If a camera (Gatso) flashes you once it's either out of film, or not loaded. It basically means, you're over the speed limit sonny, but you got away with it.
A double flash is filmed. It flashes twice with exact timing between the flashes and the photos are compared. Using the lines up the side of the road (evenly spaced) they measure how far you travelled in the time it took the camera to picture you twice. The little marks are examined, and your speed determined from a simple Time, Speed, Distance calulation; i.e that in the time it took the camera to flash, you went x lines distance thereby making your speed to be whatever it was over the limit....
It's nothing more complex than that.
Most cameras are set to 10% plus 2 miles, so a 30mph limited area witha camera in it will give you leeway of 10% (3mph) plus 2 mph for 35mph. That at least is the home office guideline.
Given that most speedos overread (they aren't allowed by law to underread) the speed you're traveling at, going past a 30mph limit camera at an indicated 35mph shouldn't trigger it. But it might.......
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