Speed limits reduce deaths, but can they also cause more pollution?

That was the question put by Leo Hickman in his Ask Leo and Lucy column for the Guardian last week :-

There an estimated 450 20mph speed limit zones across the UK, many of which are located near schools. Since the first ones were introduced in 1991, they have become an increasingly popular traffic calming measure. Bristol, Portsmouth, Leicester, Oxford and a number of other towns now have city centre-wide 20mph limits.



Although they cut down on accidents, do they trigger other unintended consequences, such as increased pollution (both carbon dioxide and particulates) and traffic tail-back further afield? As ever, please share your thoughts on this subject below and I'll be back on Friday to add my own thoughts to the discussion.
 
We replied as follows :-

First, lets deal with this myth about cars being most efficient at 50-60 mph.

A study, by Peter De Nayer, a former AA fuel efficiency expert, involved fitting cars with a fuel flow meter and testing them at Millbrook proving ground in Bedfordshire. He found that a Citroën C4 1.6 diesel achieved 99.6mpg at 20mph but only 29.3mpg at 90mph.


The average car consumes 38 per cent more fuel at 70mph than it does over the same distance at 50mph. At 60mph it uses 34 per cent more than at 40mph.


The study, commissioned by What Car? magazine and based on five cars of different sizes ranging from a 1 litre Toyota Aygo to a 2.2 litre Land Rover Freelander, found that the most efficient speed was below 40mph for all five and as low as 20mph for two.

 It was found that for most cars there was a variation of between plus or minus 10% in fuel economy between 20 and 30 mph. This was dependent upon gearing and engine characteristics. Hence for steady speeds there is probably no difference between 20 mph and 30 mph when averaged out across all cars.


However, as has been pointed out, it is acceleration and particualrly repeated acceleration that uses most fuel. By capping the speed at 20 mph the ALL acceleration between 20 to 30 mph is removed.


Whilst advocating wide area 20 mph limits for urban and residential areas we would prefer to see behaviour change used rather than speed bumps to control the speed of motor vehciles.


Where speed bumps are used it would seem perverse to reason that one only needs to travel slowly on the bumps and it is expected to speed up between them. If they exist within a low speed limit then it is actually illegal to speed up between them. Hence any argument that claims that speed bumps (where used) causes increased pollution through repeated acceleration is flawed by the fact that such acceleration is at best unwise/unnessecary and at worst illegal.


Reference can also be made to a report on pollution within Belgian 30 kph zones. This suggested that they reduce particulates and may be found at the 20's Plenty for Us website together with a wide range of other pertinent reports See :-


http://www.20splentyforus.org.uk/reports.htm


Of course the points made about modal shift to walking and cycling are very pertinent and I endorse the view that streets with 30 mph speed limits will never encourage active travel and will always by child unfriendly.


I do note that the question specifically relates to pollution caused by carbon dioxide and particulates. However maybe we should widen our perspective on "pollution" and ask how much our streets have become blighted by the presence and speed of so many vehicles in those public spaces between the houses where we live.


Rod King

20's Plenty for Us

Leo's conclusion was :-
 
So, if reducing emissions is your goal, the all-important factor seems to be what traffic-calming measures you use to ensure speeds are reduced from 30mph down to 20mph. It seems there's universal agreement, though, that speed humps are not the way to go.


I think Rod King of 20's Plenty for Us raises some very interesting and valid points on this issue. And I agree with the point made by Rod and others that 20mph limits assists in encouraging a modal shift away from residential streets being predominantly the domain of motorised vehicles and not pedestrians and cyclists. My own vote would be for more, not fewer, 20mph limits, regardless of whether evidence exists to suggest they might create marginally more emissions.

The full article may be viewed at http://tinyurl.com/34g2oey