The ‘20’s Plenty for Us’ campaign is urging people to download casualty maps from its website and use them to make a case for 20mph limits on their local roads.
Click here to download the casualty map.
For more information contact Anna Semlyen, 20’s Plenty for Us campaign manager, on 07572 120439.
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I happened to see this again after a long gap and must make these points:
Rod – your use of the word “each” in your comment that “for EACH of the bodies concerned the forces will be proportional to their mass x velocity change squared and inversely proportional to the distance over which the velocity was changed” implies that you still do not understand that all three parameters ARE NECESSARILY THE SAME for the pedestrian and the vehicle.
Newton pointed out the first a very long time ago, the Principle of Conservation of Energy determines the second and the third is perfectly obvious.
Like many others, you still fail to understand that when vehicles (inherently of much greater mass) hit pedestrians, the injuries are almost totally dependent on the impact speed, and almost nothing to do with the mass, or therefore the energy, of the vehicle. Or as I first put it when commenting elsewhere on this subject, anyone hit at (say) 40mph by (say) a 1.5 ton Ford Escort will (other things being equal) suffer virtually identical injuries if hit at 40mph by a 1.5 ton Ford Escort welded to the front of a 1,000 ton train. In other words, the mass and hence the kinetic energy of the vehicle is almost irrelevant.
Incidentally, the more I see of Peer Review the less I like it , for example https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1420798/
Jan — re “Eric – I’m surprised an obviously intelligent man doesn’t accept that a reduction in vehicular speed can help reduce casualty severity.” I hope that you have since read and understood Eric’s important point that the relationship between impact speed and injury severity is complex and that while a reduction in vehicular speed can indeed reduce casualty severity it can also increase it.
Your objection to my ECHR “right to silence” application implies that you do not understand the vital importance of that centuries–old safeguard against oppression by the authorities – or that without it, motorists now, but potentially others later, innocent of an offence but accused of it, risks being penalised simply for refusing to admit guilt. As one judge explained to a jury more than 100 years ago, if the authortities suspect anyone of a crime it is up to them to prove it, beyond reasonable doubt, not up to the defendant to convict himself out of his own mouth.”
Idris Francis, Petersfield
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Eric
You said: “You need to be aware of the work done by road safety expert Mike Natt”, but your link only goes to a press article in which Mike Natt voiced his opinion.
Can you provide the link to the “work” or “research” he has done. Thanks.
Rod King, Cheshire, 20’s Plenty for Us
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Jan: A&E Consultants can obviously comment on the effects of being hit at different speeds but they have no understanding of the wider factors that can contribute to a collision in the first place or the other results of impacts.
You need to be aware of the work done by road safety expert Mike Natt
http://www.yorkpress.co.uk/news/9903996.20mph_zones____not_the_answer______road_safety_expert/
He says:
Statistically, people were less likely to be killed if hit by a vehicle travelling at 20mph than by one at 30mph, but “this does not tell the whole story”.
At speeds above 40mph the pedestrian passes on to or over the roof of the vehicle. At lower speeds, in the 20mph to 30mph range, the pedestrian is rolled on to the bonnet, then knocked forwards and away as the vehicles brakes.
At speeds of 12mph or less, however, the pedestrian is pushed forwards down on to the road ahead of the vehicle, which then runs over them, causing serious injury or even death. Children, due to their height, are even more at risk.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Interesting to read the ensuing debate raging since returning to my office.
A few observations:
1. Eric – I’m surprised an obviously intelligent man doesn’t accept that a reduction in vehicular speed can help reduce casualty severity. Speak to any A&E consultant (as we have) and you will learn the facts. My daughter didn’t give the driver a chance to react. It was entirely her fault for stepping out. Had the driver been driving at 30 it’s far more likely she would have been killed or seriously injured.
Thank you Hugh for the comment that reading the road while maintaining an eye on speed isn’t a difficult or onerous task.
2) Idris, forgive me. I simply cannot take seriously any points that someone who went to the European court of human rights to fight for their ‘right to silence’ over a speeding conviction might make in this regard.
http://strasbourgnews.blogspot.co.uk/2007/07/judges-rule-on-speeding-fines.html
I continue to believe there is a place for 20mph limits and urge anyone who remains unconvinced to look at the 18% reduction in pedestrian casualty rates at N Lanarkshire (resulting in international road safety awards after having been scrutinised by professionals in road safety).
Jan Deans CEO Dynamic
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Commend the comments of those with a reasonable grasp of road safety and crashes. Safe driving is defined by travelling in a way that allows one to avoid a crash – and that is not determined by speed! It is a function of speed and clearance distance to a potential hazard (following distance, sight distance …). Trauma is determined by the kinetic energy of the impact speed, and the available distance over which forces can be absorbed. In 30 mph zones the principal serious trauma concern related to pedestrians (occupants of modern cars using occupant protection systems should in the vast majority of collisions survive crashes with impact speeds up to 30 mph-40 mph). Analysis of the ratio of pedestrian deaths to injuries and use of internationally recognised data on ratios shows average impact speeds in 30 mph zones are less than 6 mph. This reflects most drivers are driving in a manner that allows them to avoid crashing into pedestrians.
