Limites de Velocidad de 30kph pueden contribuir a reducir muertes relacionadas al tráfico en zonas urbanas, sin embargo ¿No causarian mayor contaminacion?
First, many thanks for a first-class response to the question posed by T Reid earlier this week. Returning to this post on Friday feels as if I’ve somehow walked into a plenary session at a joint conference of motoring engineers and urban planners.
Many excellent and constructive points have already been made about the wider issue of traffic-calming measures so I’ll try to focus tightly on the question at hand: do 20mph speed limits cause more pollution than, say, 30mph limits?
I have to say at the outset that I wholeheartedly agree with FC1967 and Snurfkin, who both point out that reducing fatal accidents, particular those involving children, is really what 20mph speed limits should be about, not reducing pollution per se. However, any reduction in tailpipe emissions is a big bonus and should be an additional compelling reason to adopt 20mph speed limits – if it can be proved that this is indeed the case.
A hefty portion of the debate so far seems to have focused on the fuel efficiency of engines at different speeds. The point was made by a few people, such aseuangray, that cars are at their most efficient when travelling at around 50mph. Others, such as wildnorthlands, disagreed. For me, this point leads us down an unnecessary cul de sac with regard to the particular question posed, because all that should concern us is how much pollution pours from a car’s exhaust at 20mph compared to 30mph when it’s travelling over a comparable distance.
Therefore, I’m thankful to DJK3 for providing a link to a report published in 1995 by the US National Research Council’s committee for study of impacts of highway capacity improvement on air quality and energy consumption. (Don’t tell me you hadn’t come across it before, either!) As DJK3 states, the report refers to some research from Germany which dates from the 1980s which showed the percentage change in emissions when a vehicle’s speed was reduced from 50kmh (31mph) to 30kmh (19mph):
Research in Germany has shown that the greater the speed of vehicles in built-up areas, the higher is the incidence of acceleration, deceleration, and braking, all of which increase air pollution. German research indicates that traffic calming reduces idle times by 15%, gear changing by 12%, brake use by 14%, and gasoline use by 12% (Newman and Kenworthy 1992, 39–40). This slower and calmer style of driving reduces emissions, as demonstrated by an evaluation in Buxtehude, Germany. Table E-1 shows the relative change in emissions and fuel use when the speed limit is cut from 50kmh (31mph) to 30kmh (19mph) for two different driving styles. Even aggressive driving under the slower speed limit produces lower emissions (but higher fuel use) than under the higher speed limit, although calm driving produces greater reductions for most emissions and net fuel savings (Newman and Kenworthy 1992, 39 –40).
[NB. “Newman and Kenworthy 1992” refers to a book called Winning Back the Cities by Peter Newman and Jeff Kenworthy with Les Robinson, Pluto Press.]
However, a much more recent study by researchers at Virginia Tech in the US came to somewhat different conclusions. In 2009, their study, which was published in the journal Transportation Research and entitled “A field evaluation case study of the environmental and energy impacts of traffic calming” (PDF, behind a paywall), reached the following conclusion:
While traffic calming measures reduce vehicle speeds on neighbourhood streets and may contribute to enhanced road safety, these measures can result in significantly higher fuel consumption and emission rates when drivers accelerate aggressively. We also found that newly installed speed lumps could be responsible for extra fuel consumption.
Traffic circles [note: not the same as roundabouts] produced the least increases in vehicle fuel consumption and emissions and the case study showed that, in general, traffic circles allow smoother driving patterns with milder acceleration behaviour when compared to speed humps and stop signs. The results also demonstrate reductions when stop signs are replaced by traffic circles. The study indicates that by eliminating sharp acceleration manoeuvres significant energy and emission savings can be achieved. Consequently, significant improvements in air quality and energy consumption may be achievable through driver education.
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.
On 19 April Leo originally wrote:
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.