TripRank
Where speed cameras are legal in the United States
Nineteen states and DC permit automated speed enforcement, ten prohibit it outright, and twenty-one have no law at all. What that patchwork means for a driver crossing state lines.
Ask ten British drivers what the speed camera tolerance is and most will say "ten per cent plus two". They are right about the number and usually wrong about what it means.
What the number is, and where it comes from
The threshold is not folklore. In an April 2024 Freedom of Information disclosure, the Metropolitan Police stated plainly that "the Met's speed camera enforcement threshold is 10% + 2mph", and that the force "observes the current NPCC guidelines which states that enforcement action should commence as soon as a speed that is at +10 +2mph".
So it is real, it is written down, and a large police force has confirmed in writing that it applies. What it is not is law.
| Posted limit | 10% + 2 mph |
|---|---|
| 20 mph | 24 mph |
| 30 mph | 35 mph |
| 40 mph | 46 mph |
| 50 mph | 57 mph |
| 60 mph | 68 mph |
| 70 mph | 79 mph |
Read that table as where enforcement typically starts, not as what you may drive. The offence is committed the moment you exceed the posted limit. Everything above that line is a decision about which offences get acted on.
The three sentences people skip
The same disclosure contains the qualifications, and they matter more than the headline number.
It can move. The Met states that "the minimum speed at which a speed camera is operated may be varied at any time", and that "speeds below those in the table may be used should the police decide to do so". There is no commitment to keep the threshold where it is, and no requirement to announce a change.
It is not a substitute for judgement. The disclosure notes that NPCC guidance itself says that "whilst tolerances exist, they do not and cannot replace police officers' discretion". A threshold governs the automated trigger. It does not bind an officer who sees you driving.
It is force-specific. This was the Metropolitan Police describing Metropolitan Police policy while observing national guidance. Policing in England and Wales is organised by force, and Scotland operates its own arrangements. Guidance is national; application is local.
Why the tolerance exists at all
Two practical reasons sit behind it, and neither is generosity.
The first is instrumentation. Every measuring device has an error band, and enforcement built on a device has to sit far enough above the limit that a prosecution does not turn on the last decimal place of a calibration certificate.
The second is the one drivers actually feel: cars are built to over-read. Under UN Regulation No 39, which governs speedometer type approval, the indicated speed may never fall below true speed and may sit up to 10% plus 4 km/h above it. A driver holding a steady indicated 70 mph is very often doing something closer to 67. If enforcement began at a true 70.1, a large number of drivers who believed themselves compliant would be caught by the bias designed into their own dashboards.
That is the honest relationship between the two numbers. The speedometer allowance is a construction rule for carmakers; the enforcement threshold is a policing decision. They point the same direction and are frequently confused, but one does not create the other.
What actually follows a trigger
Passing the threshold does not automatically mean a fine and points. Depending on the speed, the location and your record, the outcome can be a speed awareness course, a fixed penalty, or a court summons. The further above the limit, the fewer of those doors remain open.
How to think about it
The useful mental model is not "I have a buffer of 10% + 2". It is closer to: the limit is the limit, the threshold is where the machinery starts, and the gap between them is not yours.
Three things follow from that.
Do not drive to the threshold. It is discretionary, it is force-specific, and the disclosure explicitly reserves the right to enforce below it. A habit built on 35 in a 30 collapses the day the threshold moves.
Know which number your dashboard is showing you. If your car reads a few per cent high, an indicated 35 might be a true 33 — or, in a car that reads closer to true, an actual 35. You cannot tell from the dial which one you have.
Average-speed systems change the arithmetic. A fixed camera samples you at one point. An average-speed system measures the whole stretch, so a brief dip under the limit does not offset a sustained run above it.
Enforcement thresholds and penalties change. This guide reflects the Metropolitan Police disclosure linked below; check with the relevant force or with GOV.UK for the position that applies to you.