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As stated earlier, Vehicle-Actuated Signals require actuation by a
vehicle on one or more approaches in order for certain phases or traffic
movements to be serviced.
They are equipped with detectors and the necessary control logic to respond to
the demands placed on them.
Vehicle-actuated control uses information on current demands and operations,
obtained from detectors within the intersection, to alter one or more aspects
of the signal timing on a cycle-by-cycle basis.
Timing of the signals is controlled by traffic demand.
Actuated controllers may be programmed to accommodate:
- Variable phase sequences (e.g., optional protected LT phases)
- Variable green times for each phase
- Variable cycle length, caused by variable green times
Such variability allows the signal to allocate green time based on current
demands and operations.
A proper clearance interval between the green & the red phases is also
ensured.
The various advantages of actuated signals are stated below:
- They can reduce delay (if properly timed).
- They are adaptable to short-term fluctuations in traffic flow.
- Usually increase capacity (by continually reapportioning green time).
- Provide continuous operation under low volume conditions.
- Especially effective at multiple phase intersections.
The main disadvantages are as following :
- If traffic demand pattern is very regular, the extra benefit of adding
local actuation is minimal, perhaps non-existent.
- Installation cost is two to three times the cost of a pre-timed signal
installation.
- Actuated controllers are much more complicated than pre-timed
controllers, increasing maintenance costs.
- They require careful inspection & maintenance to ensure proper
operation.
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