Road Safety News

Sheffield commits to 20mph zone across the city centre

Monday 23rd October 2017

A campaign group says ‘30mph has been rejected in favour of 20mph as best practice’ following the announcement that a new 20mph zone in Sheffield city centre will encompass more than 300 roads.

The new zone, confirmed by Sheffield City Council on 17 October, will come into effect by the end of 2017.

The campaign group 20’s Plenty for Sheffield is ‘celebrating another win’ and is urging the local council to give priority to informing residents of the benefits of lower speed limits. The group is calling for a ‘full driver engagement plan for behaviour change for safer streets’.

Sheffield City Council committed to a gradual 20mph rollout in 2012, with 20mph limits now operating in around a third of the city’s residential areas. The new city centre limits will impact every street apart from a few major routes.

Richard Attwood, 20’s Plenty for Sheffield, said: “We are pleased to see a legally enforceable 20mph maximum speed limit being put in place across the centre of our great city.

“20's plenty for Sheffield now urges the council and its statutory partners to give greater priority to informing Sheffielders of the many benefits arising from lower speeds, so they may more readily choose to comply with the new limits."  

Cllr Jack Scott, Sheffield City Council cabinet member for transport and sustainability, said: “I’m pleased that the 20mph consultation was received so positively. This Labour council is absolutely determined to improve road safety throughout the city and 20mph zones play a big role in this.

“Not only do speeds reduce, there’s a reduction in the severity of the accidents and an improvement in air quality.”

20’s Plenty for us says 15 million people in the UK live in places ‘where 20mph is normal’, and that ‘worldwide 30mph is being ditched for 20mph as the emerging best practice norm where people matter more than vehicles’.

Category: 20mph limits.



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Hugh. I am answering this and no more. Some drivers would rather pay a fiver or ten quide or have it done as a favour at a locally known garage or electrician rather than spend hundreds on correcting the car's exhaust emmissions. Indeed if they were to be altered it's more than likely the car would be less efficient on acceleration and or top speed or performance in general. So they pay out rather than suffer a large bill with the resulting poorer fuel consumption and poorer performance.
Bob Craven Lancs

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Why would one bother though Bob? If there's something wrong with an engine causing it to fail the MOT emissions test, why is tweeking it for a temporary fix better than simply having it diagnosed and permanently fixed properly in the first place and possibly cheaper than having to look for and pay a dubious expert to fiddle with it, in hope that it might then pass?
Hugh Jones, Cheshire

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You are quite right Hugh, one can't do it with a scredriver. It's not illegal and there are many non-dealership garages and not MOT stations that have the computer wherewithall to tweek any system. One doesn't need a screwdriver nowadays. Just access to the vehicle's comp. Easy.
Bob Craven Lancs

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Public transport Tim?

That quaint idea of going somewhere you don’t normally go to wait for someone else’s schedule to take you to a place vaguely near your destination. And share the journey with a bunch of strangers from the rich tapestry of life. No wonder its not that popular in comparison to using one’s own transport.
Guzzi, Newport

Agree (8) | Disagree (1)

I think it's been a long time since, armed with just a screwdriver, one could adjust a car's air/fuel mixture Bob! I'm not sure if it's actually illegal, but it's not a practice an MOT station would do anyway, even if they could.
Hugh Jones, Cheshire

Agree (6) | Disagree (1)

"Not buying new cars with poor true tail pipe emissions levels would probably have the greatest single impact of any measure."

No, persuading people to drive less will have the greatest single impact of any measure. It's quite depressing that walking, cycling and public transport are akin to profanities for some of our commenters!
Tim Lennon

Agree (4) | Disagree (8)

Whilst I agree with you Pat a person looking for a car with less emissions will look at the blurb in the pamphlet and by making comparisons will choose the vehicle he wants to buy notwithstanding it isn't necessarily the most economical on emissions but the model he and his family may want.

The trouble is these emissions are all done on a test bed and not in actual driving on the road conditions. They even may go on a race track and stay in the same gear and the same speed and that will give a totally incorrect figure compared with real life usage.

So many cars can pass an MOT which checks on what is coming out of the exhaust and requires gasses emitted to be within certain limits on cars. Otherwise it will fail. It must be quite a liberal measurement however it is well known that garages can effect a temporary fix weakening the air/fuel mixture wich gives a lower emissions result.
Bob Craven Lancs

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Good to chat on this forum Rod. I think most people’s comments help to add some more depth to the articles.

There are several factors other than speed that influence air quality by a road. Traffic volume and congestion of course. Topography is another one where certain streets don’t seem to have a natural movement of air to disperse things and so polluted air just hangs around. Hafodyrynys in Wales is widely reported to be the most polluted section of road outside London.

