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Texas Highwayman Pages > Traffic Signal Cameras
In the past few years, the number
of cameras at traffic signals has increased dramatically. During
the same period, talk about red-light enforcement cameras has also
increased and, in many jurisdictions, red-light cameras have indeed been
installed..
As a result of the circumstances
above, many people believe that all the new cameras they see at traffic
signals are red-light cameras. However, this is not the case.
Below are pictures of the various cameras you'll see at traffic signals
and an explanation of their purpose.
This is the most common camera
type in use at intersections nowadays, and the one most people observe
and mistake for a red-light camera. This camera is actually a
video image vehicle detection system (VIVDS), also known by its trademark name "Autoscope".
These cameras are used by the traffic signal controller computer to determine the
presence of vehicles so that it can change the signals accordingly. This
function used to be performed by inductive loop detectors in the
pavement. However, the cost of the cameras is now low enough that
they are more cost-efficient than loop detectors. Loop detectors
are fairly expensive to install and reconfigure, have a somewhat high
failure rate, and have limited capabilities compared to video
recognition.
The way these cameras work is
fairly simple. The camera is mounted so that it has a wide view of
the approach road that it will be monitoring. Once in place, an
engineer at the system console draws "detection zones" within the view
range of the camera of the areas that need to watched for traffic.
The video computer then monitors the zones and when it detects a
significant change in the image of that area (indicating the presence of
a vehicle), it notifies the signal controller. The signal
controller then uses that data to make the necessary decisions for
changing the signals or timings.
VIVDS cameras are also used to
monitor traffic levels and perform traffic counts on freeways and on
arterial roads. Although these cameras are not red-light cameras
per se, VIVDS cameras are often used in red-light camera installations
as the mechanism to detect when vehicles have entered the intersection.
This is not even a
camera, but rather an infrared receiver. These are part of a
traffic signal preemption system, usually for emergency vehicles, but
also sometimes for public transport vehicles. These systems allow
an authorized vehicle to change the signal to green as it approaches the
intersection. The vehicle has a forward-facing mobile infrared
transmitter (MIRT). When activated, the transmitter constantly
sends out a pulsed infrared signal. As it approaches within about
2,000 feet of a preemption equipped intersection, the device you see
pictured above detects the infrared signal and notifies the signal
controller. The controller then initiates a preemptive stop phase for all directions except the one from which the preemption
vehicle is approaching. It then changes all signals (including
left turn) to green for the approaching preemption vehicle to clear any
traffic queues and give that vehicle full right-of-way. It holds the signals green until the
vehicle clears the intersection, at which point the signal returns to
normal operation.
This is an actual red-light
enforcement camera. These cameras are recognizable by their boxy
shape and will usually sit a short distance on the approach road to an intersection,
facing toward the signal. Newer red-light cameras often resemble a
large freeway traffic camera. An inductive loop detector or a
VIVDS camera
monitors the area just beyond the stop line. When the signal turns
red, the detector is activated. When it detects a vehicle passing
the stop line, it signals the red-light camera, which then takes two or
more pictures of the vehicle, usually with a telltale camera flash. The lens is focused on the violation
area so that a clear view of the vehicle, its license plate, and the
signal is visible. Additionally, the controller timestamps the
image and records other pertinent details, such as location, direction
of travel, and length of time the signal was red. These cameras may
be either film or digital, and newer cameras often also collect a short
amount of video in conjunction with the still frame photos. Periodically, the film or digital media
is collected and the photos examined. In cases where the photos
clearly show the vehicle running the red, a citation is sent to the
registered owner of the vehicle. If the registered owner was not
driving, they can submit an affidavit affirming such and naming the
person who was driving. Photos of non-violations or of situations
that are less definitive are discarded.
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