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Traffic_Signal_Systems Powered By Docstoc
					                                         facilitate   smooth      traffic  flow   rise on the County’s regional road
 TRAFFIC SIGNAL                          (progressed movement) along a            network.
                                         street is a proven technique. The
    SYSTEMS                              quality of flow along a street is
                                         basically a function of the spacing of    SPECIAL SIGNAL FUNCTIONS
                                         the signals along the street, the         CAN IMPACT TRAFFIC SIGNAL
          SIGNAL TIMING                  prevailing speed of traffic on the               OPERATIONS
                                         street, and the traffic signal cycle
                                         length.                                  Traffic Signal Preemption
All the traffic signals in the
unincorporated areas of The County       The amount of traffic and the            The transfer of signal control to a
are actuated. Detectors on the           proportion of the green time given to    special signal operation is called
approaches to traffic signals monitor    the progressed movements are also        preemption.     There   are    three
and assign the right of way on the       important. The goal of coordination is   common types of preemption, based
basis of changing traffic demand.        to get the greatest number of            on the need for preemption:
These signals attempt to assign          vehicles through the system with the     Railroad, Emergency Vehicle, and
most of the available green time to      fewest stops in a comfortable            Transit Vehicle Preemption.
the heaviest traffic movements.          manner.                                  Railroad Preemption
If a detector is malfunctioning, the     When signals are coordinated, the
signal will dwell in green even if                                                Railroad preemption is initiated when
                                         side street traffic does have to wait
there is no vehicle or pedestrian                                                 a train passes over advance
                                         sometimes over a minute in duration,
present. If that occurs, the County’s                                             detectors located on the tracks
                                         but once the side street enters the
Transportation Department should                                                  ahead of the railroad crossing. The
                                         major street it benefits from smooth
be notified so that the problem can                                               purpose of preemption is to clear
                                         traffic  progression      along   the
be corrected.                                                                     tracks of traffic stopped on them by
                                         coordinated signal system.               traffic signals.

  COORDINATION OF TRAFFIC                 WHICH TRAFFIC SIGNALS ARE               There are no traffic signals equipped
         SIGNALS                               COORDINATED?                       with railroad preemption in the
                                                                                  unincorporated areas of The County,
The greatest benefits to the public                                               but you may experience this kind of
                                         The County currently does not have       interruption   of    normal     signal
for each dollar spent on traffic
                                         coordinated traffic signal systems       operations in other nearby cities.
operations improvements come from
                                         because most of the traffic signals in
the coordination of adjacent traffic
                                         the unincorporated areas of the
signals to provide smooth movement
                                         County are either isolated or on
of the traffic through groups of
                                         lightly traveled corridors. However,
signals on an arterial street.
                                         there are plans to coordinate signals    Emergency and Transit Vehicle
                                         in the future as volumes continue to     Preemption
The coordination of traffic signals to
Emergency vehicle preemption can        WHAT DO FLASHING AND DARK
be used for any authorized                TRAFFIC SIGNALS MEAN?
emergency vehicle, but normally only
for fire engines. The purpose is to
obtain a green light for the            Flashing Traffic Signals
emergency vehicle as soon as            Traffic signals sometimes develop         TRAFFIC SIGNAL
possible or to hold an existing green   hardware problems. When they do,             SYSTEMS
light.                                  the     controller   mechanism  is
To obtain a green light, existing       programmed to turn the signal over
green lights, including pedestrian      to flashing operation.
intervals, are abbreviated. After the   According to the California Vehicle
yellow change interval, a green light   Code, when a red lens is illuminated
is given to the approach to be used     with intermittent red flashes, a driver
by the emergency vehicle.               is required to stop before entering
There are three means of signal         the crosswalk on the near side of the
preemption      from       emergency    intersection. The driver may proceed
vehicles: mobile radio, siren sensor,   subject to the rules applicable to
and modulated strobe light. A           making a stop at a four-way stop
detector at a signal receives light     controlled intersection.
modulated output of a vehicle-          Dark Signals
mounted emitter.
                                        When a traffic signal has gone dark
There is only one signal with this      due to power failure it is considered
type of preemption in the Oak Park      to function the same way as a four-
area of The County, but some of the     way stop controlled intersection and
cities in Los Angeles and Some          a driver must stop before entering
other Counties do have many traffic     the intersection.
signals with emergency vehicle
preemption equipment.                      REQUESTS AND INQUIRIES

                                        If you have questions, requests, or
                                        suggestions, please call Department
                                        at: or through the “Contact Us” link
                                        at:    Rev. 2/21/08