The Rt Hon David Cameron
The Prime Minister
10 Downing Street

A. citizen
100 Any Road
Typical Town


Date: 16th June 2010
 Dear Prime Minister

Five mile per hour incremental speed limits in the UK

 I don’t suppose that I am the only one to query why we do not have increments of five mph in our system of speed limit signage.  The only reason I can find is that they would be unenforceable because we do not have a law that requires car manufacturers to include speedometer marks to enable motorists to meet the obligatory speed limit, when that speed limit maybe 25mph or 55mph etc.. The USA have incremental speed limits and make good use of them so why don’t we?

 With the introduction of satnav, sat-nav or Sat Nav, the word and the technology is still so new as to not have a universally defined spelling, drivers are finding routes through the countryside that they have never used before. All road signage and speed limits may need to be carefully reviewed in relation to a considerable increase in traffic on minor roads. We also have a new word “traffic intimidation”. When what the people need is to keep fit by outdoor activities, what we have is people being put off walking and cycling by intimidation by motor vehicles.

 Unfortunately, traffic engineers do not have the power to use 5 mph increments. In one case, for example, Weston Super Mare council had to remove a 25mph speed limit, as it could not be enforced and knowing that 20mph would be too low had to resort to a 30mph limit!  In villages all over the country there are problems where 30mph is too high a limit and 20mph too low a limit, yet the engineers cannot install the obvious- a 25mph limit!

 Similarly, there are areas where 35mph may be better than 40mph and where a higher speed limit maybe more appropriate e.g. 55mph rather than 50mph. Although the Highway Code does state that the posted speed limit is the absolute maximum speed that a vehicle may travel at, the public behave differently. The public tend to drive at the maximum and tailgate vehicles that do not. 

The Association of Chief Police Officers (APCO) advise that police officers should allow 2 per cent + 2 mph when deciding whether to charge a speeding motorist or not. This seems fair, as there can be variations in speedometers, although I have heard they tend to be 10 per cent higher than the actual speed! The danger to a vulnerable road user is that they could be hit by a vehicle travelling at 35mph and the driver could escape prosecution. With a 25mph speed limit the vehicle could still travel at almost 30mph and not be prosecuted. (Except where there is a radar trap)

 The argument locally about introducing 20mph in villages is that the limit would be difficult to enforce, as motorists perceive that the limit is too low. Therefore, I ask again can we have 5mph increments in our speed limits so that traffic engineers can do a better job.


 Yours sincerely

 A. Citizen

The Prime Minister's office have referred my letter to the Department for Transport as they are best placed to respond to the matters I have raised. A letter from the DfT is published below.

Ian Edwards

Speed Policy Branch
Department for Transport
Zone 2/13
Road User Safety Division
Great Minster House
76 Marsham Street

Direct Line: 020 7944 2058

Fax: 020 7944 9618

GTN No: 3533 2058

Web Site: Mincom: 08/017734/10

19 July 2010

Dear Citizen


Thank you for your letter of 16 June addressed to the Prime Minister concerning incremental 5mph changes in speed limit. I have been asked to reply as I deal with speed limit policy issues.

1. The Department keeps all speed limits under constant review to ensure they remain appropriate and believes existing speed limits offer the right balance between issues of safety, the economy and the environment. The Department provides local authorities, who are responsible for setting local speed limits, with guidance to ensure those limits are appropriately and consistently set, whilst allowing flexibility to deal with local needs and conditions.

2. As I am sure you are aware, speed limits are set in multiples of 10mph from 20mph through to 70mph on motorways and dual carriageways. There are valid reasons as to why speed limits are set in this way, not least because speedometers are not sufficiently accurate to advise drivers of their precise speed. The police enforcement threshold of 10 per cent plus 2mph of the speed limit provides suitable leeway for drivers and measuring this tolerance is considerably easier when the speed limit is set in multiples of 10mph.

3. I would also like to assure you that the Department considers enforcement to be a policy of last resort. It has no desire to see drivers unduly prosecuted for minor infractions and would much rather drivers voluntarily stayed within speed limits. Maintaining the 10mph differential is a simple way to ensure drivers adopt a safe and appropriate speed within the limit. In addition, a change in the way speed limits are set would require a significant publicity campaign with a substantial lead-in time to prepare motorists in advance of the new rules. There are severe cost implications associated with these proposals and I am sure you would agree that this money would be better spent on improving the current road infrastructure.

4. It is also worth mentioning that a speed limit should not be considered as a target but as a maximum speed which may be driven in ideal circumstances. It is important to make the distinction between speed limits and vehicle speeds as the most appropriate speed on any given road is often likely to different to the posted limit.

