How do speed cameras actually work? Everything you need to know

By DPA | 5 September 2017

FRANKFURT: Just about every driver knows the feeling: You coast past a police car or a speed camera, instinctively slam on the brakes and take a quick, nervous glance at the speedometer. How much were you over the speed limit?

According to the ADAC, Germany's largest automobile club, around 2.8 million drivers per year in that country alone are caught driving above the speed limit. They can expect to receive not only fines but also penalty points on their licences, and potentially also increases in insurance premiums.

But how are speeding offences measured in the first place? The classic among speed measurement instruments, especially in Europe, is the stationary speed camera. It is usually installed in such a way that it can be used to measure speed in both directions of travel, depending on the "shooting direction."

Three brass strands containing piezoelectric crystals are placed a few centimetres below the surface of the road, about one metre apart from each other, explains Tobias Goldkamp, a traffic ??lawyer.

When a car passes over them, electricity is generated through the movement of the sensitive crystals. Using these electric charges, the speed can be calculated based on the distance between the three brass strands. "The box is only responsible for taking the photo," says Goldkamp. This enables the authorities to record the driver's licence plate and send them the resulting fine by mail.


Speed traps with radars are also used in various forms, most often as mobile speed cameras. "The devices emit radar beams that are reflected by the vehicle as it passes," says Jens Doetsch, another traffic law expert.

"When the measuring threshold is exceeded, a photoelectric device is triggered, which is usually visible in the form of a flash," Doetsch explains. Radar technology is the most error-prone of all modern measuring systems because of the wide radar waves it employs.

Another sight that is becoming increasingly common in some areas of the world is silver columns with dark rings on them. Underneath this unassuming exterior lie laser speed measuring systems.

"With laser technology, light pulses are emitted, which are then reflected by the vehicles. This allows us to calculate the speed," explains Sebastian Ramb from Vitronic, a company that specialises in manufacturing laser-based speed measurement devices. Other companies in the same sector include Jenoptik, VDS and Gatso.

The lasers can cover up to four lanes at a time. In contrast to stationary cameras, the fully digital systems do not have any film reels that need regular replacement. "The data can either be read out by authorised personnel via encrypted USB sticks, or sent over an encrypted connection," says Ramb.

The measuring range can also be deduced from the number of dark rings on the silver pole. "Poles with four rings measure in both directions, while those with three rings only cover one direction of travel," explains Ramb.

Behind the dark covering in the upper section of ??the poles, there are lighting units with the flash apparatus. The measuring equipment, including the cameras, is at the bottom of the pole.

Laser beam measurement is also an increasingly common method for speed traps. Device models like the ESO ES 3.0 have five sensors; if a car passes through the lasers, the speed can be calculated from the time elapsed between the beam interruptions, explains Doetsch. "At the same time, photo boxes are built alongside the laser devices to provide photographic evidence."

In instances when a police car is attempting to detect the speed of other cars on the road, this is usually measured using video images. "The speed of the police car is then used as a reference point," says Doetsch.

Anyone using apps to detect speed cameras or receive warnings about upcoming speed traps should be careful. "In principle, warning systems aren't allowed," says Doetsch, with a fine and penalty points being a usual punishment. "In the worst-case scenario, the device itself can be confiscated."

The purchase of such apps is not usually forbidden, however, and people who use them while in their car enter a legal grey area. Specifically, it is not yet entirely clear whether speed trap warning apps are also prohibited when used by passengers.