Some fixed road safety cameras enforce red light offences only, while others detect both red light and speed.
In-road sensors are used at intersections to detect if a vehicle crosses over the stop line against a red traffic light. This triggers the camera to take an image.
The camera is designed to take 2 images of an offending vehicle that combine to prove that it:
- entered the intersection after the light turned red, and
- continued through the intersection against the red.
The camera also captures the time elapsed between a traffic light turning red and the vehicle entering the intersection.
If a light changes to red while you are already in the intersection, you will not be issued a fine (the camera is only activated by a vehicle driving over the stop line after the light has turned red).
In-road sensors may utilise:
- piezos (when a vehicle travels over the sensor, kinetic energy from the tyres is converted into electrical energy and the electrical impulses indicate the vehicle's presence), or
- inductive loops (a series of electrified wire loops generate a magnetic field that detects the metal of a vehicle as it passes over).
The sensors are installed in individual lanes on roadways, allowing the system to differentiate between vehicles in different lanes.
Some intersections with red light only cameras still use original film processing equipment, known as wet-film technology.
These sites are tested weekly by trained officers while installing and removing the film magazines.
According to the Road Safety (General) Regulations 2009, camera test certificates are not required for wet-film sites.