John Lambert, Victoria Australia
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Idris: Some of your calculations are correct in that you can approximate the resultant speed of objects by their mass and velocity (ie momentum). However, any resultant forces will not be proportional to momentum but to the change in the kinetic energy of the objects involved.
It is the work-energy principle which decides this due to the change in kinetic energy being equal to the work (force x distance). Hence for each of the bodies concerned the forces will be proportional to their mass x velocity change squared and inversely proportional to the distance over which the velocity was changed. Of course there will be other factors influenced by speed including avoidability, braking distance, steering responsiveness, etc.
My assertion that kinetic energy is the major factor in the severity of collisions is substantiated by all the references I have given. Within the “Power model” report velocity is implicated to a far greater degree and for fatalities the exponent is estimated at 4.5.
Hence the idea that accident severity or risk is only linearly related to speed has no credibility.
Rod King, Cheshire, 20’s Plenty for Us
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I am not allowed here space to answer all Rod King’s points (I do on my web site) but because safety needs to be based on proper understanding of collisions, I must point out that:
When objects collide it is FORCE, not energy, that changes velocities and momentum (Newton’s principles of action and reaction, conservation of momentum and energy).
Momentum is proportional to velocity, hence so are forces, not exponentially or as the square of speed.
It is these forces, not energy, that cause deformation and injury, while shapes and resilience determine low forces/acceleration or high.
Anyone hit by a vehicle (inherently much heavier) accelerates almost to prior vehicle speed, even huge further increases in vehicle weight make little difference to acceleration or injurious forces.
Similarly the amount of energy transferred to pedestrians is determined by his change of speed, not remotely by the prior kinetic energy of the vehicle.
Idris Francis Petersfield
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Idris
Thank you for elaborating your anecdote.
However, I trust that you will agree that peer-reviewed academic papers are a better basis for road danger reduction than anecdotes.
Rod King, Cheshire, 20’s Plenty for Us
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I’d like to be able to run an experiment to answer the speed question effectively but with so many confounding variables present it is probably not possible.
Awareness and how people use the roads is the big issue and a large percentage of drivers probably need to improve their skills. Most drivers think they’re above average so persuading them they need to improve is a difficult task.
Speedo checks should be done when cruising with few hazards around, a driver who normally obeys limits and has eased off due to the potential presence of hazards is probably below the limit and can therefore spend more time scanning.
Dr James Whalen DSA ADI (car), Wolverhampton
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Rod – speedometers. My lucky escape I gave not as “evidence” but as an example of how someone who was not looking at his speedometer at the critical instant when I walked into his path only a few feet ahead managed to swerve to hit me (across the back and legs by the slab-sided rear quarters of the origami-styled station-wagon) not head on. Much the same as when I missed the motorcyclist.
There was no question of his braking to avoid me – he stopped at least 4 car lengths along the road. Which brings up the important point that avoiding crashes is almost always discussed in terms of braking, although swerving is often safer. I try always to have an escape path available.
Idris Francis Petersfield
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Fairly obviously (I hope), no-one should take their eyes off the road ahead if it is possible something is going to happen in the next few seconds/metres – we choose our moment, just as we would when we look in the mirrors, at the dials, through the side windows etc. This is elementary stuff surely?
If a driver hits something or someone in the fraction of a second it takes to glance down and back up again from a speedo, it’s inexcusable and should, I daresay, be virtually impossible.
Hugh Jones, Cheshire
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Dr Whalen
Speed related topics prompt comments from those of us who have realised that claims for road safety benefits from speed management (whether from cameras or lower limits) are negligible compared to the benefits from addressing other factors such as hazard perception, observation – factors where training can deliver significant benefits.
Until someone can find a collision or casualty where it could credibly be claimed that it would not have happened if the speed limit had been lower, or if a camera/hump etc had been installed, then the claimed benefits from such proposals are no more than wishful thinking – especially when the negative effects of those interventions are taken into account.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Idris
The fact that a driver used “over-monitoring of his speedo” as a defence as to why he could not avoid a collision with an elderly man is not evidence that this was actually the case, the cause of the fatality or that it was, as you imply, “one of the many deaths directly caused by the unintended consequences of flawed road safety policies”.
If you were hit by a large car in Philadephia in 1986 and feel that you escaped serious injury because the driver was looking where he was going, then surely he must have been driving at a speed which meant he had enough time to take avoiding action.
Using your anecdotes to selectively support your theories is not research or evidence.
Rod King, Cheshire, 20’s Plenty for Us
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Idris
I am still awaiting your response to the paper on kinetic energy being a factor in road casualties. Here is another one http://ec.europa.eu/transport/road_safety/specialist/knowledge/pdf/speeding.pdf
This confirms the use of kinetic energy. It also references Swedish research saying that “a change in average speed of 1km/h will result in a change in accident numbers of 4% for a 50km/h road. This result has been confirmed by many before and after studies of different speed reduction measures. This relationship is used by other Scandinavian countries and by Australian and Dutch safety engineers.”
