[Laird Scooby]

Like Canis said, there is a grid/set of lines for the caera to have a distance reference relative to the time between shots.

There are 5280ft in a mile, 60mph = 1 mile a minute or 88ft/sec, 75mph will be 110ft/sec and 90mph = 132ft/sec so you can see how they can easily relate distance to speed if they know how close the camera shots are.

If two cars are caught in different lanes by the same camera, if one is at or below the speed limit then the distance traveled between shots will be proprtional to the time between the shots, conversely if the distance traveled is far in excess of what it would be at the legal limit plus an amount for the speedo error then that car is obviously speeding.

If you were at or below the speed limit (or even a smidge over) i wouldn't worry unduly.