What is the margin for error of UK speed cameras?

How far beyond the speed limit will a camera flash? We look at the margin of error for speed cameras – and why this threshold exists.

Speed cameras UK

Speed limits are there for a reason, and you should adhere to them to the best of your ability. However, we’ve all edged a few miles per hour beyond the limit at times – occasionally followed by a heart-stopping moment when you spot a roadside camera.

All speed cameras have a margin of error that allows for small excesses of speed. This threshold varies depending on the limit itself, so what are the facts?

Speed camera tolerances

Speedcurb camera on Millbank in Westminster

Auto Express magazine did some digging on the issue, including procuring figures from many of the UK’s police forces via Freedom Of Information (FOI) requests.

Nearly all the forces that responded quoted a threshold of ’10 percent plus 2mph’. That applies for both the ‘Gatso’ and ‘Truvelo’ style cameras, along with average speed check zones using multiple cameras at regular intervals.

Doing the maths, that means ‘accepted’ speeds could be as high as:

  • 79mph in a 70mph limit
  • 68mph in a 60mph limit
  • 57mph in a 50mph limit
  • 46mph in a 40mph limit
  • 35mph in a 30mph limit

Note the quote marks, though. This threshold is optional for police forces and certainly shouldn’t be treated as a speed to aim for.

Interestingly, two forces reported a ’10 percent plus 3mph’ threshold: Lancashire and the London Metropolitan Police. So you add another mile per hour to each of the numbers above.

According to Auto Express, the reason for this higher tolerance in London is due to higher traffic volume. In Lancashire, it’s just to allow a little more wiggle-room.

Why do cameras have a margin of error?

Speed cameras UK

Different cars display speeds to varying levels of accuracy. Some will show you’re doing 60mph, when you’re actually travelling at 57mph.

Construction and use regulations specify your speedo can over-read by 10 percent, but under-read by zero percent

The threshold is there, effectively, so that drivers have no excuse if caught. If you are flashed, it’s more likely you are deliberately breaking the limit than drifting just beyond it. It serves the interests of fairness and reduces workload for the justice system.

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1 COMMENT

  1. Dear Ethan Jupp, I know several friends, driving in Essex, in Rayleigh built up areas, who have received speeding fines through the post for doing 33 and 34 mph in 30 limits. Admittedly, these fines were issued several years ago. Your latest article, using FOI requests to many UK Police Forces, suggests that these fairly tight thresholds are unlikely to be applied nowadays. I believe that at least one major Newspaper carried out a similar FOI survey, over a year ago, and found that many Police authorities declined to give exact figures, saying that the 10% +2 mph was now considered to be ‘out of date’ and that each Police Authority uses its own discretion to set the thresholds – and often considerably tighter than the 10% + 2 mph guidelines, which apparently were originally based on what a Police Patrol Car might regard as a limit when pursuing a speeding motorist. Surely, motorists have a right to know what the rules are and how they may vary as they drive from one Police Authority to another?

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