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Traffic Signals

There are 42 signalized intersections and two school traffic signals in Ponca City.  Additionally, there are 24 flashing school speed limit beacons and nine miscellaneous warning and/or four-way stop beacons in the city.  All told these signals and beacons use 1,931 light bulbs. 

 

Ponca City recently completed a system-wide conversion from incandescent light bulbs to light emitting diode lamps (LED’s).  The incandescent bulbs used previously had a life expectancy of 1 to 1 ½ years.  The new LED lamps have a life expectancy of seven to ten years and use 1/9 the power of traditional incandescent bulbs.  While the initial cost of an LED lamps is higher than an incandescent bulb, the longer life and reduced power consumption will actually result in a savings to the city.  Plus, the LED lamps are brighter across the entire signal face which provides an added degree of safety.

 

Traffic signal operation is divided into two main categories:  actuated and pre-timed.  The signals in the central business district (CBD) are operated in a pre-timed mode to facilitate traffic progression along the streets.  All the traffic signals at the outlying intersections are operated in a fully vehicle actuated mode.  This means that the signal responds to traffic by demand.

 

There are two methods by which vehicles are detected at the signalized intersections.  Most locations are equipped with in-pavement inductive loop detectors.  With inductive loops, a saw slot is made in the pavement and several wraps of wire are inserted in the slot and then terminated in the signal cabinet.  A very small electric current is passed through this wire which sets up a magnetic field above the loop.  As vehicles pass over the loops the metal of the vehicles disturb the magnetic field and amplifiers in the cabinet detect that disturbance and send the signal to the signal controller as a vehicle detection.  In-pavement loops have been reliably used for many years for vehicle detection.  However, they are susceptible to damage by pavement failures and street reconstruction activities.  When a wire breaks a constant call is sent to the controller and a green light will be given to that direction whether or not vehicles are present.

 

In recent years vehicle detection using video cameras has become more and more prominent.  Ponca City has eight intersections equipped with video detection cameras.  In these systems virtual detection zones are drawn on the video image at various locations along the intersection approach.  A processor looks for changes in the pixels within these detection zones that are caused by passing vehicles.  When a pixel change is detected the processor sends a call to controller to serve that particular direction.  As technology has improved video detection has become more reliable.  The cameras can be blinded by heavy fog or snow and will send constant calls for service to those directions until they can “see” again.  Video detection is much less susceptible to pavement failures or construction work.

 
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