Here is another paper – “Speed and road accidents – an evaluation of the power model” http://www.trg.dk/elvik/740-2004.pdf One conclusion is:-
“There is a strong statistical relationship between speed and road safety. When the mean speed of traffic is reduced, the number of accidents and the severity of injuries will almost always go down.
I would suggest that readers should look at both of these documents.
Rod King, Cheshire, 20’s Plenty for Us
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Dr. Whalen – let’s talk numbers. Looking down at the speedometer, re-focussing from near infinity to 2 feet and back takes 0.5 of a second, in which time unexpected things can happen – brake lights, pedestrian stepping into road, motorcyclist going the other way making emergency stop, losing control and falling over into my path – I say my path because it happened to me. Because I was looking ahead I was able to swerve violently so that my door mirror clipped his shoulder instead of my headlamp hitting his head. A half second speedometer reading could have killed him.
That’s why military aircraft have head-up displays and why we have the two second rule – which far too many drivers ignore and many more don’t even know.
40mph is 2 car lengths in half a second. Enough time to kill or maim.
Idris Francis Petersfield
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It is interesting how items related to speed seem to have a high number of comments, perhaps a sign that drivers like to choose their speed and don’t like their judgement being questioned?
Regardng speedometers – if drivers make rapid eye movements it is perfectly possible to be aware of one’s speed and hazards outside the car. Glances at the speedo confirm clues the brain has developed to sense speed (e.g. the rate of visual looming) so these glances do not need to take up much time – to blame a road safety policy or speed limit for a collision is an excellent example of rationalisation – people often look to outsource blame when something happens.
Dr James Whalen DSA ADI (car), Wolverhampton
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It is ironic and pertinent that the first casualty mentioned on this thread was the daughter of Jan Deans, hit by a driver doing 20mph outside a school. I cannot see how that supports an argument for 20mph limits.
Eric Bridgstock, lndependent Road Safety Research, St Albans
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Rod King – looking at speedometers.
Accidents are abnormal, so what normally happens is not always relevant. The words of a North Wales doctor at an inquest into the death of an elderly man killed when stepping into the path of his car, reported about 5 years ago:
“I was aware that this was a speed enforcement area so I was monitoring my speed carefully. When I looked up, there he was.”
Just one of many deaths directly caused by the unintended consequences of flawed road safety policies.
I was hit by a large car in Philadelphia in 1986 because I looked the wrong way. I escaped with a sprained wrist because the driver was looking where he was going.
Idris Francis Petersfield
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Again to Rod King – this time to agree. That so many accidents do not occur in clusters does indeed mean that wide area interventions are needed.
Whether he realises it or not, this is a very good argument against speed cameras, whose sites typically cover only 1% of rural roads and 3% of urban. It also explains regression to the mean – that after an accident has happened at point A, the next accident will more than likely happen at point B instead.
Not that this stops those who install speed cameras at point A claiming credit for reduced accidents at point A. And there, in a nutshell, is the central fallacy behind speed camera claims, and how falls in (reported) accident numbers at camera sites far exceed the proportion of accidents that ever involved speeding in the first place.
Idris Francis Petersfield
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To Rod King – the raw data for the 20mph area I show was supplied by Portsmouth CC under FoI.
The national data I compare it with is taken the DfT’s annual Reported Road Accidents Great Britain, and the primary comparison I made is with Urban road data. However trends for All Road roads are not significantly different.
My graphs are based on the above official data.
Idris Francis, Petersfield
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Idris
Thank you for presenting your calculations on momentum.
It seems to me that the approach you have taken is to reference an “extreme” example regarding the laws of conservation of momentum and then extropolate this down to a “normal” situation.
However, I don’t think you must have read my post where I pointed to kinetic energy rather than momentum as being the major factor in casualty severity. I also gave a reference to an academic paper supporting this. Was this helpful or should it be dismissed as incorrect?
Rod King, Cheshire, 20’s Plenty for Us
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Idris
Police have been asked to do more with less resources. Without getting into the politics of this, the light touch random enforcement of 20mph limits gives a great return on compliance for the effort involved. Portsmouth did show skewing of speed reductions with highest reductions on fastest roads. Something which the DfT has noted and referenced, especially where previous ave speeds were between 24 and 29mph.
20mph limits are never based upon just signs but include democratic debate and decision-making as well as post-decision engagement. The latter is a role that Public Health is playing a part in increasing compliance. See our latest briefing sheet on multi-agency collaboration on compliance at: http://www.20splentyforus.org.uk/BriefingSheets/Raising_Compliance_and_the_Role_of_Enforcement.pdf
Rod King, Cheshire, 20’s Plenty for Us
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Idris
Please remember that many drivers do actually know what it is like to drive at 20mph and can do it fairly consistently without continuous reference to a speedometer.
Referring to your Portsmouth 20mph v GB SI, I note that you do not reference the source figures. I wonder if your GB “scaled” figures include motorways. Ie are they similar roads to Portsmouth which has one of the densest urban populations in Europe?
