycles.
16.4.7 Costs
Implementing signal coordination plans and upgrading signal control equipment
generally involves considerable costs. Costs also include installing real-time traf
c
sensors to update signal timing based on actual traf
c demand.
Sunkan [
6
] estimated the cost to retime signals in the range of $2,000
$3,100
per intersection; while the reported costs of computerized synchronization systems
in several cities ranges between $30,000 and $60,000 per intersection.
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