Red Light Camera Signal


I have been wondering how the information is sent, and am sure someone here has the answer.
When the camera snaps you photo, how does the signal get back to the police headquarters, or where ever it goes? Is it a wireless signal, or sent via TV cable, or sent over a phone line? I have read that some are still pictures, and some are movie, but have not seen how it's transmitted.

Garmin 660

Red Light Cam Techno

Here you go all you need to know about the Red light cams per How stuff works P.S. I'm also a truck driver....
To see the pictures it talks about go to this http...

Red-light Camera Technology
Red-light camera monitoring systems are automated computer-camera systems designed to photograph, under any weather or light condition, vehicles in violation of a red traffic signal. Through sequential photographs, the red-light camera provides indisputable evidence of red-light violations.

Choosing to capture frontal, rear, or both images is usually mandated by state law. Rear photographs are frequently made in states (e.g., New York and Virginia) where owner onus is the law, that is, the owner of the vehicle is responsible regardless of who the driver is. This is usually the case in parking citations. In states where driver identification is required (e.g., California), frontal photography is required. In states requiring positive driver identification but not requiring front license plates (e.g. Arizona), both front and rear photographs are required.

How a Red-Light Violation is Captured
Automated cameras capture red-light violators on 35mm photographic film. The cameras are computerized and usually record date, time, and location on each frame in a data bar or block. The cameras are usually tied directly to the traffic signal system and are active only during the red phase of the traffic signal cycle. The photo at left is typical. It is part of a pair of photographs from New South Wales. Notice that the traffic signal is clearly red in both photographs as the white car passes through the intersection.

A pair of Electromagnetic induction loops are embedded in the crosswalk area of each lane to be monitored. These serve as the triggering mechanism for the red-light camera. The red-light camera is connected to the loops and to the red phase in the traffic signal pole. During the green phase of the traffic signal cycle, the unit is deactivated and photographs can not be taken. During the amber phase the unit is automatically switched to stand-by, and with the start of the red phase the camera is activated after an initial delay specified by a jurisdiction. – often .3 seconds.

When a vehicle passes over the loops during the red phase, the first photograph is taken documenting on the film the red phase, the vehicle’s prior point relative to the intersection and a number of supportive data in a data field of the picture and optionally on a memory card in the unit. A second photograph is taken within a predetermined time frame based on the speed of the vehicle (usually .5 - 1.1 seconds) documenting the vehicle’s position in the intersection.

As mentioned above, a data bar or block is usually placed on the negative at the same time it is exposed. The data block used on the Gatso camera is described by the Oxnard, CA Police Department, "On every photo is a "Camera Data Block." This block (enlarged from the above photo) can tell us many things about the violation. This picture was taken on July 7, 1997 at 5:50 p.m. and was the eighth photo citation of the day at this intersection. This is the second of two photos taken of this car. The first photo was exposed 0.5 seconds prior to this shot. At the time of this photo, the light had been red for 0.9 seconds and the speed of the car is 58 miles per hour.

The film is collected and processed on a regular basis, often daily. Any change to the system or the cameras – including vandalism -- noted during film retrieval should be documented and presented to the jurisdiction

Not all photographs can be used to issue citations The number of photographs that result in citations occurring during any one calendar month could be affected by:

Weather (Snow, Ice)
Number of days in the month
Seasonal variations in traffic patterns
Technical problems (Camera malfunction, loop damage)
Exempted Vehicles (Police, Fire, Diplomat, etc.)
Red-light camera systems are usually composed of three main subsystems:

Control mechanism
Violation Detection
Red-light Cameras
Automated red-light cameras usually capture images of red-light violations on 35mm photographic film in B&W or color. Similar to standard 35mm cameras but manufactured to more rigorous standards, red-light cameras are able to withstand years of continuous operation in a variety of conditions. While most systems in place today use what is called the "wet film" technology available since before the turn of the 20th century, some companies are distributing Digital systems.

Some of the digital systems use digital or Video Cameras to record the violation. Some systems provide an optional real-time video system to monitor traffic and record collisions on video tape. A video system may be connected to a central dispatch facility in order to provide immediate information and expedite appropriate response and notify law enforcement personnel. Several companies such as SD Scicon, Econolite, and Peek Traffic Systems offer video systems that can be used to monitor intersections. While a violation may be detected with video cameras, sophisticated computers and software using a trigger known as a "photo loop", a still camera is usually required to capture the violation because of the placement of the video camera (high above the intersection) and a desire to use film - instead of videotape - as the evidentiary media.

Red-light Camera Control
Standard red-light cameras include computers which record the date, time, seconds into red, location code, superimposed onto each photograph. Lane number, vehicle speed, and seconds of amber may also be recorded. The red-light camera computer also handles the triggering function.

Red-light cameras are activated when a vehicle crosses a trigger during the red phase of a signal. The trigger is usually specially designed loops but may include Piezo strips or even radar beams. The trigger activates a camera placed behind the vehicle, in front of the vehicle, or sometimes both.

Red-light cameras are placed and controlled at the discretion of a jurisdiction. Since they are connected to the red phase of the traffic signal (preferably through opto-isolation circuitry) and only active during the red phase, the probability of false triggering is remote. Additionally, a jurisdiction may choose to add a speed threshold and activation delay. A speed threshold is used to prevent the false triggering of vehicles which sometime creep over the loops. This is usually 5-15 mph. An activation delay may be used by a jurisdiction if it wishes to give the motorist a grace period after the signal turns to red. When used, this delay is usually not more than 3/10ths of a second.

