Correct- the reason there is a row of lines on the road is so they can calculate the more or less exact speed you were going by counting the number of lines passed between the two pictures- very simple basic arithmetic, but it won't work without the lines; with no lines all you have is two pictures of a car in slightly different places-it may have been doing 35, it may have been doing 55, but without series of fixed reference points there's absolutely no way of knowing if it was 35,55, 75, or whatever, so no, you won't get prosecuted if it's am old type box on a pole two-flash gatso.