Speed Camera Tolerance in the UK

What Is the Speed Camera Tolerance?

The speed camera tolerance — often referred to as the "10% plus 2" rule — is an enforcement threshold guideline used by most police forces in the UK. It suggests that drivers should not be prosecuted for speeding unless they exceed the posted speed limit by 10% plus 2 mph. For example, on a 30 mph road, the threshold would be 35 mph (30 + 3 + 2 = 35). On a 70 mph motorway, it would be 79 mph (70 + 7 + 2 = 79).

However, this is a guideline, not a law, and there are important nuances that every driver should understand.

The NPCC Guideline Explained

The tolerance guideline comes from the National Police Chiefs' Council (NPCC, formerly ACPO). It was introduced to account for the fact that no speedometer is perfectly accurate. Vehicle speedometers are legally required to never underread — they must always show a speed equal to or greater than the actual speed. This means most speedometers slightly overread, often by around 2–3 mph.

The tolerance threshold provides a margin to account for:

• Speedometer inaccuracy: Your car's speedometer may show a higher speed than you're actually travelling
• Camera calibration margins: A small tolerance accounts for any measurement uncertainty in the camera itself
• Proportionality: Prosecuting drivers travelling at 31 mph in a 30 zone would be disproportionate and would overwhelm the courts

Tolerance by Speed Limit

Here's how the 10% + 2 guideline translates across common UK speed limits:

• 20 mph zone: Enforcement from 24 mph
• 30 mph zone: Enforcement from 35 mph
• 40 mph zone: Enforcement from 46 mph
• 50 mph zone: Enforcement from 57 mph
• 60 mph zone: Enforcement from 68 mph
• 70 mph zone: Enforcement from 79 mph

Does the Tolerance Apply Everywhere?

No — and this is the most important point. The 10% + 2 rule is a guideline issued by the NPCC, but individual police forces are free to set their own enforcement thresholds. Some forces strictly follow the NPCC guideline, while others enforce at lower thresholds, particularly in high-risk areas.

Key exceptions and variations include:

• 20 mph zones: Several forces enforce more strictly in 20 mph zones, particularly near schools and hospitals. Some enforce from as low as 22 mph
• Variable speed limits: On smart motorways, some forces enforce variable speed limits more strictly than the national limit, as the variable limit is set for specific safety reasons
• Roadworks: Temporary speed limits in roadworks zones may be enforced at or very close to the posted limit to protect workers
• Scotland: Police Scotland has historically used slightly different thresholds in some areas

Regional Variations

Freedom of Information requests have revealed that enforcement thresholds do vary between police forces. While most follow the NPCC guideline for standard speed limits, there is no obligation to do so. Some notable variations:

• Metropolitan Police (London) generally follows the NPCC guideline but has been known to enforce more strictly in 20 mph zones
• Some rural forces have lower thresholds on country roads with high collision rates
• Welsh forces may apply different thresholds following Wales's adoption of a default 20 mph limit on restricted roads

Why You Should Never Rely on the Tolerance

There are several compelling reasons why relying on the speed camera tolerance is a bad idea:

• It's a guideline, not a law: No police force is legally obligated to apply any tolerance. You can legally be prosecuted for exceeding the speed limit by even 1 mph
• Thresholds can change: Forces can alter their enforcement thresholds at any time without notice
• Your speedometer may be inaccurate: If your speedometer overreads by 5% and you're driving at the tolerance threshold, your actual speed may be higher than you think — or lower, meaning you have less margin than expected
• Tyre wear and pressure: Changed tyre sizes, low pressure, or significant wear can alter your speedometer's accuracy
• Safety: Speed limits are set based on the conditions of the road. The limit itself is the maximum safe speed, not a target. Exceeding it by any amount increases risk

Speed Awareness Course Thresholds

The threshold for being offered a speed awareness course (instead of points) is typically narrower than the prosecution threshold. As a general guide, courses are offered when the detected speed is between the enforcement threshold and the threshold plus a further margin — roughly between 10% + 2 and 10% + 9. Above that upper threshold, a fixed penalty or court summons is more likely.

For example, on a 30 mph road:

• Below 35 mph: No action
• 35–42 mph: Speed awareness course likely to be offered
• Above 42 mph: Fixed penalty or court summons

These thresholds are approximate and vary by force. The speed awareness course option is also only available once every three years — if you've attended one within the last three years, you'll receive a fixed penalty instead.