There are a few different types of red light camera triggers. The main purpose is the same for all of them. Detect if and when a vehicle has crossed over a certain point or detection zone on a street.
An induction loop is implemented at red light camera intersections by laying electrical wire in concentric rectangular loops resting on top of each other, underneath the asphalt of the street. This is done by cutting a 1/4" to 3/8" wide groove in the asphalt with a saw. The wire is laid inside the groove 2.5" to 5" deep and covered with a rubber compound, such as 3M Elastomeric sealant. Wire commonly used for this application is #14 XHHW-XLP but other types are also used. The inductance of each loop should remain between 100-350 MicroH and not allow a variation of more than 15% between the loops. The wire is then hooked up to a power source and a loop detector to measure the magnetic field being generated by the induction loop. The loop detector usually has a four channel scan ability, auto tuning, variable sensitivity, fault detection and presence / pulse modes. It will detect changes in inductance in the loop wires caused by high mass objects such as vehicles. Altering the layout of the wires or using a different metal within the wires will affect the loop's inductance. The loops are placed .5 meters to 5 meters apart depending on the usual or anticipated speed of traffic. Loops for higher speed traffic may be placed even further apart. They are switched on and off in series quickly. This will reduce interference from adjacent loops. This is known as a fast scan mode. When enabled, the loops using a fast scan mode can accurately measure speed and the direction of a vehicle. Speed sensitivity can be set to only photo a vehicle moving at a minimum speed over the induction loops. Vehicles that come to rest at a red light, over top the intersection line, or trigger zone will not set off the red light camera because the induction loops will sense the vehicles speed to be at 0 mph.
The piezoelectric effect is a principle of physics which is the generation of an electrostatic voltage as a result of compressing a quartz crystal. Piezoelectric strips are laid on the street. When a vehicle drives over them, they compress. This generates voltage that will trigger a timer. The timer will measure the time it takes for a vehicle to compress one strip before compressing another in front of it. The red light camera system will measure the time difference, determine the speed, and apply the calculated number to the red light camera ticket photo. Piezoelectric strips can be combined with induction loops to provide a very accurate speed calculation.
Simple in concept and implementation. Air tubes are simply rubber tubes placed across a street full of compressed air. When a vehicle crosses over an air tube the wheels press down on the air tube which compresses the air further. A pressure sensitive switch is triggered and sends a signal to the red light camera. The signal will be ignored unless the the light has turned red. The limited lifespan of air tubes make them a very short term type of red light camera trigger.
A radar monitors a given length of street by continuously emitting radio signals at all vehicles driving through an intersection. The speed of the vehicle is calculated by measuring the wavelength of the returned radio signals. As a vehicle approaches an intersection the red light camera radar will record smaller wave lengths. It compares the previous measurement with the current one and performs a calculation to determine the speed of the vehicle. This can show the difference in speed from one red light camera photo to the second proving that the driver was increasing, decreasing or maintaining their initial speed.
Laser radar is considered to be one of the most precise tools in measuring speed, when calibrated correctly. The laser red light camera system emits light pulses over specific intervals. It compares the previous measurement with the current one and performs a calculation to determine the speed of the vehicle. This can show the difference in speed from one red light camera photo to the second proving that the driver was increasing, decreasing or maintaining their initial speed.
Main reasons why red light camera tickets are not issued are below. These are examples only and are not rules or regulations regarding the issuance any traffic tickets of any kind.