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How do speed cameras work?


What is the technology behind speed cameras, how is the system implemented, how do radars work?

Why are speed cameras where they are?

Officialy, the criteria are :
- areas where accidents frequently occur;
- areas where speeding is usually one of the main causes for the accidents;
- areas where human police controls are difficult.

Speed cameras sensitivity

Sensitivity is usually set up at 5% of the maximum speed. However when maximum speed is below 100 km/h (31 mph) it is set at 5 km/h (3 mph).

Maximum speed             Margin                     Fined if ...
- 50km/h (31 mph)          5km/h (3 mph)        56km/h (34 mph)
- 60km/h (37 mph)          5km/h      "              66km/h (40 mph)
- 70km/h (44 mph)          5km/h      "              76km/h (48 mph)
- 80km/h (50 mph)          5km/h      "              86km/h (53 mph)
- 90km/h (56 mph)          5km/h      "              96km/h (59 mph)
- 100km/h (62 mph)        5km/h      "            106km/h (65 mph)
- 110km/h (69 mph         6km/h (4 mph)       117km/h (73 mph)
- 120km/h (75 mph)        6km/h      "            127km/h (79 mph)
- 130km/h (81 mph)        7km/h (4.5 mph)    138km/h (85 mph)

How does the system work?

The vehicle is speeding. As it passes by an automatic radar, speeding is detected and a digital picture of the vehicle is taken. The picture is crypted then sent to a national treatment center via internet networks (adsl connection). At the center the picture is uncrypted and the registration of the car is taken, as well as all the relevant data (time, speed, etc). The registration is compared to the database of stolen cars and rented cars. Police officers supervise this stage. The fine is then mailed to the owner of the car. A contestation is possible if the owner was not the driver.

How are foreign registered vehicles concerned?

A law passed 12 june 2003 makes it possible to go after the owners of vehicules whose registration is not french. This of course depends on the exchange of data and cooperation between European police forces, but we have to admit that this kind of cooperation is on the rise, especially regarding data sharing procedures.
That law gives the owners of foreign registered cars one month more to pay their fines.

What else could you be fined for?

Automatic radars are or will be developped/improved to also control:
- Security distances between vehicles (work in progress).
- Respect of traffic lights (implemented).

How does a radar detect the speed of a vehicle?

A radar uses electromagnetic waves to detect objets and provide informations about them (distance, speed...).

The radar sends radio waves (electromagnetic waves) at a given frequency and listens for their return (echo). If an object is in the beam, some waves will bounce off it and return to the radar. By dividing the time it took for the waves to travel back and forth by the speed of the waves (the speed of light) it is then easy to determine the distance.

To determine speed, radars use the doppler effect. They measure and scale the frequency variation between the radio wave sent and the radio wave received (echo). If the object is moving away, the frequency will be lower, if it is getting closer, the frequency will be higher. Depending on how much lower or higher the echo frquency is, it is possible to know at what speed the object is moving from the radar position. This technology is now quite old and easy to jam.

A newer system called LIDAR (light detection and ranging) does not use the doppler effect. It only does calculate the distance at which the object is, but does so very often. From the change in distance observed in a given lapse of time it is simple to extrapolate the speed. The frequencies used by Lidar are higher (laser lights) and more dificult to jam. To be even more jam-proof, some systems use various frequencies, making jamming systems more complex and expensive. This Lidar system is now extensively used in the new police speed detection equipements.

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