5. Clearly drivers have a vital role to play in setting local speed limits as vehicle speeds are a significant factor in determining an appropriate limit. Naturally local authorities take a number of other factors into consideration such as accident data, the type and volume of road users and the nature and function of the road in question.

6. in your scenario where you state that "35mph may be better than 40mph" the local authority would need to determine whether 35mph was in fact the ideal speed to be travelling. If so, a 40mph speed limit would be thoroughly appropriate as this would provide the driver with sufficient leeway before enforcement would begin.

7. Most importantly, though, drivers have a responsibility to themselves and all other road users to drive safely and should not be thinking about how quickly that can arrive at their destination as much as making sure they arrive safely.

I hope this is helpful.

Ian Edwards

Paragraph numbers have been added to the DfT's letter above

1.  I can't understand how the DfT can provide "flexibility to deal with local needs and conditions" when a local authority may only set either a 20 mph or a 30 mph limit, the option to select 25mph is denied to them by the DfT's regulations at present.

2. Is the DfT seriously suggesting that the police would have difficulty making a calculation as to whether a motorist should be prosecuted or not?   Surely that is insulting the intelligence of our police force. All that needs to be done is to add 0.5mph to the calculation, and why not round it up? Incremental speed limits of 5 mph allow for greater discretion in deciding whether to charge a motorist or not. At present a police officer has the discretion not to charge a motorist who is travelling at 35 mph in a 30 mph limit, based on 10 per cent plus 2 mph. In a hypothetical 25 mph speed limit the calculation would be 2.5 mph plus 2 that's 4.5mph round it up to 5mph and the motorist would not be charged if he was travelling at 30mph. Rather appropriate isn't it?

3. I don't agree that there needs to be a substantial cost in introducing incremental speed limits of 5 mph. I am not proposing that all existing speed limits need to be changed, am I? All that would change is that where appropriate, highway engineers would be able to set not only safer speed limits but more sensible speed limits. The costs and benefits  of the introduction of a maximum 55 mph speed limit in the USA some time ago has been investigated in this document:  Speed Limits how should they be determined

4. Exactly, but unfortunately, motorists are travelling at the posted speed, the posted speed limit. The advice of the Highway code  although, stressing that the posted speed limit is the absolute maximum speed limit is being widely ignored.   Drivers are travelling at the posted speed limit and tailgating those motorist who are not. Now, that suggests that either the posted speed limit is too low, or that motorists feel that they may safely drive at that speed. But whatever the reason for such behaviour in many situations it leads to traffic intimidation and a danger to other road users.

5. Clearly drivers do have a vital role to play in setting local speed limits. In villages across the country there is a growing anger towards drivers who, although not breaking the speed limit, are driving at a speed that is perceived to be dangerous to the public. The highway engineer at present has no choice it's either a 30 mph or a 20 mph speed limit. In many settlements there is a great difficulty in setting a 20 mph speed limit as it would be regarded by most drivers as being too slow. The police if  consulted will state that imposing a 20 mph limit would be difficult to enforce,

6. No, I disagree, I am not suggesting an "ideal speed" but a posted speed limit of 35 mph. if the most appropriate speed were to be assessed by highway engineers as 35 mph then a motorist travelling at 41 mph would be unlikely to be charged by police for speeding. That is the motorist's "leeway" a 40 mph limit would mean that a driver could be travelling at 46 mph and not be charged with a speeding offence. No, if the highway engineers decided that the most appropriate speed limit were to be 35 mph then that should be the posted speed limit.

I can't read a driver's mind but I think that the majority of drivers see a speed limit as signifying that they may drive at that speed, and that is safe.  The DfT won't be able to change that, what it can do is make sure that highway engineers may fine tune the posted limit, they cannot do that at present.

Returning to the observation in paragraph 2 of the DfT's letter.

 I quote: " There are valid reasons as to why speed limits are set in this way, not least because speedometers are not sufficiently accurate to advise drivers of their precise speed"  Agreed, speedometers are not sufficiently accurate to advise drivers of their precise speed, in that case it does not matter whether the speedometer is graduated in increments of 10 mph or 5 mph they are still inaccurate. And in fact, no mention has been made in the DfT's letter, that manufacturers tend to ensure that their speedometers are inaccurate as they display a higher speed! The opposite- displaying a lower speed would lead to them being responsible for a motorist being mislead as to the speed his vehicle was travelling at.

If a motorist is unable to see a 5 mph increment then he should not be driving and as mentioned in my letter the USA has incremental speed limits of 5 mph and I can find no reason in the letter from the DfT why the UK doesn't introduce them.