Road Safety News

Slower speeds are better for health: Danny Dorling

Thursday 27th March 2014

Professor Danny Dorling certainly pulled no punches in the online seminar he hosted last week (27 March) on the Academy website.

Professor Dorling was answering Academy members’ questions about 20mph limits and zones, which he has described as “the most effective thing a local authority can do to reduce health inequalities”.

Below is a full transcript of the seminar - with the questions in red.

How can we be assured that speeds in such areas do not exceed 20mph without the use of ISA (Intelligent Speed Adaptation), which, it would seem, the Government is failing to promote?
Almost all areas which should be getting 20mph currently have 30mph. In such areas we cannot be assured that speeds do not exceed the 30mph limit. However, because there is a 30mph limit drivers become used to driving no faster than that limit. They know it is illegal to do so. And most drivers drive at or below the limit which has the effect of forcing drivers behind them who might want to drive faster than 30mph to have to obey the limit.

There is no reason to suppose that the same process does not operate in 20mph zones. When speeds in such zones have been monitored, local councils have found that they are successful in encouraging drivers not to speed. Just as 30mph zones are successful in encouraging large numbers of drivers not to drive above 30mph in those zones. The problem is that 30mph is simply too fast in most residential areas, near shops and schools. It is an anti-social speed to drive at, but by zoning these areas as 30mph drivers are being encouraged to drive towards that speed. It is quite hard to drive at 20mph in a 30mph zone. You get tail-gated very quickly.

There is now a huge amount of evidence on the 20’s Plenty website as to why councils should be ashamed if they have not joined the growing numbers who already think their citizens are worth enough to be able to live in quality neighbourhoods without such fear of traffic. See: and, if you want to know why this matters above improving the general quality of neighbourhoods, see:

If 20mph schemes have a positive impact on health inequalities and deliver wider health benefits, at a local level who should be paying for the schemes? Should it be highways/transportation or public health?
In a time of austerity it is hard to think of a policy which represents better value for money than replacing 30mph signs with 20mph signs. It would be cheaper if there was a national move to make the default residential speed limit 20mph, and that will occur at some point. Already around 100 local authorities have adopted 20mph – at some point there will only be a minority which have not. However, before we get to that point, there is the question of who pays.

In the North West of England public health authorities have led the way by providing budgets to local authorities to help them implement authority-wide 20mph schemes. It is worth asking why this is not happening (if it is not happening) where you live? It is not just the loss of life that is prevented by slower speeds, or the savings made to the NHS from having to deal with fewer accidents on the roads. People walk more and cycle more when they are less afraid of traffic, which is good for health. Pollution from cars is reduced, which is good for health. Sitting in a car burning almost no calories is not good for your health and sitting in car going at speed is the most dangerous thing most people do in any day – other than trying to cross the road.

There are currently many contentious issues in public health over what works best: statins/never drinking/drinking moderately – and so on. There is no contention over whether slower speeds are not better for health.

With the results of the DfT’s study into the effectiveness of 20mph limits not expected until 2017, what should local authorities do in the intervening period?
There are already enough results showing the effectiveness of 20mph limits that local authorities can act on. Perhaps the single most devastating study is the following paper which was published in the British Medical Journal on 2 January 2010: “Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis”.

Here are its results for London:
“The introduction of 20 mph zones was associated with a 41.9% (95% confidence interval 36.0% to 47.8%) reduction in road casualties, after adjustment for underlying time trends. The percentage reduction was larger in children (48.5%, 41.9% to 55.0%) and also larger for the category of killed or seriously injured casualties than for minor injuries. There was no evidence of casualty migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8.0% (4.4% to 11.5%). The study estimates that 20 mph zones prevent around 200 casualties a year, of whom 27 would be killed or seriously injured and 51 would be pedestrians.”

I can see no reason why, until the DfT study is released, local authorities should not take this British Medical Journal paper as being the single most important study published to date. I also hope that the DfT study is peer reviewed and refereed to the same level of quality as that BMJ paper. It should at the very least include analysis of the same quality, but for all of the country and a longer time period.

