Photos
Gatso Cameras take two pictures, normally 0.5 seconds apart. This time interval should be indicated by the two photographs. If you have both photographs you can judge the distance travelled in the time and hence derive the speed of the vehicle at the time of the offence. If the photographic evidence shows that contrary to the radar evidence you were not exceeding the speed limit, then you are in the clear.
The first problem is obtaining the photographs. Many speed camera pratnerships will simply refuse to send them out unless you plead not guilty, but you cannot make that decision until you have seen the evidence. They want to force the maximum possible inconvenience upon you in the hope that you will simply give up and pay the fine. Justice has nothing whatsoever to do with it. But you should persist, engaging the help of a solicitor if necessary.
Examine the photographs and count the number of marks passed by the vehicle between the two photographs. One way of doing this is to count the number of marks visible behind the car in the second photograph and subtract the number of marks visible behind the car in the first photograph. It may not be a whole number, so you will have to estimate any fraction.
Calculation
You need the following pieces of information:
Number of marks passed
Interval between photos in seconds
Distance between marks in feet
You can calculate speed in mph as follows:
Speed = Gap * Marks * 0.6818 / Interval
Example:
Suppose the vehicle passed 7 marks 5 feet apart in 0.5 seconds. Calculate as follows:
7 marks * 5 feet * 0.6818 / 0.5 seconds = 47.7 mph
You may need the following conversion factors:
Mph to feet per second: multiply by 1.46667
Feet per second to mph: multiply by 0.6818
Feet to metres: multiply by 0.3048
Metres to feet: multiply by 3.2808
