What is the margin for error of UK speed cameras?

Motoring Research
·2-min read
Speed cameras UK
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
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
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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