Reducing traffic speeds below 40mph may increase toxic pollution, says Transport Research Laboratory report

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

by Sarah Barth   February 3, 2013  


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:


25 user comments

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"The reason for the air pollution is the increased amount of acceleration and braking in stop-start driving"

And that is a MASSIVE caveat which basically means the stand-out message is complete and utter bollocks: stop-start driving increases emissions, lower speed doesn't. No surprise to anyone.

Properly controlling traffic speeds often leads to *less* "stop-start" driving - anyone who's been on a stretch of road with average speed cameras should attest to this.

Bez's picture

posted by Bez [420 posts]
3rd February 2013 - 22:04


Your headline is totally wrong.

You should be asking: "Why would a 50kph (30mph for the barbarians) speed limit increase stop-and-start driving?"

And if a default 30kph speed limit in urban areas is adopted, the Government should work with the motor industry to force them to make cars run efficiently at that speed.

posted by andreacasalotti [4 posts]
3rd February 2013 - 22:30


Curiously I saw a report of the same research today that said reducing speed limits would reduce air pollution (which is also supported by previous research based on monitoring air pollution while varying speed limits). I get the feeling that the story is being spun by the motoring lobby. Either way, I will find out when I go to the Edinburgh Centre for Carbon Innovation Seminar on Wednesday.

posted by Kim [132 posts]
3rd February 2013 - 22:31


Just found a ref for the statement above, according to Professor James Curran, the chief executive of the Scottish Environment Protection Agency “Reducing speed really will reduce emissions”.

posted by Kim [132 posts]
3rd February 2013 - 22:40


The logic of this seems to be a little flawed.

Surely Stop-Start driving in a higher speed limit area would mean more time spent accelerating to get to that higher speed.

Also emissions are linked to engine speed as higher revs lead to incomplete combustion, they needn't be linked to the travelling speed of the car. Its only gear ratios.

If the speed limits are set to keep people safe, the vehicles can easily be designed to minimise pollutants at those speeds. Road safety and environmental policy should drive vehicle design, not the other way around.

posted by Mike_Hall [15 posts]
3rd February 2013 - 22:40


Have you checked the commissioned report itself? You don't seem give a link to it. I note that The Edinburgh Centre for Carbon ­Innovation is listed at Companies House. Its web site is a bit light on practical accounts of who they are and what they do for a living. I'm not suggesting one thing or another. But when I read an item about "commissioned research" I normally want to skim the data and methodology before I give it credence - especially, perhaps, when the headline seems to be counter-intuitive or controversial.

posted by Sam Saunders [20 posts]
3rd February 2013 - 22:58


I see that the TRL did a literature review called Traffic calming and vehicle emissions: a literature review in 1997. (payment required for download)

posted by Sam Saunders [20 posts]
3rd February 2013 - 23:09


it's always a good idea to provide a link to the original research, so that people that can read and think would be able to make their own minds up rather than follow statements from someone else.

posted by mhtt [43 posts]
4th February 2013 - 1:13


If speed is reduced, then it will take longer to travel to your destination, which in turn will mean more time spent driving, which means more congestion, which in turn leads to even slower speeds, and more pollution. Thinking


posted by nbrus [281 posts]
4th February 2013 - 6:32


nbrus wrote:
If speed is reduced, then it will take longer to travel to your destination, which in turn will mean more time spent driving, which means more congestion, which in turn leads to even slower speeds, and more pollution. Thinking

That's not universally true. The variable speed limits on the M25 and other motorways have demonstrated that slowing traffic down increases road capacity. That's because it cuts out the random slowing to a halt and speeding up that spread like waves over miles.

Similarly, the shared space in Poynton allows traffic to continue moving instead of the stop/start in a queue at traffic signals.

posted by Campag_10 [153 posts]
4th February 2013 - 8:57


Less speed = less fuel used = less pollution.


antonio's picture

posted by antonio [966 posts]
4th February 2013 - 8:57


is the result actually "there's less stop-start driving on the sort of roads that are appropriate for 40mph and higher speed limits"?

posted by Al__S [597 posts]
4th February 2013 - 9:08


antonio wrote:
Less speed = less fuel used = less pollution.

