How to check the accuracy of a Gatso speed camera from the photos
 
This page explains how to check the accuracy of a speed measured by a Gatso speed camera. The accuracy of such cameras, and the competence of the officials checking the photographs is in doubt following several high profile cases.
 
Gatso speed cameras are the most common type of speed camera in the UK. They are often painted grey, but other colours are used, and they may now have yellow reflective panels or stripes on some sides. The cameras are usually accompanied by a series of white lines painted on the road, also known as Dragon's Teeth. There are two possible configurations of these markings:
 
Centre of the lane
 

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Centre lane markings on Holt Road, Wrexham
 
Edges of the lane

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Lane edge markings on the B5129 Pentre, Flintshire.

At the time of writing it is not clear that all calibration marks are the same at all sites around the country, however, the most common interval appears to be 5ft between each minor mark, or 25 feet between the major marks.
 
To be certain you will need to revisit the site and measure the distance between the marks.
 
If the lane markings are in the centre of the road please take great care when doing this. It is best done at a quite time of day, with an assistant watching out for traffic. To be safer still, take some chalk with you and place chalk marks on the kerb directly in line with two of the road marks, then measure the distance between the chalk marks.
 
When measuring the marks, it may be more accurate to measure several gaps between marks rather than just one. Then divide by the number of marks. Be sure to measure from the same side of each mark.
 
No Markings?
 
Some Gatso speed cameras have no road markings at all. This may be because the camera housing is just a dummy, or the road may have been recently resurfaced. If you obtain photographs showing no markings, you can still estimate the distance travelled by returning to the location with the photographs and placing chalk marks on the kerb in line with the back of the vehicle in each photo. Measure the distance between these chalk marks.
To improve your evidence, place noticeable objects at these two points and take a photo from precisely alongside the speed camera lens, so that your photo and your measurement can be compared with the original photos.
 
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:
  1. Number of marks passed
  2. Interval between photos in seconds
  3. 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

 
 
Further Reading