to the Insurance
Institute for Highway Safety, 22 percent of all traffic
accidents in the United States are caused by drivers running
red lights. Every year, these accidents kill some 800 people
and rack up an estimated $7 billion dollars in property
damage, medical bills, lost productivity and insurance hikes.
And this sort of traffic violation seems to be on the rise. In
many areas, red-light violations have increased by 10 percent
or more since the 1980s.
These days, even traffic lights are keeping
an eye on
To curb this trend, more and more cities are installing
red-light cameras. These fully automated devices
collect all of the evidence authorities need to prosecute
light-runners. If a camera catches you speeding through the
intersection, you can expect a ticket (along with a photograph
of the violation) to arrive in your mailbox a month or two
later. In this edition of HowStuffWorks,
we'll look at the basic elements in these systems to find out
how they catch drivers red-handed.
Red-light systems rely on some
sophisticated technology, but conceptually they are very
simple. The system includes only three essential elements:
- One or more cameras
- One or more triggers
- A computer
In a typical system, cameras are positioned at the corners
of an intersection, on poles a few yards high. The cameras
point inward, so they can photograph cars driving through the
intersection. Generally, a red-light system has cameras at all
four corners of an intersection, to photograph cars going in
different directions and get pictures from different angles.
Some systems use film
cameras, but most newer systems use digital
Multiple cameras are mounted high above the
intersection to get a full view of any traffic
There are a number of trigger technologies, but they all
serve the same purpose: They detect when a car has moved past
a particular point in the road. Red-light systems typically
have two induction-loop triggers positioned under the
road near the stop line (more on this later).
is the brains behind the operation. It is wired to the
cameras, the triggers and the traffic-light circuit itself.
The computer constantly monitors the traffic signal and the
triggers. If a car sets off a trigger when the light is red,
the computer takes two pictures to document the
violation. The first picture shows the car just on the edge of
the intersection and the second picture shows the car in the
middle of the intersection.
Most modern red-light-camera systems use
digital cameras. Older ones use 35-mm cameras, in which
case the film has to be collected for development
In some states, a ticket is issued to the car's owner, no
matter who's actually driving. In these states, the red-light
camera only needs to photograph the car from behind, since the
authorities only need a clear view of the rear license
plate. In other states, the actual driver is responsible
for paying the ticket. In this case, the system needs a second
camera in front of the car, in order to get a shot of the
driver's face. The ticket is still sent to the car's
owner, but the authorities have the information available if
there is any disagreement down the line.
In the next couple of sections, we'll look at the major
components of this system in greater detail.
As we saw in the last
section, the main trigger technology used in red-light systems
is the induction loop. An induction-loop trigger is a
length of electrical wire buried just under the asphalt.
Usually, the wire is laid out in a couple of rectangular loops
resting on top of each other (see diagram below).
This wire is hooked up to an electrical power source and a
meter. If you've read How
Electromagnets Work, you know that when you send
electrical current through a wire, it generates a magnetic
field. Positioning the wire in concentric loops, as in any
electromagnet, amplifies this field.
When a car drives over an induction loop, it
disturbs the loop's electromagnetic field. This changes
the total inductance of the loop
This sort of field affects not only objects around the
loop, but also the loop itself. The magnetic field
induces an electrical voltage in the wire that is
counter to the voltage of the circuit as a whole. This
significantly alters the flow of current through the circuit.
The intensity of this induction depends on the structure
and composition of the loop; changing the layout of the wires
or using a different conductive material (metal) will change
the loop's inductance. You can also change the inductance by
introducing additional conductive materials into the loop's
magnetic field. This is what happens when a car pulls up to
the intersection. The huge mass of metal that makes up your
car alters the magnetic field around the loop, changing its
Construction crews cut into the asphalt to
install loop sensors. You can see where a loop was
installed at this intersection.
The meter in the system constantly monitors the total
inductance level of the circuit. When the inductance changes
significantly, the computer recognizes this shift and knows
that a car has passed over the loop.
This is the most common trigger mechanism, but it's not the
only one in use. Some areas have had success with radar, laser or
One emerging trigger mechanism is the video loop. In
this system, a computer analyzes a video feed from the
intersection. As the computer receives each new video frame,
it checks for substantial changes at specific points in the
image. The computer is programmed to recognize the particular
changes that indicate a car moving through the intersection.
If the light is red and the computer recognizes this sort of
change, it activates the still cameras. The main advantage of
this system is you don't have to dig up the road to install
it, and you can adjust the trigger areas at any time.
Essentially, it is a virtual inductive-loop trigger.
The trigger mechanism isn't worth much if it isn't
connected to a central brain. In the next section, we'll see
how a red-light system's computer puts everything together to
construct a case against any traffic violators.
As we saw in the previous
sections, a red-light-camera system is controlled by a
computer. To see how these computers bring everything
together, let's look at a typical intersection and a typical
The central control box houses the computer,
the brains of the system. The computer activates the
cameras based on information it receives from the
traffic lights and
For simplicity's sake, we'll only consider traffic moving
in one direction through this intersection. When the light is
green or yellow for incoming traffic, the computer ignores the
triggers and does not activate the cameras. The system doesn't
"turn on" until it receives a signal that the light is
red. If you're already in the middle of the
intersection when the light turns red, the system will not
activate the cameras (this is not a traffic violation in most
areas). Some systems wait a fraction of a second after the
light turns red, to give drivers a "grace period."
In most systems, the computer will not activate the cameras
if a car is just sitting over the induction loops. To trigger
the cameras, you have to move over the loops at a
particular speed. In most systems, there are two loop
triggers for each lane of traffic. When the triggers are both
activated in quick succession, the computer knows a car has
moved into the intersection at high speed. If there is more of
a delay, the computer knows the car is moving more slowly. If
the car activates only the first trigger, the computer knows
it is stopped at the edge of the intersection.
When a car activates both triggers after the light is red,
the computer automatically takes a picture. This first shot
shows the car just as it is entering the intersection. The
computer then hesitates briefly and takes another shot. This
catches the car in the middle of the intersection. The
computer calculates the length of the delay based on the
measured speed of the car. It's important to get two
pictures of the car to show that it entered the
intersection when the light was red and then proceeded through
With all this information and photos of the infraction, the
police have everything they need to charge the driver. In most
areas, the police, or a private firm hired to maintain the
system, simply look up the license plate and send the ticket
in the mail. The driver (or car owner) can pay the fine
through the mail and be done with it or he or she can try to
contest the ticket in court. Of course, the police send the
photos along with the ticket, so most drivers end up just
paying the fine.
Red-light cameras have been around for more than 40 years,
but they've only gained widespread popularity in the past
decade. Police forces all over the world claim that these
systems are a great addition to their communities. They serve
as a deterrent against traffic violations, and they help
police keep track of the worst offenders. Additionally, they
are a good source of government revenue. It doesn't cost much
to maintain the system once it's installed, and it works 24
hours a day, seven days a week, systematically catching
violators and sending out revenue-generating traffic tickets
that are really hard to contest.
To find out more about this technology, as well as some of
the issues surrounding its use, check out the links on the