Speed Camera Tolerance UK: The 10% Plus 2 mph Rule Explained

Most UK drivers know the phrase "ten percent plus two" — the informal idea that speed cameras will not catch you unless you are more than 10% over the limit plus 2 mph. It is one of the most widely repeated rules of thumb in British motoring. It is also one of the most widely misunderstood. This article explains what the threshold actually is, where it comes from, where it does not apply, and what it cannot do for you if you receive a fine.

What the 10% plus 2 mph threshold actually is

The threshold originates from guidance issued by the Association of Chief Police Officers (ACPO) in 2011 — now superseded by equivalent guidance from the National Police Chiefs' Council (NPCC). It recommends that enforcement action should commence when a vehicle's speed reaches 10% above the posted limit plus 2 mph. The original ACPO document was explicit that the guidance "does not and cannot replace a police officer's discretion."

It is a prosecution guideline — a tool used by forces to decide when to act, not a legal threshold that defines when an offence is committed. Under Section 89 of the Road Traffic Regulation Act 1984, exceeding the speed limit by any amount — including 1 mph — is an offence. The statute makes no reference to the NPCC guidance whatsoever. As one road law barrister practice puts it: the guideline "has zero legal status." (Road Law Barristers)

Why the threshold exists: two technical reasons

1. Camera and speed gun measurement tolerance

The "+2 mph" element reflects the accuracy limits of speed detection equipment. Type Approved speed enforcement devices must produce a positive error no greater than 2 mph for speeds below 66 mph, or 3% for speeds above 66 mph. This means that a camera reading of 33 mph might represent an actual speed of anywhere from 31 mph upward — so prosecuting at exactly 31 mph in a 30 zone risks a conviction where none is justified.

The tolerance ensures police can only prosecute on the basis of reliable evidence. If a device's margin of error were ignored and a driver successfully challenged the reading in court, the prosecution would fail. Building the device tolerance into the guideline protects the integrity of enforcement.

2. Vehicle speedometer regulations

The "10%" element relates to how vehicle speedometers are permitted to behave. Under the Road Vehicles (Construction and Use) Regulations 1986 and UN/ECE Regulation 39, a speedometer must never show a speed lower than the actual speed, but it is permitted to show a speed up to 10% higher than true speed (plus a small additional allowance). In practice this means a speedometer showing 70 mph may correspond to a true speed anywhere between 63 mph and 70 mph.

A driver genuinely relying on their dashboard reading of 30 mph might be travelling at as little as 27 mph — or they might be at exactly 30 mph. The 10% element in the guideline accommodates this permitted speedometer inaccuracy when deciding whether to pursue enforcement.

The thresholds by speed limit

Applying the 10%+2 formula to each standard UK speed limit gives the following typical enforcement starting points. These are not guaranteed — they are what most forces apply in normal circumstances.

Note the distinction between the two columns: at the prosecution threshold, you may be offered a speed awareness course as an alternative to a fixed penalty notice and points — if you have not attended one in the previous three years. Above the course band, you move straight to a fixed penalty notice (£100 and 3 points) or a court summons for higher speeds. See how UK speeding penalties work for detail on the bands.

What it is not

Avon and Somerset Police, addressing this directly in a 2025 public statement, put it plainly: This is not an official allowance or 'safe margin' — it is a technical threshold used to ensure consistency and account for equipment calibration. (Avon and Somerset Police, 2025)

The guideline is used to decide whether to initiate enforcement. Once a Notice of Intended Prosecution has been issued, the guideline is irrelevant — it played its role in the decision to prosecute, and it cannot be used to argue that the prosecution should not proceed. Valid grounds for challenging a NIP are procedural: whether the notice arrived within 14 days of the alleged offence, or whether it contains incorrect details. "I was within 10%+2" is not a valid challenge.

