| Traffic Signal Cameras
|This page last updated January 6, 2022
the past couple of decades, cameras at traffic signals have become
ubiquitous. During the same period, red-light enforcement 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 enforcement 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.
Traffic detection camera
is the most common camera type in use at intersections nowadays, and
the one most people (including the media) 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 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.
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 enforcement cameras per se, VIVDS cameras are often used
camera installations as the mechanism to detect when vehicles have
entered the intersection. They may be dedicated to that purpose (i.e.
separate from the signal controller cameras) or they may
serve both purposes. However, it's important to remember that the
presence of these cameras in most cases does not mean the
intersection has photographic red-light enforcement; see the
"red-light enforcement cameras" section below to see what those cameras
cameras are not effective during periods of inclement weather, so in
many cases, they are now being replaced with acoustic, microwave,
infrared, radar, and ultrasonic detectors.
Signal preemption receiver
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.
many places, these are being retired and replaced with GPS-based
Red light enforcement cameras
are actual red-light enforcement cameras. The camera at the left is a
typical modern red light camera, while the green one to the right is an
older style but still in use in some places.
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 now often also collect a short amount of video in
with the still frame photos to help validate violations.
Images and recordings are then retrieved by or transmitted to the law enforcement agency with jurisdiction over that
intersection or their contractor for review. In cases where the photos
and/or video clearly
show a violation, a citation is sent to the registered
owner of the vehicle. Photos of non-violations or of situations that
are less definitive are discarded.
In Texas and many other places, a sign similar to the one shown above is required to be
placed on the approach to an intersection with red-light
Red light camera violations in Texas are civil offenses, much like
a parking ticket, with the fine capped at $75. In 2019, the Legislature
passed a law banning enforcement cameras, and many cities have
since scrapped their red-light camera programs. But cities with
existing contracts are allowed to continue their use until the contract
expires. Some cities with long-term contracts will therefore continue
to operate them for many more years.
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