Another major factor is real world vehicle emission rates. The recent BBC video article indicated that both Mercedes and the Nissan Qashqai met the Euro 6 technical standard but in real world testing at the tailpipe, PEMS results showed the Mercedes was still delivering the same result but the specific model of Qashqui tested was 18 times more polluting than on test.

Which? magazine also undertook some in-depth testing earlier this year and concluded that some manufacturers engines are fundamentally much dirtier than others although all are designed pass the standard Euro 6 testing regime and are therefore approved for use.

Seems that many car buyers either don’t know or are not concerned what the real world emissions of the cars they are buying are. Not buying new cars with poor true tail pipe emissions levels would probably have the greatest single impact of any measure. It would certainly be more effective than penalising users of older cars as has just happened in London.
Pat, Wales

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What Adrian has come up with is quite interesting. On the face of it, 146 KSI collisions on '30' roads compared to only 16 on '20' roads is good news as a fact taken in isolation, but what would be more significant though, is a comparison between collisions on roads when they were a 30 limit and any of those same roads which later became a 20. Also, if it were possible, knowing what the average speeds were on the 30 roads (the 146 KSI roads) and the speeds on the 20 roads i.e. is the collision reduction due to actual slower speeds induced by the lower limit.
Hugh Jones, Cheshire

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Thanks for that info Adrian but first of all as you know there have been some years of increased 20 mph areas and one cannot judge on just one year alone.

Further Sheffield's main arterial roads are like the spokes of a wheel and there are many of them. They have many estates lining these roads miles away from the TC. but still in Sheffield and therefore many of these residential areas are really far away from the dangers of congested traffic where the majority of collisions will occur. See THE CUBE. I would imagine that the road usage on these 20 is plenty areas can be considered somewhat minimal compared to some other areas of 30 mph on main arterial roads leading in and out of the town. So comparisons are odious and cannot be correlated as such. Further if a vehicle travelled out of a 20 mph road and hit, or was hit, by another vehicle that was travelling on a 30 mph arterial road that accident would be considered to be on the 30 mph one wouldnt it?

What is needed is the accident figures for the last 3 to 5 years and to correlate these figures in line with the increasing areas of 20 mph. It could also be that there have been no accidents etc in many of the areas that were 30 mph. but are now 20 mph areas.
Bob Craven Lancs

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That's good Adrian, no fatalities is always good. By the way, How many miles of 20mph roads are there in Sheffield? And how many miles of 30mph roads are there in Sheffield?
James, Weston B, Herefordshire

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To Bob Craven. You asked for Sheffield casualty stats.

In 2016:
Fatalities on 30mph roads = 5; on 20mph roads = 0

Seriously injured:
Total 30mph roads: 141; 20mph roads: 16,

of which:
Cyclist 30mph: 27; 20mph: 0
Pedestrian 30mph: 47; 20mph: 9

Looks encouraging for Sheffield residents when more 30mph roads become 20mph...
Adrian, Tunbridge Wells

Agree (9) | Disagree (12)

Once again Rod you have quoted from a paper that suppports your slow speed mitigated injury campaign or intervention and does little to actually alleviate the causaton of the accident but merely mitigates its consequences. I would argue that by actually reducing the speed limit one is enabling more accidents to happen due to the lack of safe space that drivers will afford themselves and others in such a slow moving environment.

As regards the conclusions from the Cube they are in error as it does not follow that when the mean speed increases there will be a subsequent increase in accidents and injuries. Obviously the degree of injuries may be greater but the fact that there will be more of them is a wrongful presumption. Most accidents are actually at low speeds and are never ever reported to the authorities, to the police or the hospitals. Of those that are, some 60 %+ occur in slower speed environments within conurbations. The remaining 30%+ occur out on the open road in the country. Stats prove that year on year so it's not as the CUBE would wish us to believe.

The Cube has not improved road safety by not appreciating and not identifying the dangers of tailgating. Or should I say in more general safety terms a lack of ‘Safe Following On Distance’ or indeed ‘Safe Space’ or ‘Safe Driving Distance'. Something apparently easily identified by a large % of ordinary motorists but not recorded at all as a danger by the Cube. A grave mistake that seems to be self perpetuating. People will now say as its not a danger identified by the 'Cube' it doesnt exist but it does. It's just that authorities and countries do not identify it as being in any way important or of concern preferring to label it a SPEEDING concern or DRIVER ERROR and so its not been identified to the Cube and that is the error..and it needs correcting.
Bob Craven Lancs

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Hi Pat

Indeed, older average speed models were where a range of sites were monitored for average speed and average pollution levels. In such cases the average speed was influenced by the degree of congestion rather than any speed limit. Whilst it included roads with average speeds of 20 and below the average speed was an output alongside emissions as a result of the congestion. There was correlation between the two but only as outputs. That's why the very next sentence after the one you picked noted: "However, the stop-start nature of traffic in central London means that such a method may not be suitable, and further investigation is required."

