Traffic Signal Cameras
This page last updated January 19, 2019
Over the past couple of decades, cameras at traffic signals have become ubiquitous. During
the same period, red-light cameras have been
installed in many locations and there has been much publicity about them. As
a result, many people believe that all the cameras they see at
traffic signals are red-light cameras. However, this is usually not the
case. Below are pictures of the various cameras you'll see at traffic
signals and an explanation of their purpose.
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 cameras are more cost-efficient and flexible 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.
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.
cameras are not effective during periods of inclement weather, so in
many cases, they are now being replaced with acoustic, microwave,
infrared, and ultrasonic detectors.
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
In many places, these are being retired and replaced with GPS-based preemption.
are actual red-light enforcement cameras. The camera below is an older
version; the one to the left is a newer type. These cameras sit a short
distance before the intersection on the approach road, facing toward
the intersection. 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.
Many cameras often also collect a short amount of video in conjunction
with the still frame photos to help validate violations. The images are
then sent to the law enforcement agency with jurisdiction over that
intersection for review. In cases where the photos and/or video clearly
show the vehicle running the red, a citation is sent to the registered
owner of the vehicle. Photos of non-violations or of situations that
are less definitive are discarded.
jurisdictions, a sign similar to the one below is required
to be placed at or on the approach to an intersection with
red-light enforcement cameras.
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