What would you suggest is the most effective way to ensure 20mph speeds are achieved if a 20mph limit is introduced?
I think it is a question of winning over hearts and minds. At some point in the future we may have “Intelligent Speed Adaptation” and all manner of other technological advances which would make us safer. Human beings didn’t evolve to safely steer a ton of metal and glass at speed. It is a stupid thing to get a human to do. But, in the short term, we are where we are - and we need to quickly move many more areas to slower speeds.

Take an analogy. How do you stop parents hitting children? It used to be common for parents to hit children, often in public. Teachers used to be allowed to hit children as well when corporal punishment was legal. Now we consider hitting children as child abuse. Hearts and minds were won over.

There will come a time when driving at 30mph in a 20mph will come to be seen as being as anti-social, as small minded, and as inconsiderate to others as smoking in a confined space is now viewed, or as randomly striking out at a stranger might be seen, let alone hitting children. If you think the analogy with hitting children takes things too far ask yourself what damages, kills and seriously injures children the most in the UK today – adults who strike them with their fists, or adults who strike them from behind the wheel of a vehicle?

We got people wearing seat belts through hearts and minds campaigns, and to largely stop drink driving. We are currently helping people to realise that they are considered an idiot if seen talking on their mobile phone while driving. Most things in society work because most people follow the rules, including a lot of unwritten rules. A few people are psychopaths, a slightly larger number lack much empathy, but the vast majority of people are what is called “pro-social” in their attitudes. When it comes to behavior change and predisposition this is a rapidly evolving area of research, but we are getting a clearer and clearer picture that suggests only a few people have severe antisocial behaviour problems and can’t be persuaded not to harm others.

Some road safety professionals feel that all the attention/funding that is being focused on 20mph limits is deflecting attention/resources from bigger road safety issues such as young drivers/motorcyclists etc. What would you say to them?
It is not an either/or issue. Would you suggest that campaigns telling people not to smoke detract from campaigns suggesting that it is bad to drink too much as it might harm your liver? There is also an obvious end point to 20mph campaigns. Perhaps in those areas of the country that have them campaigners are now more free to concentrate on the problems that are left – including coping with those few people who are arguing for even faster motorways.

Men and motorcycles is certainly a big problem. Young people getting access to cars is similarly a big issue. Anybody reading this forum will know that the worst birthday present you can give an 18 year old is a fast car. Sadly many rich people do not yet understand that. You have to be rich to afford to make such a mistake.

There are other issues too. Why do we not have a public transport system more like that which works so well in Japan? Why do we have such polarized and segregated cities that we commute so much further than most other people in Europe do (because it is harder to live near where we work)? Why can’t we see that we need far more cycle friendly cities such as they have in the Netherlands and Denmark?

There are always many things to consider. But of all these, the simplest question to answer is – is there any good reason not to have 20mph in residential areas, near shops or schools? The answer is that there is not a single convincing reason for carrying on treating other people’s welfare as being of so little value.

There have been many cases where societal values have developed and as a result laws changed to reflect those values. Whilst there may be majority support (as in case of 20mph limits) universal support (and hence compliance) may take longer. The role of enforcement always plays a part in levering up (or down) the rate at which new values are accepted. It is often done with more progressive communities (cities or countries) taking the lead and eventually others following towards universal acceptance. I would ask if Prof Dorling could put the movement towards lower speeds in urban and residential environments into such a context and compare with similar movements for change.

OK – see my answer above to James – but let’s make the context much wider. Here is a list of things that we made law. At each point there was opposition to it becoming a law – and then, not much later, people wondered how on earth they could have ever opposed the change in the law:

1)      Factory acts limiting the working hours of children
2)      Laws requiring the building of sewers in towns
3)      Extending the vote to people who did not own property
4)      Intruding a 30mph speed limit in the first place
5)      Introducing a right for ramblers to climb hills
6)      Banning fuel/coal that causes smog in cities
7)      Banning teachers from using the cane on children
8)      Rules to prevent people owning or carrying guns
9)      Domestic violence becoming seen as a crime
10)    The introduction of 20mph local speed limits 

I’ll have got some of these out of order. It is simply a list off the top of my head. You may think some are more important than others (I chose not to mention slavery, the death penalty, or abortion). Often the gaining of rights is continuous. It is never easy. If it was easy it would have happened by now. But the campaign for 20mph in residential areas, by shops and schools is part of a wider campaign to be safe and happy living in your city, town or village. It’s a campaign to worry far less when your children play out, or when your elderly parent walks home, or when you don’t want to have to be so afraid – including many people who are afraid of driving at current unsafe speeds.