Rod King, Cheshire, 20’s Plenty for Us
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Mr King,: sorry, you are still wrong. As words have not persuaded try numbers:
a/ 1 tonne Escort hits 0.07 tonne pedestrian at 40mph.
Pedestrian on 1.07 tonne combination is then at speed X such that momentum is unchanged (Newton)i.e. (1.07 * X) = (1 * 40) units so X = 40/1.07 = 37.4mph.
b/ 1 tonne Escort on 2,000 train hits 0.07 tonne pedestrian at 40mph.
Pedestrian on 2,001.07 tonne combination is then at speed Y such that (2,001.07 * X) = (2001 * 40), Y = 40 * 2001/2001.07 = 39.986mph
c/ 1 tonne Escort hits 0.07 tonne pedestrian at 60mph.
Pedestrian on 1.07 tonne combination is then at speed X such that momentum is unchanged (Newton) i.e. (1.07 * X) = (1 * 60) units so X = 40/1.07 = 56.1mph.
In b/ kinetic energy of vehicle 4 million times greater but acceleration of pedestrian 7% greater.
In c/ kinetic energy of vehicle 2.25 times greater, acceleration of pedestrian 50% greater.
Idris Francis Petersfield
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Duncan
From looking at the crashmaps, we usually find that one of the largest %age is actually the “unclustered” casualties. These really need wide-area interventions because they are unlikley to re-occur in exactly the same place.
This is one reason why local authorities have found wide-area 20mph speed limits together with all the community engagement a useful tool in addressing those unclustered incidents.
Rod King, Cheshire, 20’s Plenty for Us
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To Rod King. All Police forces make it clear that they do not have the resources – especially now – to enforce 20mph limits.
As Portsmouth and elsewhere show, 20mph areas based only on signs typically result in speeds falling by only 1mph on average. But it (as engineers know) averages can be both misleading and dangerous because they ignore the spread of the numbers.
Portsmouth highlighted significant falls on many roads, but more or less ignored that the increases elsewhere that led to little average change. That speed fell by (say) 5mph on one on road but rose by 3mph on another does not necessarily mean improved safety, because (a) the latter might have had more traffic and (b) driving faster than previously thought safe is likely to cause more problems than driving slower than previously thought safe is likely to eliminate. Once again, analyse all the evidence, not just what seems obvious or convenient.
Idris Francis Petersfield
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An alternative view of Jan Dean’s daughter’s accident is also possible – the driver who hit her at 20mph would not have hit her at all if he had been looking at the road ahead instead of his speedometer, trying to keep to 20mph.
Once again I ask that we all review as much evidence as possible, ask the relevant questions and only then try to assess the benefits or otherwise of this or indeed any other policy.
A good place to start is http://www.fightbackwithfacts.com/portsmouths-20mph-area/ and in particular the Portsmouth 4 year resuls showing a steadily worsening gap between their SI and national urban trends.(document 24)
Never forget the law of unintended consequenes, it has its wicked way any time, any place.
Idris Francis Petersfield
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The benefits of reduced vehicular speeds within residential areas are irrefutable. It doesn’t take much imagination to consider the difference hitting a child at 20mph to hitting a child at 30 or 40. It seems disingenuous even to be debating this.
My beloved 15 year old daughter was run down outside school recently (ironically enough when I was speaking at RSGB’s national road safety conference). She survived with minor cuts and bruises and hopefully a healthier respect for the dangers of cars (far greater impact than my daily nagging). Why? Because the driver in question was doing 20mph outside the school.
Let’s not debate any more…lets get 20mph rolled out nationally to reduce further casualties!
If you need any evidence of efficacy…look no further than North Lanarkshire Council’s award winning ‘Twenty’s Plenty’ initiative…
Jan Deans CEO Dynamic
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I welcome the 20s plenty casualty maps in that they at least might help councillors and residents understand where collisions are most likely to take place in the future (based on the collision history), rather than just relying on their own perceptions of danger. While perceptions are important too (the feeling that a road is unsafe can be a barrier to more walking and cycling), there is a need to ensure that casualty hotspots are tackled. I regularly send a link to the http://www.crashmap.co.uk website to residents and councillors so they can see for themselves.
Duncan Knox, Surrey
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I have read all the previous posts and viewpoints with great interest. Without making reference to studies, case histories, physics or any personal history in implementing these, one element that hasn’t been referred to so far is the change of attitudes by pedestrians as a result of introducing 20mph zones.
I am sure that as part of the implementation, there would have been consultations, publicity, community discussion and education/debate in local schools etc, which the local authorities may or may not have been involved in. This would have created a heightened awareness of the safety issues and probably create a temporary change in behaviour among all road users in the area. This could explain the claims by some on this forum that the benefits were not sustained in some areas.
Once the initial novelty wears off and the publicity, discussion and education subsides, does behaviour then revert to the ‘norm’? Is it the publicity and education aspect of 20mph zones that was the difference here, not the engineering enforcement measures?
Andy, Medway
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Hugh Jones asks “The fundamental question remains though – a lower speed limit is fine in principle, but how is it achievable in practice?”
