The lines on the road are actually there in order to calculate your speed. All the Gatso does is trigger at a set threshold, say 35mph, so if you're doing 40, it only knows you're going faster than 35. It doesn't know how much faster. So in order to prosecute you it needs to calculate your exact speed. It does this with the 2 pictures it with a defined gap in between thus the 2 flashes. From the markings on the road it can then calculate how far you travelled in the 0.5 seconds between the 2 pictures and work out the speed. So with no markings they can't prosecute you.
It's all very manual using analogue film which has to be collected from each device, which is why they don't install traditional Gatso's anymore. Most of them are now just dormant or have been set to flash at anyone breaking the speed limit without taking any pictures, just as a deterrent.
Also the front facing Truvelo camera's still need markings on the road in order to issue a NIP.