Do you think engineering schemes are pretty much a necessity in order to get people to comply with 20mph limits?
The most important thing to understand is that speed enforcement in the main works by convent and people adapting to what becomes normal. The introduction of speed humps is not favoured by many of those advocating 20mph in all residential areas because of the cost and the annoyance, the damage to vehicles and so on. Similarly with road narrowing.

There will always be places were engineering is needed to improve road safety as well as the liveability of cities. But these will always be a minority of areas. Similarly there will be areas where cameras help, but these too will be a minority of areas. Just as for 30mph today, 20mph is becoming the new norm in many areas and will become so in many other places.

It is when drivers (and cyclists) calm down, and are helped to calm down by being told that 20mph is the maximum safe speed to travel in an area that they slow down and start to see the people and places they are driving past. This is not just an issue of immediate road safety. Slower speeds would mean that far more parents would be happy with their children playing out and visiting friends on their own, going to school on their own at ages, 8, 9 and 10.

The ultimate enforcement goes when a large majority of people come to see that a particular action is anti-social. In those towns and cities where 20mph is already normal you can begin to see drivers changing more than just their speed. They become more courteous overall. Cyclists find it more embarrassing to undertake and to act in other ways when car drivers are driving more slowly. Pedestrians see cyclists and drivers as less of a threat, partly because at 20mph or less it is easier to look people in the eye.

Professor Danny Dorling

Danny Dorling is Halford Mackinder Professor of the School of Geography and the Environment at the University of Oxford. He is a widely published and highly respected expert with a special interest in differential life expectancy between socio-economic groups.

Recently, in a paper published by the British Academy, he suggested that introducing 20mph limits is the most effective thing a local authority can do to reduce health inequalities. In his paper Professor Dorling suggests that “introducing 20mph zones would save lives, prevent injuries and reduce health inequalities in the process". He describes it as “a low cost measure and a devolved power that can only easily be enacted at the local level”. He goes on to suggest that “death is much less likely if a pedestrian is hit by a car travelling at 20mph, than at 30mph or more, and cyclists are far safer if travelling with traffic that does not exceed 20mph”. He also says that “lower traffic speeds bring many other benefits: less congestion; less air pollution and CO2 emissions; stronger communities; more walking and cycling; and reduced obesity”.

Road Safety GB Academy
The Road Safety GB Academy is the professional development arm of Road Safety GB. Complimentary membership of The Academy is offered to road safety practitioners employed by local authority members of Road Safety GB – up to a maximum of 10 places per authority. Applications are also welcomed from practitioners working in the private and voluntary sector, subject to meeting the membership criteria. The annual cost of membership for these practitioners is £35.

Since its introduction in July 2013, the monthly online seminar programme has proved popular with Academy members. The seminars are free and open to all members, and questions can be submitted in advance or during the session.

For more details or to apply for membership visit The Academy website or contact Emma Norton, head of membership.


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"An unenforced 20mph speed limit must be better than an unenforced 30mph speed limit."

Absurd. Speed limits must be respected, if there is a problem with compliance and enforcement with 30, all that a 20 limit will achieve is lower compliance and further erosion of respect for speed limits as a whole.
Dave Taylor, Guildford

Agree (9) | Disagree (4)

I would suggest the biggest change in Oxford has been the reduction in traffic as much of it has been pedestrianised. That would make much more of a difference than a speed limit reduction from 30 to 20 and would obviously reduce volumes in approaching and surrounding roads. Are you seriously suggesting cyclists go slower in 20 limits? I see very poor compliance with 20 limits as confirmed by the 1mph drop in Portsmouth.
Dave Taylor, Guildford

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20mph speed limits got strong endorsement this week from both London Assembly’s Feet First report and PACTS’ Achieving Safety, Sustainability and Health Goals in Transport.