Violation Detection
In order to be able to photograph a traffic violation, some mechanism must be in place to accurately determine when and if a violation has occurred. Traditional enforcement has often relied on the eyes and judgment of a uniformed police officer. The use of automated detection equipment, however, is much more accurate and cost effective. Detection equipment ranges from the simple air tube to complex Radar or laser.

The enforcement application, of course, determines the detection method used. Speed violations are usually detected with radar. Many red-light cameras are capable of measuring speed as well and can be combined with red-light monitoring to provide more intersection monitoring and enforcement. The technique is used successfully in Great Britain.

Triggers are mechanisms that activate the recording equipment when a violation occurs. Many devices can be used. Some common ones include air tubes, loops, Piezo strips, RADAR, and Laser.

One of the simplest trigger is the air tube. These are rubber tubes (much like a garden hose) that are placed across a roadway. When a vehicle crosses over the tube, air is compressed inside the tube and a pressure sensitive switch at one end is triggered. While simple, air tubes are almost never used for monitoring intersections for more than a brief period due to the limited life of an air tube. However, they are very effective when coupled with a traffic counter such as the Mitron 3000 (shown at left) to determine the need for a red-light or rail crossing camera.

A vehicle detection system is frequently composed of a digital loop detector and inductive loop wires embedded under the road surface. This combination provides a highly sensitive detection field for sensing offending vehicles within the detection zones. A rectangular loop detector zone is used to ensure that all vehicles entered the zone along the same axis, and that the intensity of the magnetic field was equal throughout the zone. This configuration has proven to be very successful in detecting and photographing vehicles that are running a red-light or going under or around a rail crossing arm during the red signal sequence.

The digital detection system usually allows the system to accurately monitor both the direction and speed of vehicles traversing the detector loops. The direction sensitivity allows the unit to require that a given loop be tripped before another in order for a violation to be recorded.

Loop detectors used in photo-enforcement are usually of the scanning type. That is, they are switched on and off in series very quickly. This reduces the amount of interference from adjacent loops since no two loops are actually active at any one time. A fast scan ensures that they always appear on. Many are able to accurately measure speed as well. This technique is used to allow the unit to monitor traffic from various directions. For example, direction sensitivity is achieved when the central processing unit (CPU) receives a message from the loop detector that a vehicle was sensed encroaching on loop 1 or loop 2 followed by loop 3 or loop 4 respectively. Vehicles traveling in the other direction should not activate the unit.

Speed sensitivity allows the service personnel to set the system to only photograph violators exceeding a minimum speed threshold. This sensitivity is especially useful in determining the speed of a vehicle as it is illegally crossing the grade or intersection, or simply to help preserve film by not photographing vehicles that just happen to rest on the detection loops during the red phase. Thus, if the service personnel set the minimum speed to 5 mph, the unit will only photograph violators passing over the loops at a speed of 5 mph or above. However, scanning detectors are not often used when accurate speed measurement is required because the scan rate may affect the time increment measured from loop to loop.

Speed sensitivity is of great importance since each violation has recorded in the data box or bar the speed at which the vehicle passed through the highway-railroad intersection. In most cases, this information serves as additional evidence that the offending driver had sufficient time to stop the vehicle before entering the crossing, or that the driver simply was proceeding through the highway-railroad intersection at a high rate of speed with no intention to stop. An additional benefit of setting a minimum speed, is that vehicles that come to rest on the detection zone during the red phase will not cause the unit to be triggered since their speed is 0 mph.

Correct installation of loops is critical. Placement, choice of materials and thorough testing are very important.

Inductive loops are usually composed of three - five turns of #14 XHHW - XLP (or other) Loop wire buried in a 1/4 to 3/8 inch saw cut 2.5 - 5 inches deep. The inductance of each loop should be between 100-350 microH, with no more than 15% variation between loops at any one site. Lead-in cable normally has an inductance of approximately .22 microhenries (uh) per foot as shown in the first equation below. The second equation below can be used for approximating the inductance of a single square or rectangle loop. (Detector Systems, 1). Loops should be sealed with a 3M Elastomeric sealant or equivalent.


Lc=Inductance of lead-in Cable (uh)
lc=Length of lead in Cable
Lu=Unit inductance (uh/ft)


L=Inductance (uh)
P=Perimeter (ft.)
N=Number of turns in loop
Loops are usually placed .5 m to 5 m apart front to back on center depending on the anticipated speed of traffic. Loops should be placed no closer that 1 m side to side. Loops for use with higher speed traffic area usually placed farther apart. Standard 6' x 6' diamond or round loops are not suitable for red-light camera triggering. While these are a standard shape for traffic monitoring needing only simply presence detection, smaller loops are more effective for passage detection required for red-light camera triggering (Farradyne, 2).

Loop wire is connected to loop detectors. They are used to detect changes in inductance in the loop wires caused by the presence of high mass objects such as cars. These are usually standard NEMA, four channel scanning detectors with autotuning, variable sensitivity, fault detection, and presence and pulse modes. They are usually included with the red-light camera system.

Marshal Brain's outstanding web site, How Stuff Works recently answered a question about how loops work.

Red Light Camera

In one city I live in I have been told they go out daily to each location of the red light camera and download the info to a computer. There only about 10 in that city.