That doesn't necessarily follow. Most cars are most efficient at about 60-70km/h.
To illustrate the point think about a car travelling a kilometre at 60 km/h and one travelling the same distance at 0.1km/h.

The first takes a minute to cover the distance - the second car takes 10 hours. The first will use a lot less fuel than the second.

Smooth flowing of the traffic also makes a difference, fewer traffic lights and more roundabouts allow some speed to be maintained across junctions when traffic is light, thus saving fuel.


posted by NorthernRouleur [26 posts]
4th February 2013 - 9:29


There may be a factr which has not been mentioned - driving at less than 30mph(51kph, not a dinosaur!) in most modern cars means using lower gear than top, so engine revs are a bit higher, this may mean the engine is running efficiently as it would around 60/70 kph in top gear, so one is balanced by another? As I run a diesel, in town I rarely go further tha 4th of 5 gears, often 3rd at 30mph in traffic. But I know from checking that a steady 113kph (70mph) on motorways is by far the most efficient at over 50/55 mpg, in a 1500kg car. Confusing, isn't it? It's acceleration and braking that uses the fuel, and a smooth driving style changes both mpg and pollution levels. Pollution = boy racers flying up to hazards and braking hard, using up pads, discs, tryres, then tearing off using loads of fuel and tyres.
Less pollution = people who can drive properly, easing into hazards and moving easily away, using hardly and brakes, tyres, and minimising fuel consumed. Which seems to be simple common sense.
Perhaps the solution is proper driver training, not just be taught to pass a once in a lifetime test?


posted by doc [167 posts]
4th February 2013 - 10:05


I'm always wary of studies that focus on CO2 emissions without considering the wider context. What percentage of the time do cars drive at the posted speed limit anyway? My gut feeling is that during busy periods, average speeds are well under (it's taken me an hour to drive a mile across Bristol city centre before, and that's not particularly unusual).

If more people were cycling or taking the bus, there would be fewer cars, less emissions, and average journey times would decrease. But that's not going to happen while roads are threatening places where cars rule untamed.

posted by Mr Agreeable [145 posts]
4th February 2013 - 12:49


the new set up in poynton is great, however if you have ever ridden through there (as many manchester readers may have on the way out to the peaks) you'll find the major failing for cyclists is that the materials used in its construction are when wet %100 lethal. I've gone down twice on that roundabout, once without realising the danger and a second time even when trying to be careful.

Nice idea but a bit of a fail in execution as far as cycling is concerned (i might add both of these accidents occurred on nearly new GP4000s tyres).

posted by edf242 [36 posts]
4th February 2013 - 13:42


The starting point for any discussion on which road speed is the most appropriate should be this -

"Results show that the average risk of severe injury for a pedestrian struck by a vehicle reaches 10% at an impact speed of 16 mph, 25% at 23 mph, 50% at 31 mph, 75% at 39 mph, and 90% at 46 mph. The average risk of death for a pedestrian reaches 10% at an impact speed of 23 mph, 25% at 32 mph,50% at 42 mph, 75% at 50 mph, and 90% at 58 mph. Risks vary significantly by age. For example, the average risk of severe injury or death for a 70‐year‐old pedestrian struck by a car travelling at 25 mph is similar to the risk for a 30‐year‐old pedestrian struck at 35 mph." ( Is an extra 10 mph really worth a 100% increase in the risk of a pedestrian being killed?