We very much agree and that is why we are working with communities and authorities to ensure that compliance is maximised. Later 20mph schemes have been accompanied by much greater public engagement which is key to behaviour change and drivers recognising the large community benefits and minimal personal dis-benefits from driving slower where people are.
We are also working with police and road safety partnerships for enforcement to play a balanced role in compliance.
I am not aware of any traffic authorities who have implemented wide-area pilots of 20mph limits who have not gone on to make them even wider and usually across the whole authority.
Rod King, Cheshire, 20’s Plenty for Us
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As an enginer (BSc Automobile Engineering) I am afraid that Idris makes the wrong case regarding momentum being the critical factor in collisions. It is the total energy that is dissipated that is key. His extreme case of a 40mph car hitting a pedestrian proves nothing as both car alone and car plus train would likely kill the pedestrian. If you add on the speed effecting the ability of participants to avoid collisions then speed plays an exponential role in contributing to total casualties.
But why not reference a more academic report and see the implication of “kinetic energy” in crash and casualty severity http://www.monash.edu.au/miri/research/reports/muarc229.pdf
Rod King, Cheshire
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The Newsfeed report says: “(it will)….help councillors understand the need for wide 20 mph limits to prevent road crashes”. With all due respect to 20s Plenty, Councillors have always been able to get this info from their own staff if they want and in far more detail and with analysis if required.
That aside, there is no doubt that in principle, lower speeds are desirable in urban areas, however casualty stats on their own do not necessarily reflect the benefits, as there are many incidents that do not appear on stats: damage only (to vehicles); damage to private property and street furniture etc., death and injury to wildlife and domestic animals etc. It’s also quieter and less stressful for local residents.
The fundamental question remains though – a lower speed limit is fine in principle, but how is it achievable in practice?
Hugh Jones, Cheshire
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After driving 1m miles and 12 years studying road casualty policies and statistics I agree throughout with Eric and see nothing from the pro-20mph side to persuade me otherwise.
As an engineer, I find it worrying that those who think they understand physics in fact don’t – the reference to momentum varying exponentially with speed and (Rod King, DriveTech, and others) who think that it is kinetic energy (which does rise as the square of speed) that results in much worse damage at speed. Nonsense – what matters is the change of momentum of the person hit, and the forces involved: A 40mph Escort welded to the front of a 2,000 tonne train would causes the same injury to a pedestrian as would the car alone, because in both cases the pedestrian is accelerated suddenly to 40mph. Massively greater kinetic energy has nothing to do with it.
Idris Francis Petersfield
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This debate started with the suggestion that 20’s Plenty says that the maps can be used to “help councillors understand the need for wide 20 mph limits to prevent road crashes”. The debate, although interesting, has gone away from this point. Where I live Councillors went ahead with a 20 scheme although the maps showed very small numbers of accidents, 3 in 5 years if I remember correctly, all minor, and no fatalities. It would be very hard for a 20 limit to reduce this rate of accidents, yet the council wasted lots of money putting the scheme in. I do not understand how the maps can help councillors. Do they show the cause of the accidents, and I wonder in how many speed was a factor?
Bobbio Chiswell Green
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It’s a sure fire certainty that a debate based on assumptions against evidence will generate a one sided ‘sword fight’ that will only end in tears. The assumptions are that reducing average speeds by X mph will reap Y percent reduced casualties. Mathematically the claims that have been previously made would mean that all casualties would cease if we all drove around at 10mph. It is nonsense that such could or would be achieved.
How can a real reduction in casualties be achieved? By being aware of others, respecting their environment and presence, and acting accordingly i.e. safely. 20mph in residential areas might sound a good idea, but which areas and under what circumstances, for they cover vast amounts of the UK. There are a great many places where 20 is unattainable due to the narrowness of streets or the traffic thereon. How can any constructive claim be made that 20 is plenty when 20 is not achievable? Indeed, how does one “construct a non-event”? That has me puzzled.
Derek Reynolds, Salop.
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Rod
There is nothing unrepresentative about Portsmouth – a flagship 20mph experiment which has been an unmitigated disaster.
You have failed to answer numerous challenges in this thread (1mph/6%, your flawed physics, your flawed logic, what does cause increased injuries in 20mph limits, and so on).
Eric Bridgstock, Independent Road Safety Research, St Albans
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Eric
We have already commented on the few specific incidents which you seem to keep alluding to and shown them to be unrepresentative or not significant.
Personally, I am keener to work with local authorities, police, communities, government and other agencies to maximise compliance and the success of Total 20 implementations.
I think that most people on this site will agree that lower vehicle speeds means reduced danger on our streets and that is our objective.
So far local authorities and communities in many of our most iconic cities and places seem to agree.
Best regards
Rod
Rod King, Cheshire, 20’s Plenty for Us
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That’s all very interesting Rod – so how do you explain the increased casualties in 20mph limit areas/zones? What do you think the causes are?
Eric Bridgstock, Independent Road Safety Research, St Albans
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Let’s look at your “negative effects” in detail:-
Roads feel safer leading to less care. Where is the evidence of this? Care by pedestrians/cyclists is determined by real risk from cars not the speed limit. Or are you suggesting that pedestrians take more notice of speed limits than drivers?
