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But slower cars cause fewer crashes, and the roads are safer for cyclists and pedestrians too

Reducing the speed of traffic in Britain's towns and cities would increase pollution, a study by a Scottish carbon reduction group has found.

The Edinburgh Centre for Carbon ­Innovation (ECCI) has commissioned research by the Transport ­Research Laboratory (TRL) that showed that, while reduced traffic speeds encourage walking and cycling and reduce the number of crashes, they are linked to higher levels of toxic emissions.

It said: “Reducing urban speed limits has concomitant health, community and safety benefits, but is likely to raise greenhouse gas emissions..”

The report shows that cutting speeds to below 40mph was likely to increase pollution, such as nitrogen oxides and particulates. Carbon dioxide emissions would also go up.

The reason for the air pollution is the increased amount of acceleration and braking in stop-start driving, although crucially, these could be reduced if a way was found for traffic flow was smoothed (see the video below for how this was successfully achieved in the Cheshire town of Poynton). The report said 40mph was the optimum speed for minimising vehicle emissions and pollution.

According to government information, nitrogen dioxide is pretty nasty stuff:

    Nitrogen dioxide (NO2) is one of a group of gases called nitrogen oxides. Road transport is estimated to be responsible for about 50% of total emissions of nitrogen oxides, which means that nitrogen dioxide levels are highest close to busy roads and in large urban areas. Gas boilers in buildings are also a source of nitrogen oxides.

    There is good evidence that nitrogen is harmful to health. The most common outcomes are respiratory symptoms such as shortness of breath and cough. Nitrogen dioxide inflames the lining of the lung and reduces immunity to lung infections such as bronchitis. Studies also suggest that the health effects are more pronounced in people with asthma compared to healthly individuals.

    In recent years the average level of nitrogen dioxide within London has not fallen as quickly as predicted. This largely appears to be the result of diesel cars creating more nitrogen dioxide than was anticipated.    Nitrogen dioxide also reacts with hydrocarbons in the presence of sunlight to create ozone, and contributes to the formation of particles*   

    *tiny bits of solids or liquids suspended in the air, that can settle in the airway and deep in the lungs and cause health problems.

Unsurprisingly, the news has pleased motoring groups.

The Institute of Advanced ­Motorists welcomed the report, saying that not enough research had been done into the consequences of bringin down speed limits.

Neil Greig, its Scotland-based policy and research director, told the Scotsman: “We have not previously had an open debate about the impact of speed limits on the environment.

“But when you start to mix speed limits, road safety and emissions, it gets very ­complicated.”

And even if speed limits do get reduced, it's not all good news for vulnerable road users. The report found reducing speed limits alone usually cut average speeds by only one quarter of the change in the limit.

For one town's take on slowing speeds, cutting emissions and making things safer for vulnerable road users, see this video:

 

After an unpromising start, having to be bribed by her parents to learn to ride without stabilisers, Sarah became rather keener on cycling in her university years, and was eventually persuaded to upgrade to proper road cycling by the prospect of a shiny red Italian bike, which she promptly destroyed by trapping a pair of knickers in the rear derailleur. Sarah writes about about cycling every weekend on road.cc.