Where the threshold is lower — or zero

The NPCC guidance is a recommendation to chief constables, not a binding rule. Forces can and do apply lower thresholds in specific circumstances:

• Roadworks speed limits — temporary limits through active roadworks are often enforced with reduced or no tolerance, particularly where workers are present. Average speed cameras in roadworks are common and routinely enforce at lower margins.
• School zones and 20 mph limits — some forces apply stricter enforcement near schools, particularly during arrival and departure times.
• Accident black spots — areas with a recent history of speed-related collisions may be subject to tighter enforcement.
• Scotland — Police Scotland applies zero tolerance. Any speed above the limit is prosecutable, regardless of the NPCC guidance which applies only to England and Wales forces.
• Some individual forces in England and Wales — freedom of information requests have revealed that some forces, including the Metropolitan Police and Lancashire Constabulary, have used a 10%+3 threshold rather than 10%+2. Others decline to publish their threshold at all.

Average speed cameras: a tighter margin in practice

Average speed cameras — the SPECS system and similar — calculate speed by recording the time taken to travel between two points, then dividing distance by time. They do not depend on a single instantaneous reading, and their manufacturers claim a margin of error as low as 0.1%. This removes most of the measurement uncertainty that gave rise to the +2 mph element in the first place.

In practice, average speed camera systems are used extensively in roadworks and on some permanent roads (notably the A9 in Scotland and several motorway sections). The practical effect is that the cushion is considerably smaller. Drivers who coast at 79 mph through a gatso-enforced zone and then slow for the next camera are unlikely to escape an average speed system that has recorded their overall journey time. See how different types of speed camera work for more detail.

Vehicle-specific limits: the threshold applies to your limit, not the road limit

This is widely misunderstood and matters practically. The enforcement threshold applies to the legal limit for your vehicle on that road — not to the posted limit for cars.

The clearest example is towing. A car towing a caravan or trailer on a motorway is subject to a 60 mph limit, not the 70 mph car limit. The NPCC prosecution threshold for that driver is therefore approximately 68 mph — not 79 mph. A driver towing at 70 mph on a motorway is 10 mph over their legal limit. The same logic applies to large goods vehicles, buses, and other vehicle categories with lower statutory limits. See towing speed limits in the UK for the full breakdown.

The NPCC's own warning about misuse of the threshold

In 2018, Chief Constable Anthony Bangham — then NPCC lead for roads policing — wrote publicly that the informal tolerance had created a problematic cultural expectation. His argument was that the guideline had been misread by drivers as permission to exceed the limit, and that this undermined the purpose of speed limits entirely.

The limit is set for a reason, he wrote. There should not be a 'comfort zone' over the speed limit where it is considered safe to speed. He confirmed that police would not pursue prosecutions for 1 mph over — not proportionate or achievable — but was clear that the tolerance was never intended to signal that any speeding up to that point was acceptable. (CC Anthony Bangham, NPCC Roads Policing, 2018)

The NPCC guidance has been periodically reviewed since then. Technology has advanced — average speed cameras with sub-1% error margins and AI-assisted enforcement reduce the measurement uncertainty that originally justified the +2 element. The direction of travel, if anything, is toward tighter rather than looser enforcement.

Why relying on the threshold is a bad idea

Even setting aside the legal position, using 10%+2 as a target speed is practically unreliable for several reasons:

• Your speedometer may not be accurate. Speedometers are permitted to over-read by up to 10%. If your speedometer shows 34 mph and you believe you are within tolerance for a 30 zone, your actual speed may be as low as 31 mph — or it may be exactly 34 mph. You are not reading a calibrated instrument.
• The threshold varies by force and situation. You cannot know in advance whether the camera ahead is operated by a force applying 10%+2, 10%+3, or zero tolerance in that specific location.
• Average speed cameras do not play by the same rules. The single-reading tolerance that underpins +2 mph is largely irrelevant to a system measuring average speed over a kilometre or more.
• The threshold may be lower than expected. School zones, roadworks, and accident black spots are all candidates for tighter enforcement, often without signage indicating this.
• It creates no buffer for the underlying cause of most speeding. Most speeding is unintentional — speed creep, missed limit changes, unfamiliar roads. See why drivers speed without realising. Targeting 34 mph in a 30 zone leaves no margin at all for that drift.

Disclaimer: This article is for general information only and does not constitute legal advice. UK driving law, speed limits, and enforcement practice are subject to change and content may become inaccurate over time. You should not rely on this article when making any legal or driving decision — consult a qualified solicitor for advice specific to your situation. Always observe posted signs, road markings, and the Highway Code. Speed Angel is a driving awareness aid only. It does not replace your legal obligation to observe speed limits.