And 20mph limits are being rolled out, and not just on residential roads. This summer the City of London will start an 18 month pilot, with all roads including Blackfriars Bridge and London Bridge. Only Chancery Lane will be exempt.

On enforcement. Jenny Jones’ London’s Lawless Roads highlighted how few camera activated speeding FPNS were paid. And RoadPeace has documented how rare it is for MPS officers to enforce 30mph. An unenforced 20mph speed limit must be better than an unenforced 30mph speed limit.

No one will be surprised to hear that RoadPeace, the national charity for road crash victims, supports 20mph limits. They were a key call in our 1997 parliamentary manifesto, and we believe the case for them has only grown with our waistlines and deficits.
Amy Aeron-Thomas, London

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Dear Dave,
I happen to find the statistics in the reports that have been published credible. But I do think this is more an argument about entrenched views more than statistics, so maybe an annecdote helps. I have just cycled across Oxford to the train station. As you probaby know Oxford now has 20mph on most roads. I counted 20 repeater signs on my journey. I looked out for the other possibilities you mentioned. There was hardly any new road engineering along my route as compared to when I cycled along this same route as a child. The traffic volume was not high. Cars and cycles could have travelled faster but were choosing not to. There was no road calming along my route and the road markings often looked as if they were as old as me! Some cycle lanes had been added, but not along most of my route. In short, all that had changed consistently along the route were the little round signs containing 20 in a red circle.

It is, of course, possible that people travelling calmly and all below 20mph was due to some publicity campaign in the past. Or maybe something has been added to the water supply of the city to calm people down? But I do think that the most obvious answer is probably the right answer. The big surpise for me, having moved back here in the last year, is watching cyclists not travelling at speed, as they don't need to keep up with cars. I have become used to cars, vans and lorries driving at 20mph - or below.

I don't think it's hard to see and measure the effect of the little road signs. What is hard to know is what would happen if, instead of all these signs, there were simply a few large signs on entering the city declaring that the speed limit was 20mph, and the only other signs you then saw were telling you when you had entered or were exiting one of the few 30mph roads? That is a difficult question to answer. But either reading the reports showing speeds declining, or seeing it yourself in areas with many 20mph signs, is not that difficult. I can see that it is easier to understand that the signs work if you live where there are signs. It is a bit like understanding that the smoking ban in public buildings worked after having lived through and experience the effect of that ban. Of course some people complained when that ban came into force; a few still do, but they are a minority. Compliance is largely voluntary, but gaining compliance to a measure which the majority of people support has tended to work in the past and works now in existing 20mph areas.

All best wishes,
Danny Dorling, Oxford

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Danny - you would do well to treat with care any claims made by Brake or indeed 20's Plenty - see and

My detailed analysis at the first URL shows that by the 4th year in Porstmouth SI continued its worsening trend relative to national urban figures and was 50% higher than had it followed national trend. Which is why Brake and 20's Plenty choose to highlight total casualties not SI.
Idris Francis Fight Back With Facts Petersfield

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Mark - that the kinetic energy of a moving vehicle has any significant relationship to injuries is a fallacy - it is the forces involved in suddenly changing the speed of the injured party that matter.

The kinetic energy of a train doing 40mph may well be 1 million times greater than of a Fiat Panda doing the same speed, but the injuries sustained by being hit by either, in an otherwise identical impact, will be virtually identical as in both cases the individual is accelerated very rapidly to the same speed as the vehicle.

This is no more than Newton's Laws of Motion, what astonishes me is that so many commentators on road accidents don't seem to understand them.
Idris Francis Fight Back With Facts Petersfield

Agree (7) | Disagree (3)

Were there other factors involved in those accident reductions? Traffic volumes, calming, road markings, publicity, trend, etc? It's just as easy to find articles which are not so complimentary, such as:

If you ask someone if they want safer roads of course they are likely to say yes. I certainly want safer roads and I am a regular cyclist. I certainly agree that speed is a problem but you can't make that problem go away just by putting numbers in circles and making those numbers lower and lower.
Dave Taylor, Guildford

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On Portsmouth, and others areas, and to quote from the website report of today's evidence and survey from the charity 'Brake':

"It is citing case studies in Portsmouth and Camden, where a reduction to a 20mph limit led to a drop in accidents by 22% and 54% respectively. Other areas introducing 20mph limits include Birmingham, Newcastle and Edinburgh.

