| The normal operation of any
traffic signal controlled intersection is designed for
the maximum and efficient throughput of vehicular traffic.
A common occurrence at any intersection is traffic back
up, which can require many signaling cycles to clear.
Without the ability to change the operation of the traffic
signals themselves, police and emergency response vehicles
can also be forced to sit in traffic, thus dramatically
increasing their response times to crime scenes and fire
or medical emergencies. Even without heavy traffic, a
police or emergency response vehicle entering a traffic
signal controlled intersection at a high rate of speed
places all motorists (and sometimes pedestrians) at extreme
risk.
Traffic signal preemption is an optical communications
system that allows preemption-equipped vehicles to alter
the normal operation of preemption-equipped traffic signals.
Priority Green products act as the "transmitter"
to allow emergency personnel to communicate with a traffic
signal that is equipped with a preemption "receiver"
unit. All Priority Green products are compatible with
3M Opticom and Tomar traffic signal preemption receiver
units.
An overview of a typical emergency response scenario:
- A fire truck is dispatched to an emergency.
- The fire truck is also equipped with a preemption
transmitter which, in operation, is a high intensity
forward-facing strobe light that is flashing at a rapid
rate - much faster than normal attention-getting lights
on the fire truck.
- When the fire truck approaches within 2500 feet (line-of-sight)
of a preemption-equipped traffic signal controlled intersection,
the preemption detector (normally mounted on the cross-arm
that suspends the traffic signal) "sees" the
fire truck’s preemption transmitter and locks onto
its flashing strobe.
- Once the traffic signal "sees" the fire
truck, it begins to initiate a "preemption sequence"
of the actual traffic signal that is different from normal
operation.
- If the fire truck already has a green light, the
light will remain green. Any other direction that also
has a green light (usually the opposite direction) will
first get a yellow light, then red. When all of the other
directions are then red, and the fire truck’s direction
is the only one that is green, the left turn arrow will
illuminate (if one exists), and a brilliant white flood
lamp mounted near the traffic signal will begin to flash.
This flood lamp indicates to the driver of the fire truck
that he now has control of the intersection, and complete
right-of-way.
- If the fire truck has a red light, any other direction
that has a green light will transition to yellow, then
red. When all the directions (including the fire truck’s)
are red, the traffic signal facing the fire truck will
then turn green, along with the left turn arrow (if one
exists), and the brilliant white flood lamp will begin
to flash.
- Once the fire truck has passed through the intersection,
optical communication with the preemption detector (on
the traffic signal) is lost. At that time the traffic
signal will default back to normal operation. Conversely,
until the fire truck passes through the intersection,
it will have a green light, regardless of the time duration.
- If several intersections are within the 2500 foot
range of the fire truck’s preemption transmitter,
they will all respond accordingly to the above operational
description.
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