Hence the research.

Regarding NOx for petrol powered vehicles, the 7.9% higher is indeed correct. But as a diesel vehicle emits 10 times as much NOx as a petrol vehicle under the same conditions then the 7.9 increase in petrol NOx is dwarfed by the 80 reduction in diesel NOx. Thats why when you extrapolate the figures for UKs mix of petrol and diesel vehicles the change to a 20mph limit is equivalent to taking nearly half the petrol vehicles off the road in terms of NOx emissions.

Emissions is just another of the many boxes which 20mph limits tick for authorities. Whilst it also includes danger reduction its the breadth of many other benefits which provide the basis for adoption of authority-wide 20mph defaults.
Rod King, Warrington, 20's Plenty for Us

Agree (8) | Disagree (10)

Is there much pollution from vehicles in quiet residential roads anyway, regardless of the speed of those vehicles? Once a vehicle has passed, do the exhaust gases not simply rise and dissipate fairly quickly? I would think residents are far more concerned about the speed reduction than air quality, simply because the latter is not as noticeable, if at all.
Hugh Jones, Cheshire

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Hi Rod
I can cherry-pick quotes as well. The opening paragraph of The Imperial college report says “Average speed models suggest that a lower speed limit in urban areas may result in higher pollutant emissions.” It goes on to define driving modes covered in the test which do not include congestion. So the report isn't comprehensive and does not pretend to be.

The report also states NOx emissions for petrol cars tested are 7.9% HIGHER in 20mph speed limits than in 30mph. You forgot to mention that.

The project finding is very cautious :”It is concluded that it would be incorrect to assume a 20mph speed restriction would be detrimental to ambient local air quality". That cautious statement hardly qualifies air quality improvement to be stated as a benefit for 20s.
Pat, Wales

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May I suggest you refer to the recent RSGB article on the Safety Cube DSS. Here you will find Speed and Road Accidents: the Power Model

"This report deals with the evaluation of the Power Model. The main findings are: 1) 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. When the mean speed of traffic increases, the number of accidents and the severity of injuries will usually increase. 2) The relationship between changes in speed and changes in road safety holds for all speeds in the range between about 25 km/h and about 120 km/h. 3) The relationship between changes in speed and changes in road safety can be adequately described in terms of a power model, in which the relative change in the number of accidents or accident victims is a function of the relative change in the mean speed of traffic, raised to an exponent. 4) The relationship between speed and road safety can to some extent be modified by the road environment, by vehicle-related factors, and by driver behaviour, but the effects of speed on road safety appear to be remarkably consistent across different contexts"


The formula for the energy required to reach a given speed is well established as 1/2 mv^2. Hence to reach 30mph requires 2.25 times as much energy as that to reach 20mph. Whilst there may be minor differences in steady state friction and air losses between steady state speeds it is the acceleration which is the dominant factor in an urban environment.

Imperial College found an 8% reduction in NOx and PM10 for diesel vehicles when operating at a 20mph as opposed to 30mph cycle. They also found a complete elimination of all emissions for a driver electing to leave the car in the garage and walk or cycle.
Rod King, Warrington, 20's Plenty for Us

Agree (10) | Disagree (12)

"...15 million people in the UK live in places ‘where 20mph is normal’ No doubt true, but a lot of them need reminding that living in such roads does not make them exempt from having to comply with the limit any more than those 'just passing through' or 'just making a delivery'.
Hugh Jones, Cheshire

Agree (7) | Disagree (1)

The alleged link to air quality improvement going from 30 to 20 is unproven. The speed reduction could actually make it worse but that is also unproven. The best bet is to consider the speed changes neutral to air quality.
Pat, Wales

Agree (10) | Disagree (5)

So Sheffield has had 20 mph schemes ever since 2012 and they now operate in some 30% of residential areas. Can they then actually state catagorically that there have been pedestrian incident reductions in those areas. It could be that they could say that there has been a 30% reduction in child deaths or accidents as the 20 is Plenty Campaigners do in their literature but that may mean 2 deaths and not three.

Or as in paragraphs 7 council member Jack Scott spoke about there not only being a reduction in speed but a reduction in the severity of the accidents. Now at last a person in power has spoken about injury reductions but can he put some meat on that statement or is he talking rhetoric and propaganda taken from someone else's pamphlet.
Bob Craven Lancs

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