The survey also found that 72% agreed that their town needed to be made safer, with improved facilities for pedestrians and cyclists, with a similar number (79%) agreeing that more people would be encouraged to get out of their cars in favour of other methods of transport, provided that safety was improved.

81% of those surveyed also said that they felt traffic was travelling too fast on some or most of their local roads."

Brake's report can be found here:

All the best,

Danny Dorling, Oxford

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Rod, Danny:
As has been seen in other areas like Portsmouth, in line with common sense too, the average speed reduction actually achieved by 20 limits is in the order of 1MPH. How can this really make any difference whatsoever to anything, no matter how many nice words you associate with it like “behaviour, environments, movement and liveability, public health, community” etc.

I can tell you that when I need to drive the slowest (picking up kids from school or in busy shopping areas) the very last place I’m looking is my speedo, my eyes are constantly scanning everywhere to look for hazards and my speed is walking pace. Strangely and fortunately the vast majority of other drivers are the same. Those that don’t really won’t give a damn about a 20 limit and anyone believing that they are doing the right thing by just obeying a 20 limit would be knocking people down all over the place like 10 pins.

Responsible driving amongst vulnerable users is a complex task, achieved best with the right attitude, and to believe that the solution to that complex task is to put a number 20 in a circle is quite ridiculous, and to rely on it as a safety solution is therefore dangerous.
Dave Taylor, Guildford

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Dear all, if you are interested in a referenced and refereed version of what I had to say please see this report which was published by the British Academy earlier this year, it gives many more details:

The select group who have to be influenced are the minority of people who regularly drive. Without 20mph limits in force it is very hard for them to slow down. Just as it was hard for an employee to pay the minimum wage before the minimum wage became law. Their competitors could out-compete them, but compliance is largely social as the number of recent prosecutions of employees for not paying the minimum wage can be counted on one hand.

Humans have developed social systems of condoning some actions and not accepting other behaviour for millennia, probably not for as long as we have been able to swim, but at least for as many thousands of years as we have been doing maths. Cars and lorries have only been around for a few generations. That is why the rules over how best to live with them are still so contentious. But do you really think that after 20mph are introduced in residential areas, around shops and schools, there will be people campaigning to raise the speed limits again?

Other roads are, of course, very important as the majority of people who die on British roads now die on faster roads, but there I don't think the solutions are as obvious, or cheap, or so workable, as 20mph.
Danny Dorling

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To a certain degree I agree with Tim that this whole debate is about whether we can change behaviour. But I disagree in that either coercion cannot be used or that it is the only method that is effective.

This is really a debate about our urban realm and how we value it. Should that be based on the perspective of people in cars, people outside of cars or even road safety professionals? And when we do revalue that public space between homes and buildings that we call streets how can we deploy the necessary societal, psychological and coercive measures to maximise compliance.

Traffic authorities and governments are changing their attitudes to such roads and many realise that city expansion cannot be met by simply expanding car usage. Our cities need repopulating from the inside to maximise their economic success and create more vibrant living environments. The town planners and those seeing the “bigger picture” understand the need to win back “streets for people” and balance the need for motor and people movement and liveability.

And in this respect the social observers like Danny who identify such aspects as gender distribution, inequality and public health are bringing a fresh and valuable perspective on how we can revalue those places. With increasing acceptance of 20mph as the right limit, more effective policing (NDORS courses) and cheaper signage and administration then soon we will be at the tipping point where we have established that new social norm. It won’t be perfect (as any speed limit) but certainly a community that sets 20 as being plenty on its residential roads will be a far better place to be than one that endorses 30mph.
Rod King 20's Plenty for Us

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What an interesting exchange:-

EB “I suggest that for a given scenario of, say, a pedestrian emerging from between parked cars the impact speed of an alert driver at nominally 30mph could be lower than a less alert driver at nominally 20mph.”

