Speeding: what is the “10% plus 2” rule and what does it mean?

Most motorists, when discussing speeding, will have no doubt heard tales of the “10% plus 2” rule.  But what is this rule, and what effect (if any) does it actually have in practice?


What is the ‘rule’?

The ‘rule’ itself is quite straightforward: if the speed limit is (for example) 30mph, the rule states that you won’t get a speeding ticket unless you are going 10% plus 2 mph faster than the limit.  In this example, this would mean that you would have to be travelling at 35mph or faster in order to receive a speeding ticket.

However, most people will have heard tales of friends and colleagues who have been given tickets for exceeding the speed limit by much smaller margins – in some cases by only a couple of miles per hour.  What happened to the rule in these cases?

The truth is that the “10% plus 2” rule isn’t actually a rule at all: it is merely a guideline.  The ‘rule’ originates from speed enforcement guidance issued by the Association of Chief Police Officers[1] which sets out the strategy that police forces should use when enforcing speed limits.  At paragraph 9.6 of this document, a table is provided which sets out the recommended outcomes for different levels of speeding:


Device Tolerance

Fixed penalty when education is not appropriate

Speed awareness if appropriate

From                      To

Summons in all other cases and above

20 mph

22 mph

24 mph

24 mph

31 mph

35 mph

30 mph

32 mph

35 mph

35 mph

42 mph

50 mph

40 mph

42 mph

46 mph

46 mph

53 mph

66 mph

50 mph

52 mph

57 mph

57 mph

64 mph

76 mph

60 mph

62 mph

68 mph

68 mph

75 mph

86 mph

70 mph

73 mph

79 mph

79 mph

86 mph

96 mph


The guidance recommends prosecuting drivers only where their speed exceeds the thresholds set out in the table.  It also sets out whether you are more likely to receive a fixed penalty, a speed awareness course, or a court summons.

A closer analysis of the table will reveal that the “plus 2 mph” relates to device tolerance, to take account of the fact that the speed guns used by the police are not always 100% accurate.  However, paragraph 9.7 of the guidance reveals that, at speeds below 66mph, the tolerance level is +/- 2mph, whereas for speeds above 66mph the tolerance level is +/- 3%.  This distinction is evident on the 70mph row on the table, where the fixed penalty threshold is 79mph, rather than 78mph as would be the case if the 3% device tolerance figure was not used.

What effect does the guidance have in practice?

The guidelines themselves state very clearly that they “do not and cannot replace a police officer’s discretion”,[2] and express reference is made to circumstances in which an officer decides to depart from the guidelines set out in the table above.  Therefore, you should not rely on the table as a set of ‘replacement speed limits’, as you may still be prosecuted.

The main speeding offence is contained within s89 Road Traffic Regulation Act 1984, which clearly states:

“A person who drives a motor vehicle on a road at a speed exceeding a limit imposed by or under any enactment to which this section applies shall be guilty of an offence.”

In court, this is the standard to which drivers will be held, and you will notice that there is no reference here to any threshold other than the signposted speed limit.  According to the statute, if you are even 1mph over the speed limit, you can legally be convicted for speeding if a police officer considers it appropriate in his discretion.  No reference is made to the guidance in the statute whatsoever: it has zero legal status.

What is important, however, is the column of device tolerance figures in the table above.  All the police need to do to convict you for speeding is prove beyond reasonable doubt that you have exceeded the speed limit.  However, given that the speed guns used by the police have a tolerance of +/- 2mph (or +/- 3% for speeds over 66mph), it is doubtful that they would be able to convince a court that a recorded speed of 31mph (for example) is a truly accurate reading: the driver in this example could actually have been driving at 29mph, which is clearly within the speed limit.

However, drivers should be aware that the Scottish police have decided not to follow the guidance at all, and will prosecute drivers for exceeding the limit by even 1mph, regardless of any device tolerance.[3]  There is also talk of a review of the guidance, which was first issued in 2011, to take account of developments in speed enforcement technology.

It is questionable whether the device tolerance figures outlined above apply to the average speed cameras widely used across the motorway network to enforce speed limits in road works, which calculate a driver’s speed based on the time it takes for their vehicle to travel between two points set a measured distance apart.  The manufacturers of SPECS average speed cameras suggest that their equipment only has a margin of error of 0.1% due to the fact that they measure speed over a longer distance than conventional fixed speed cameras.[4]  This clearly makes it easier for the police to prove that a driver was speeding in the even the most marginal of cases.  This may explain why Bedfordshire police made the decision in 2015 to apply a zero tolerance approach to speeding on the motorway.[5]


Whilst it is obviously advisable to stick to the speed limit, if you are caught speeding it is not the end of the world.  With the expertise of a Road Law Barrister on your side, you have the help you need to get the best possible outcome in court.  Back in March, we represented Mr C in Leeds Magistrates court.  He had been caught travelling at 56mph in a 30mph zone, and faced the prospect of at least 56 days disqualification, according to the sentencing guidelines.  However, having heard persuasive submissions from his Road Law Barrister, and having received evidence in support of these submissions, the Magistrates decided that 6 penalty points would be an appropriate sentence in this case.  Mr C therefore left court with his license and livelihood intact.

If you are caught speeding, or are arrested for any other driving offence, then please do not hesitate to contact us today for your free consultation.


The contents of this article should not be relied upon in isolation.  Each case is fact specific and this article should not be treated as legal advice or as a substitute for legal advice.



[1] See https://www.cambs.police.uk/roadsafety/docs/201305-uoba-joining-forces-safer-roads.pdf

[2] Para 9.7

[3] http://www.driving.co.uk/news/just-1mph-too-fast-and-youre-nicked-new-zero-tolerance-approach-to-speeding/

[4] http://www.driving.co.uk/news/just-1mph-too-fast-and-youre-nicked-new-zero-tolerance-approach-to-speeding/

[5] http://www.independent.co.uk/life-style/motoring/motoring-news/speeding-fines-bedfordshire-force-to-adopt-zero-tolerance-approach-towards-motorists-travelling-a6721846.html

How can we help you?