Roymondo:
Wires across the road? How quaint! Yes, we used to use those back in the day for speed checks, before the widespread introduction of hand-held radar. Similar kit is still used for unmanned vehicle counting/monitoring (although I think they are usually pneumatic tubes rather than the co-ax cables we used). Wires/tubes pegged down with steel spikes hammered into the road surface.
No, the Truvelo cameras I am on about use multiple sets of piezo sensors buried in the road surface. These generate a voltage in response to the pressure of a vehicle driving over, with each set giving an independent speed reading. If they all agree on the speed (and it’s over the limit, of course!) a single photo is taken a set time afterwards (I think it’s half a second). There is a line painted on the road and, if the vehicle has passed the line by the time the photo is taken, it confirms it was exceeding the limit. Still current, and still used all over the country. This is what they look like:
Yeh, there’s one 300 yards from my house. I’ve only ever seen it go off when emergency vehicles fly past it doing over 50! 
Dunno of anyone around here who’s “had a citation” for setting it off - that’s for sure.
The sensors under the tarmac are like two wires coming out from the kerb. They are not “wireless” sensors in other words.
just found this news report from april last year, says the speed camera’s on the M25 between A3 and M40 haven’t worked since 2009 nor the average cam’s on the M3 off of the 25 due to technical problems 
itv.com/news/meridian/story/ … dont-work/
Winseer:
The sensors under the tarmac are like two wires coming out from the kerb. They are not “wireless” sensors in other words.
Never said they were wireless - I said they were not two wires across the road (and they are not). Besides, my primary disagreement was with the assertion that “If a speed camera does not flash twice, then it cannot measure your speed” which is absolutely NOT true. Those Truvelo units only flash once.
Well, it still can’t measure your speed then. The decision to “flash”" or “not flash” is a simple signal from the seperate “yes/no” signal that is flawed, because it doesn’t allow for different wheelbases on various vehicles.
Back to the Bristol cameras, out of all the managed motorways these seem to be the most trigger happy. You’re almost guaranteed to see a flash on the opposite side. Not just when the signs are lit either.
Heading home just the other week a camera zapped some poor bugger hooning down the outside. Bloody hell mate you’re taking the ■■■■ I thought, wonder what the speed is set to? But looking in the mirror after I passed, all 4 signs were blank. Seems these cameras are live all the time.
Wheelbase is irrelevant. The sensor pairs are only a couple of centimetres apart and could, if they really wanted to, be set to record the speed of each axle independently. They don’t of course - they are triggered by the front wheels then allow a second or two for the rest of the vehicle to pass. There are four sensors, connected in two pairs (1 is paired with 3, 2 is paired with 4), and if the speed recorded by the two pairs differs by more than +/- 2mph the readings are discarded. If the speeds match, and are over the preset threshold, the camera is triggered a preset fraction of a second later. There is a line painted on the road 1.8m after the sensors, with two more lines 18cm before and after it. The photograph shows how far past the 1.8m line the front wheels were when the camera flashed, which confirms by how much the limit was being exceeded.
Thanks for all your replies folks
Paul
Yesterday, being a lovely day I saw bikers everywhere but one group really got my attention, 30 limit, 3 bikers, front one points up at a static yellow camera to let his mates behind know in case they didn’t spot it, one rider had no gloves on. Now I am a believer in doing the speed limit and I’m glad I do but obviously these morons were speeding then slow for the camera, to me this is what is wrong with drivers these days, they are more worried about getting caught than the actual reason a speed camera is put up, other people, no worries for his hands either, I guess he doesn’t need them to eat, cook or wipe his arse.