NR “Are you able to substantiate your assertion that drivers are more likely to be alert when driving at 30mph than at 20mph?”

EB “My assertion is actually that drivers are more alert when driving at around the 85%ile speed. That is based on personal experience, observation of other drivers (including as a passenger).”

The quoting of the mythical 85%ile is interesting because in a comment on a previous topic EB said “The 85%ile speed is the speed which 85% of drivers would not exceed if there were no speed limits”

Hence we have an assertion being substantiated not by research simply “personal experience and observation”. And that “assertion” is based upon drivers adjusting their speed to a level that could only be determined after analysing the behaviour of drivers on that road if it did not have a speed limit.
Rod King 20's Plenty for Us

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Curious comparison to make but I would have to observe that introduction of the Living Wage like all those other societal changes mentioned by Prof Dorling is not done by influencing the population at large but by influencing a select group, in this case employers. I can only say again that in spite of this spurious distraction over gender, all the benefits claimed of this initiative whether accruing to women, the poor, or anyone else will not arise if speeds remain largely unchanged.
Tim Philpot, Wolverhampton

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We didn’t evolve to be capable of maths or swimming but most us can do these things naturally. And in fact we do have a natural built in understanding of speed, probably from the need to be able to spear fast moving animals etc. In any case, 20mph is within the capabilities of a man running, does it seem strange to you that this is possible without them being unable to avoid running into trees etc? Of course these speeds and far higher are within human capabilities. If they are not, why are you not asking for 20mph limits on all roads? Do you really think that a number in a circle makes everything safe?
Dave Taylor, Guildford

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Adding to the gender balance here, I work for 20's Plenty for Us so am clearly in favour of 20mph.

Let me compare this debate to the minimum and Living Wage. At the bottom of the heap, very low wages were common until regulated by the minimum wage (with the state picking up the safety net cost in tax credits etc). Wide area 20mph is like bringing in the Living Wage. We know it's the right thing to do. We know it helps people to live better lives. The detractors are like those people still only grudgingly paying min wages. The smart places like Oxford, Cambridge and York have worked out that better quality public realm means change and have committed to giving 20mph to most of their residents. Like the Living Wage this brings better equality, dignity, retention (fewer deaths and injuries in the road safety case), self esteem and sustainability. Yes there is a transitional up front cost. But its well worth it. And yes, like the Living Wage, 20mph benefits women and the poor more than men.
Anna Semlyen

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I agree with Hugh and Tim and find the vagaries of "It is natural to be afraid of heights. Unfortunately it is not natural to be afraid of speed".

Heights vary from a kerb to many hundreds or even thousands of feet. People vary widely in what they feel comfortable with. I was content to be inside the top of the Shard on Friday evening but would not have climbed into a window-cleaning cradle.

Similarly, speeds [in cars] vary from around walking pace to well over 100mph. Different people feel comfortable (and, I suggest, are safer) at different speeds. This is why you will find vehicles travelling at 50mph on motorways.

Like Tim, I am disappointed by lack of rigour, in favour of wide generalisations and rather meaningless assertions.
Eric Bridgstock, Independent Road Safety Research, St Albans

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Anyone else puzzled by Mr Dorling's - perhaps unneccessary - references to gender?
Hugh Jones, Cheshire

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So now as well as being the wrong sort of respondent according to a major Local Highway Authority I am now the wrong gender to have any serious views on this matter. Well that's me told. Prof Dorling once again misses the point that it is not speed reduction I oppose, it is the belief that this will be achieved by simply changing the speed limit. I had hoped some academic rigour would be brought to this discussion by the involvement of a representative of a world-renowned university, but I can only say I am disappointed - most of all at the way I am dismissed for being a man.
Tim Philpot, Wolverhampton

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Dear Eric, Derek, Dave, Roger, Mark, Tim, Nick and Hugh, thank you for your comments.

You obviously know my views, but thought I'd point out that the proportion of women taking part here is not very high (so far its zero guessing from your names).