This also explains how fuel efficiency is affected by speed -

posted by spen [87 posts]
4th February 2013 - 19:41


Just got round to watching that Poynton video, and it's very interesting stuff. Although I can't help wondering if the benefits they're waxing lyrical over are, to use As Easy As Riding A Bike's metaphor, just the result of getting the hockey players to go into the ice rink in single file.

posted by Mr Agreeable [145 posts]
4th February 2013 - 23:22


It's appalling that the report relies on dishonesty -Comparing apples with oranges to 'prove' a falsehood is utterly disgraceful.

The answer is to keep cars out town-centres and allow only restricted access for motorised traffic.

The first benefit would be the regeneration of the high-street, which was severely damaged by the private motor-car and out of town shopping centres.
Then there would be a dramatic drop in transport-related air pollution which kills ~35,000 people per year.
Source: House of Commons Environmental Audit Committee Air Quality Fifth Report of Session 2009–10 Volume I

By excluding cars from town centres way we'd short journeys by car (with their appalling fuel-inefficiency) with bicycles which are at least 25 times as fuel-efficient.
Source: Sustainability Without the Hot Air McKay.


It's about time motorists paid the full costs of motoring, instead of whining about 'road-tax' and the cost of fuel and accusing cyclists of not paying their fair-share.

Each registered vehicle in the UK is subsidised by 2000 Euros per year, (on average, 2008 figures).

The True Costs of Automobility: External Costs of Cars Overview on existing estimates in EU-27
Final Report
TU Dresden
Dresden, October 12th, 2012

posted by Recumbenteer [149 posts]
5th February 2013 - 19:08


Surely less speed = perceived safer roads, thus encouraging more people onto bikes and decreasing pollution?

posted by paulfg42 [377 posts]
5th February 2013 - 19:50


All this misses the truth that motor vehicles keep society going and without them we would die pretty rapidly. Not at all healthy. Witness. The faster mankind got so life expectancy went up. Unfortunately the population explosion that went with it and thus the vicious circle of more people,needing more vehicles to sustain it all.

But our study, backed with results in areas that have adopted blanket 20 zones, is that accident rates have gone up in them. So no ,they're not safer for cyclists or pedestrians. This is all explained on our site and include the confession to us by the DfT & TRL that they had grossly exagerrated chid accident figures by using percentages to justify them.

Road safety 'experts' are often folk who's CV doesn't cut the mustard.

posted by Sedgepeat [64 posts]
6th February 2013 - 1:39


Most people don't want to get on bikes and so do not do so. What an authoritarian attitude! I like doing it so every one should do it. I know what's good for people and they dont so we will force them to cycle.

Cycling is only temporary and short lived for most people. They stop doing it because it's of very limited use to them, uncomfortable, cold & wet and they, quite justifiably, don't feel safe among all these bits of heavy machinery operated by complete strangers of varying ability.

Road safety 'experts' are often folk who's CV doesn't cut the mustard.

posted by Sedgepeat [64 posts]
6th February 2013 - 1:49


Sedgepeat, you are wrong about lower speed limits (and a few other things besides). The reports of increased accidents in 20mph areas were based on figures that failed to take account of the larger numbers of roads which are now 20 mph. Measured by distance travelled, the areas covered by 20mph zones are safer. Not by much, but better enforcement and road design would probably sort that out.

If you read this website regularly instead of just dropping in to try and direct traffic to your appalling dog's dinner of a page, you'd be aware of this.

posted by Mr Agreeable [145 posts]
6th February 2013 - 11:03



andrew miners's picture

posted by andrew miners [46 posts]
6th February 2013 - 12:12


Sometimes I cycle to work and sometimes I drive a car to work.
When I drive it takes me longer to get there because of the big queues of traffic I become part of.
Despite part of the journey being in a 40 mph speed limit I never manage that speed and rarely get close to 30mph, so in my real life experience, reducing the speed limit would make absolutely no difference to the speed I'm actually able to drive at and therefore make no difference to the emissions spewed from my car.
It might just slow down some of the nutters that use the residential streets around my neighbourhood as rat-run racetracks though.

Chris's picture

posted by Chris [109 posts]
6th February 2013 - 13:09