Slower vehicles make less noise. Not really an effect when the car is coming towards you. Far more likely when the car is going past.
Speeds lower than natural. As you may have gathered most people according to surveys think 20mph is the right “natural” speed for residential roads. And surely anyone overtaking a car travelling at or less than the speed limit is not a reason to increase speed limit?
Human brains have been tuned over millions of years to cope with the fastest thing within their environment. And that is 20mph. The suggestion that you see things better if they are moving more than 20mph is ludicrous. There is evidence that children and elderly have difficulties in recognising oncoming speeds above 20mph. I can provide references.
Rod King, Cheshire, 20’s Plenty for Us
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Rod
Thank you for quoting my paper.
Those points all apply – lower speeds (or actually lower speed limits) can and do create a sense of safety in pedestrians and the negative effects you have kindly listed. I have never stated or assumed that higher speeds would have a positive effect, and have not had reason to consider that scenario.
A implies B does not mean NOT A implies NOT B. Simple logic.
You are proposing a change – 30mph limits reduced to 20mph. That change must be assessed for all possible consequences – negative and unintended as well as beneficial. You consistently ignore all except the [possible] beneficial.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Eric
The following is taken from a document produced by yourself and handed to me personally :-
Negative and hence undesirable effects of lower speeds, which increase accidents:
• Roads “feeling safe” lead to less care/attention by pedestrians/cyclists
• Slower vehicles make less noise and are therefore less likely to be noticed
• Speeds lower than natural increase frustration and lead to inappropriate overtaking.
• Human brains being tuned to pay attention to faster moving objects in peripheral vision, slower vehicles are less likely to be noticed
• Speeds lower than “naturally safe” lead to lower concentration levels by drivers
• Driver attention diverted to checking speed limit signs and speedometer
• Driver priorities shifted from being safe to the belief that legal is safe
So, are you denying that you have said that “lower speeds increase accidents”?
And are you denying that this does not imply that you assume that “higher speeds reduce accidents”?
Some clarification would be useful.
Rod King, Cheshire, 20’s Plenty for Us
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The ABD reference gives a fully argued case against the 1mph fabrication, which is worth reading. There are others available, showing that the authors were not able to support their “findings”. I am relying not solely on my own observations but on the results reported.
My causal links (based on road users taking less care) are evident, even though you are in denial about it.
And please do not misquote me – it was not an assumption about “more speed means fewer casualties” – read my original posting again.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Eric
You may trust measured data, but it is your blind faith in your claimed “causal links” which I find difficult to trust.
You seem to be able to “give” many causal links but everyone is still waiting for any evidence to prove causality.
Somehow relying on your own “observations” is simply not credible.
Your assumption that more speed means fewer casualties is ludicrous and no amount of referencing ABD will convince people otherwise.
Rod King, Cheshire
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Rod
The 1mph = 5-6% reduction has been discredited on many occasions (http://www.abd.org.uk/onemph.htm is one example). To quote that ABD assessment: “Any survey has to be questioned if it comes up with results that are inconsistent with trends observed in the whole population. These trends show a steady fall in accident rates and casualty rates throughout this [20th] century despite huge increases in free flowing traffic speeds. More specifically, injury accident rates fell by 30% in the UK during the 1980s whilst road speeds increased.”
There is no evidence or sound argument to believe a 1.5mph decrease in av speed will lead to any casualty reduction, let alone 9% – the evidence is that casualties increase, for the reasons stated.
As a safety engineer, I trust measured data over theoretical modelling studies.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Eric
What you have done is to “suggest” a causal link but provided no evidence of causality.
There is already established evidence quoted by DfT and many others that for each 1mph reduction in average speed then a 6% reduction in casualties can be expected in urban environments. You make the mistake of saying that a 1.5mph reduction in ave speed is inconsequential, yet how can a 9% consequent reduction in casualties be inconsequential? Of course there may be skewing factors such as more pedestrians on streets as they become less dangerous. But the key to creating better communities is reducing danger rather than creating safety by pedestrian and cyclist exclusion.
Whilst accepting the benefits of good driving I think I can rely on Newton and his laws with greater confidence.
Rod King, Cheshire, 20’s Plenty for Us
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Rod
Once again you twist statistics to hide the truth.
It’s not that Portsmouth’s 20mph suddenly turned bad in 2011 – it was resulting in increased casualties (when adjusted for traffic volume and national trend) from Year 1.
I have given the causal link between 20mph and increased casualties on many occasions – pedestrians lulled into a false sense of safety, take less care, while traffic is generally travelling much as before (average speed reduced by 1-2mph?). It can be witnessed in any 20mph area, any time.
Attempting to reduce road safety to the laws of physics underlines how little you appreciate the contribution of good drivers to road safety.