On what a 'natural speed' to drive is - it might be worth noting that we didn't evolve while driving cars. It is natural to be afraid of heights. Unfortunately it is not natural to be afraid of speed, or to be able to judge the speed of fast unnatural objects (such as cars). This is one reason why so many parents don't allow children out to play and so many elderly pedestrians and other vulnerable road users are deterred from using their local communities as freely as they might. A majority of those adversely affected are women or girls.

Thanks again for reading, all best wishes, Danny.
Danny, Oxford

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Mark, a good driver is unlikely to hit anything and a bad driver is likely to hit things, whatever the speeds, even if the good driver is much faster where it is plainly obvious that it is completely safe to do so. Limits far below natural safe speeds don’t only reduce some impact speeds, they increase driver frustration, fatigue, journey times, conflict between harmless behaviour and the law, conflict between drivers who do not exceed the limit at all and those that do, are most ignored by those who should most obey them, require considerable attention to the speedo instead of the road in front, etc etc.
And yes the faster something hits you the more it hurts, but we clearly cannot use this in isolation to determine what limits should be, as it would tell us limits need to be zero. Indeed it is difficult to have a collision without some speed, but until we can change the laws of physics we are stuck with it so we must be intelligent about it and move on from the simplistic idea that reducing limits is the solution to everything.
Dave Taylor, Guildford

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I don’t assume that travelling speed is the same as impact speed unless the pedestrian runs straight out in front of the car, but it is the basis for all events afterwards. If someone steps out some distance ahead of a car (say 15 metres) of the car and the drivers are equally alert (and there is no real reason to assume otherwise based on my observations) then the 30 mph driver will still be moving when they get to the person whilst the 20 mph driver will have stopped.

I am not going to get into momentum vs energy (I can see your point where a car hits a pedestrian and carries on would be mostly momentum – but two cars heading towards each other would be mostly energy dissipation) but even then a vehicle at 20 mph has 33% less momentum than one at 30 mph.
Mark, Caerphilly

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Th "impact speed of an alert driver" seems like a contradiction. An alert driver shouldn't hit anyone in the first place. Also, the assertion that drivers are more alert when driving at the 85th%ile is based on no more than a wishful stab in the dark as neither can be measured by the casual observer.
Hugh Jones, Cheshire

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My assertion is actually that drivers are more alert when driving at around the 85%ile speed. That is based on personal experience, observation of other drivers (including as a passenger). Pedestrians are positively encouraged to feel safer in 20mph areas which will make some less alert to the dangers. Again, evident when driving in any 20mph limit.
Eric Bridgstock, Independent Road Safety Research, St Albans

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Eric - you say:
"the impact speed of an alert driver at nominally 30mph could be lower than a less alert driver at nominally 20mph. The fact that a pedestrian is likely to be more alert in a 30 zone will also influence the outcome and may prevent contact altogether."

Are you able to substantiate your assertion that drivers are more likely to be alert when driving at 30mph than at 20mph? And that pedestrians are likely to be more alert in a 30mph zone than (presumably) a 20mph zone?
Nick Rawlings, editor, Road Safety News

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Mark assumes that impact speed is directly related to travelling speed (actually, he seems to think that they are the same).

I suggest that for a given scenario of, say, a pedestrian emerging from between parked cars the impact speed of an alert driver at nominally 30mph could be lower than a less alert driver at nominally 20mph. The fact that a pedestrian is likely to be more alert in a 30 zone will also influence the outcome and may prevent contact altogether.

Danny Dorling has clearly not considered these possibilities and also assumes that higher travelling speeds always lead to higher impact speeds.

And, by the way, normally damage is directly proportional to momentum (mass x impact speed) not energy (mass x impact speed squared).
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (8) | Disagree (10)

The heart of this debate is whether people can be persuaded to change behaviour without coercion. It's refreshing to see such optimism and perhaps only old cynics like me who doubt. But I've had a lot of contact with road users who aren't interested in public good, only their own preferences. As professionals we think in principles but as road users people think in practicalities. Much of the successful societal change that Prof. Dorling mentions has been done not by changing populations but by applying legal pressure to key individuals – public authorities, employers, landowners, premises managers etc. and making them liable for compliance. Trying to use the law against a population that might accept the wisdom of it but regard themselves as exempt is like herding cats by comparison. I could comment more but Prof. Dorling has told me I should be ashamed for thinking objectively about this matter rather than just jumping on the populist bandwagon. Oh, one other thing. The BMJ report cited – is about traffic calmed zones, not sign-only limits. No wonder its results are so good.
Tim Philpot, Wolverhampton

Agree (17) | Disagree (5)

I know Eric disagrees with 20 mph as the default Urban limit so no argument in its favour will get his support.

