ScoobyChris wrote:Just doing a bit of reading up on how they work. It seems that they detect the speed first (either via radar or sensors embedded in the road) and then snap the vehicle twice if it's above the set threshold while it is on the painted markings on the road. Presumably, the ones with sensors in the road are the ones that can easily detect HGV's?
It seems that the only distinction is between cars/vans and HGVs and so it wouldn't be able to tell if you were in a car towing a trailer, caravan, etc?
Interesting stuff!
Chris
Out RoADA group had a talk by the head of Lothian and Borders safety camera parnership a few years ago and some of use accepted an invite to visit their HQ and view the equipment.
It seems the Gatsos in The Scottish Borders project two sets of radar beams at an angle across the road. One beam is low and is set to the car limit. The other beam is set high where it is only triggered by large vehicles and is set to the HGV limit. The ones in the Scottish Borders are often triggered erroneously by high top vans and buses and a lot of the operators time is taken up filtering out these fales detections.
Suprisingly the radars do not use doppler shift but are projected at an angle across the road. The sensors measure the deflection of the beams to determine the vehicles speed. This type of system is immune to the problems associated with the doppler radar versions which can be confused by reflections from vehicles travelling in the other direction. The older cheaper doppler units are still used with the dummy flashes, which is why you can still set off units while travelling the other way.
They are also trialling a Speed Curb unit that uses detector strips in the road. They tell the difference between large and small vehicles by measuring the time between the different sets of wheels and counting the axles (I'm wondering how it can work out the difference betwee a many axled vehicle and multiple cars?). They should be able to identify trailers and tell the difference between buses and LGVs.
In the future the digital units will be able to use ANPR to identify the vehicle and determine its speed limit accurately (currently under test at one of the SPECs test beds at the M8 in Glasgow).
The trial sites are not currently being used for detection, but offenders who exceed the limit by a wide margin to get a warning letter.