I will agree that casualty maps are useful as a historical record – but they do not provide any rationale for wasting thousands of pounds implementing 20mph schemes.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Eric
You said “Causal link is necessary in any convincing safety argument” but it seems that this can be put on one side for your own arguments. You have made great play of the increase in casualties in Portsmouth in 2011 from 2010 and jump to the conclusion that 20mph limits in 2008 are the causal factor. But you cannot explain why :-
1) Other towns, such as St Helens & Bridgend, not implementing 20mph limits had higher increases. Or that Bracknell Forest, Reading, Monmouthshire, Stoke-on-Trent, Gloucestershire, West Berkshire, Coventry, Rotherham, Wrexham & North Ayrshire had rises in SI of 25% or more against a national trend of 2%.
2) The increase in casualties from 2010 to 2011 was lowest on Portsmouth’s 20mph roads.
3) Why the 2008 implementation took 3 years to suddenly take effect.
It is quite correct to recognise that there could be other causal factors such as Stats 19 reporting where we have evidence that this only partly reflects the casualty levels. It is interesting that Hampshire, Isle of Wight and Southampton all recorded increases in Serious Injuries of 17%, 18% and 21%. All way above the national trend of 2%.
Those truly interested in road safety when given statistics will look beyond them to understand trends.
Rod King, representing campaigners in 175 UK communities
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“Reduced vehicle speeds necessarily induce greater margin for error on the part of all participants, reduce exponentially vehicle momentum and therefore stopping distance and therefore ability to avoid collision”
1. Like many commentators you assume only the positive benefits from reduced speed – and margins of error help only when an emergency situation has developed. Better to avoid emergencies by concentrating on conditions, not speedo.
2. Momentum is mass x velocity (linear), there is no exponential. (energy is mass x velocity squared and relates to braking distance).
You finish that paragraph with a variation on “hitting people at slower speeds kills fewer of them”, which is at the heart of the 20’s Plenty campaign and illustrates just how little they know about the keeping vulnerable road users safe.
Eric Bridgstock, Independent Road Safety Research, St Albans
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But my case does not. Creating non-events and preventing events are in my mind the same thing spelt differently. Vehicle speed does not have to be the primary cause of a collision for speed reduction to be a successful solution. Reduced vehicle speeds necessarily induce greater margin for error on the part of all participants, reduce exponentially vehicle momentum and therefore stopping distance and therefore ability to avoid collision, also force of impact and potential severity of injury if collision is unavoidable.
I’d like to see a research study into carelessness and 20mph zones which proves unequivocally that the carelessness is the product of the zone rather than from any other source. On the off-chance that there is one, all I can say is how much better the results would have been in Hull and everywhere if we could solve the problem of carelessness as well. But as my figures from my last post showed, there are plenty of drivers who can’t even take care of their own safety when given free choice.
Tim Philpot, Wolverhampton
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Tim
You say “casualty reduction is about creating non-events”. Not so. It is about preventing events, which is achieved by removing the causes or conditions that led to the events. If no events were due to speed > 20mph (and I’m confident that that were the case), then simply implementing 20mph cannot be responsible for the reduction. Causal link is necessary in any convincing safety argument, and it does not exist for 20mph schemes. But carelessness induced by 20mph is evidently a cause (I’ve seen it numerous times). My case rests, for now.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Sorry Eric, I saw the phrase fightbackwithfacts recently and thought I’d give it a try. West Mids ped cas 1994-2775, 2001-2443, reduction <10% compared to Hull's 40%. Hull's figures 2004-2011 mirror other socioeconomically similar locations (see MAST). I don't know why I'm bothering because you will still say "aha, but you can't actually prove this is the reason". That's because casualty reduction is about creating non-events and no-one, not even you can prove unequivocally why a non-event "happens". Yes we could remove all speed limits and speed measurement devices and trust to the expertise of motorists to keep us all safe, but before we do, can we talk about the 28,984 car occupant casualties caused in the UK in incidents involving single cars in 2011? Doubtless these are not the product of incompetent driving but all our fault as well for distracting drivers by drawing their attention to the needs of others.
Tim Philpot, Wolverhampton
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Tim
Pure numbers out of context prove nothing. As you know, casualties reduce due many factors – better vehicle design, road engineering, faster/better medical response, etc. How did Hull 20mph compare to national trend? Or to other towns with 30mph? What has happened to traffic volume in Hull over the period concerned? Did the causes of casualties change? Ultimately, what net benefit, if any, can be attributed to 20mph?
All too frequently, there are attempts to give 20mph (and other speed management interventions) credit for road safety improvements achieved by other means.
Eric Bridgstock, Independent Road Safety Research, St Albans
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I must say that Eric has a point and backs his claims with evidence rather than emotional opinions. In the area I covered the 20mph zones were ignored through the signed elements and followed only by force through the rest of the area because of traffic calming and poor residential parking. I have little faith in the 20mph zones now and would prefer schools being encouraged to teach their parents about the dangers they cause through questionable parking rather than spend their time campaigning for a false sense of security outside their gates.
Liam, Essex
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I am not a spokesperson for RSGB but as a member on behalf of a local authority, I am satisfied that RSGB fosters debate about this topic and ensures that creditable information about it can be made publicly available through the Knowledge Centre. If Portsmouth showed one thing it is that the results of sign-only 20mph limits vary widely. In this context it is inappropriate for there to be a national position, and decision making should be at a local level. For the record, Hull had 388 ped cas in 1994, 240 in 2001 and continued to show steady reduction between 2004 and 2011 to around 140. Speaks for itself really.