Eric seeks to dismiss the BMJ's analysis on the basis of “spurious precision” whilst cutting out the confidence limits. Would “associated with a 42% (95% confidence interval 36% to 48%)” be more acceptable?

He also dismisses as “not credible” the reduction in stopping distances from 23m to 12m and the energy in a vehicle dropping by 55% (1 - (20^2 / 30^2))% as having anything to do with the casualty reductions and assumes it must be something to do with other unspecified, area wide, changes that have not occurred elsewhere.
Mark, Caerphilly

Agree (9) | Disagree (8)

Where is the evidence that pollution is reduced? This flies in the face of the findings of the Radcliffe Group which demonstrated that, at 20mph, catalytic converters do not work/are not effective and thus increase pollution.
Roger Harding, Stratford-upon-Avon

Agree (10) | Disagree (8)

In some 20 limits in our area it would be insane to drive more than 10 sometimes - and in some, perfectly safe to drive at 30. A fixed number cannot possibly provide some magical threshold where everything becomes wonderful. The kinds of incidents likely to be reduced by many of the 20 limits seem to be severely limited. And the other benefits suggested such as reduced emissions, congestion, obesity etc. seem to confirm the absurdity of the whole case. I agree with Eric - just wishful thinking, but dangerous wishful thinking as once our resources have been spent on reducing limits, they will not be available for spending on improvements which might have worked, perhaps educating drivers who demonstrate an inability to adjust their speed to the actual circumstances rather than a number in a circle, and perhaps at least a small number of the other driving errors and faults that don't involve speed.
Dave Taylor, Guildford

Agree (11) | Disagree (10)

And when 20mph limits are applied and adhered to, what next – a 10mph limit based upon the same research that propounds a 20 over a 30? “It will save lives”. The argument is never ending. Perhaps we should all crawl around on our hands and knees. Might be more difficult to see, and accidents may rise if we did, and I feel sure there would be statistics made available to just give up and die.

A great many roads and streets are main arteries where road safety is the responsibility of all who use them, not just drivers and riders, and on most of those roads 30mph is the long adopted limit. True, it does not stop some from exceeding that limit, and in some cases that may be safe to do so – and in others not so. The same would and does apply to 20mph limits. But in the main they are on roads and streets that already are reduced in width, non-thoroughfares such as housing estates and older town centres, and where speeds in excess of 20mph are not normally adopted by drivers in general, so the statistics so often expounded to and used by Councils seemingly shamed into either providing the desires of twenties plenty et al or not, will never be conclusive.
Derek Reynolds, Salop.

Agree (11) | Disagree (9)

Difficult to know where to start – so much wishful thinking.

“…because there is a 30mph limit drivers become used to driving no faster than that limit. They know it is illegal to do so. And most drivers drive at or below the limit which has the effect of forcing drivers behind them who might want to drive faster than 30mph to have to obey the limit. There is no reason to suppose that the same process does not operate in 20mph zones.”

There is every reason to expect drivers to exceed a speed limit set well below the 85%ile, as evidenced by simply watching what happens.

“The introduction of 20 mph zones was associated with a 41.9% …”
Apart from the spurious precision, the clue here is in the words “associated with”, not “attributable to” and that is the case for all such reports. There are always multiple confounding variables affecting the results, such as road engineering changes, traffic volume, etc. Also, taken literally, that 41.9% fall must be due to law abiding drivers who were having crashes heeding the 30mph limit, who are no longer having those crashes because they are now heeding the 20mph limit. That is simply not credible. Something other than the 20mph limit is causing the reductions, and who has assessed if those falls would not be greater if the 30 limit were retained?
Eric Bridgstock, Independent Road Safety Research, St Albans

Agree (19) | Disagree (29)