Tim Philpot, Wolverhampton
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Once again, Rod King evades the issue of increased injuries in 20mph areas, preferring to use emotional and meaningless language (“feared”, “elitist”, “loving it”). The public and councils have been, and continue to be grossly misinformed by the misguided and dangerous 20’s Plenty campaign – and the same public are the casualties of that policy.
And when will an RSGB spokesperson declare their position regarding 20mph?
Eric Bridgstock, Independent Road Safety Research, St Albans
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It is important to be precise in these matters. There are 10 serious and 50 slight injuries for every fatality, and it is important to assess each severity separately, not mask K and SI within the much larger totals of Slight.
It is also important to compare results not with previous years in the same place but with national trends in similar (urban) areas.
Freedom of Information requests should provide relevant data – as mine did for Portsmouth’s 3rd and 4th years which they had not analysed themselves.
The DfT warned Portsmouth before they started that signs alone achieve trivial changes in speeds – as indeed happened, but seem not to have noticed that SI trends were strongly adverse.
See: http://www.fightbackwithfacts.com/portsmouths-20mph-area/ and particularly the 4 year data near the end of the list.
Idris Francis Petersfield
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Hull – “widely reported success”. Not the same as real success as we have found with the specious claims of success in Portsmouth.
I have read various Hull accounts and it is impossible to separate any benefits attributable to 20mph from numerous other factors (statistical and long term trends, engineering changes other than speed limits and traffic calming, changes in traffic volume, etc). Their reports also concentrate on subjective surveys of residents feelings, rather than measured results.
Finally, I note that no-one is reporting what has happened since 2002. What are the measured results over the last 10 years? Portsmouth stopped reporting when the casualty trend went upwards – same in Hull? Seems very likely.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Of course physically calmed schemes are effective and relatively cheap. But one thing that they do is put across the idea that its only “here” that you need to slow down. They in fact create isolated calming rather than a community wide respect of the needs of vulnerable road users. In so many urban areas we have already tackled the clustered casualties and now need a wide-area tool that makes cycling and walking safer not only for those who currently do so, but also for those who are yet to do so. That’s why and how 20mph limits at a 50th of the cost of physically calmed and isolated zones are transforming our whole attitude to creating better public spaces (ie streets) in our communities. And that’s why their spread is so feared by those who would prefer the elitist freedom to drive at whatever speed they feel is safe.
20’s coming and people throughout the land are loving it.
Rod King, representing campaigners in 175 UK communities
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Hull installed 20mph zones across the city in the late 1990s and early 2000s with widely reported success. The evidence suggested casualty reductions over and above the general trend, for children, cyclists and pedestrians. The below links provide more information, with a case study of Hull at pages 27/28 of the Streets Ahead report.
http://www.publications.parliament.uk/pa/cm200102/cmselect/cmtlgr/557/557ap80.htm
http://www.ippr.org/images/media/files/publication/2011/05/streets_ahead_1266.pdf
Tanya, Suffolk
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Eric
I’m not actively involved at LHA level with either engineered or sign-only schemes so I’ve not got access to any data, one way or another.
As a road safety professional I seek evidence to support my professional judgement. My judgement is that:
• signed only schemes are ineffective or (at best) poor value for money [I very much doubt that making “noise” about such schemes “increasing accidents” is a productive way to proceed unless there is a long term body of evidence – the Portsmouth work was inconclusive, but had enough info to beg the question why LHAs are keen to invest so heavily
• engineered schemes are cheap and effective (close reading of the Portsmouth work summarises positive experiences of engineered schemes elsewhere)
pete, liverpool
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Pete
Every scheme I looked at, and reported about at the 20mph Places Conference last May, had increased serious injuries, especially among pedestrians.
My challenge is back to you. Find any 20mph scheme (with or without “calming”) where serious injuries have reduced after allowing for reduced traffic volume and compared to national trend.
Idris Francis has four years of data from Portsmouth and it is evident why Portsmouth City Council stopped publishing the results.
Liverpool is spending hundreds of thousands of pounds on 20mph under the claim that road safety will be improved. Why?
Eric Bridgstock, Independent Road Safety Research, St Albans
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Whilst I have seen many pages of debate about the Portsmouth experience, I am intrigued by Eric’s statement about “every 20mph scheme” having serious accident increases. Can links be provided to the evidence? Is he including engineered (calming) schemes in this statement – I would be surprised if this is the case.
pete, liverpool
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There is no evidence that a 20mph limit would have prevented any of the previous collisions or casualties and no reason to suppose that they will prevent future casaulties.
In fact, 20’s Plenty are well aware that serious injuries have increased year on year in every 20mph scheme, including “flagship” schemes such as Portsmouth – why do they ignore such valuable evidence? The reasons are obvious – all road users take less care in 20mph zones.
Councillors and road safety officers need to resist the dangerous 20mph bandwagon.
Eric Bridgstock, Independent Road Safety Research, St Albans
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