20mph Limits Offer A Toxic Diesel Fume Reduction Equivalent to Taking Half of All Petrol Cars Away

Diesel emissions dominate urban road pollution with about ten times the toxicity of petrol fumes. As 20mph limits reduce the most toxic diesel fumes, setting a wide 20mph limit is equivalent to removing nearly half of all petrol cars. This massive air quality and public health gain must be implemented urgently and nationally to reduce the 30,000 annual UK deaths from air pollution.


When the City of London Corporation evaluated a city-wide 20mph limit they commissioned the respected Imperial College London to evaluate emissions effects[1]. They found that for diesel vehicles there is a substantial reduction in harmful Nitrogen Oxides (NOx) and Particulates (PM10) emissions from implementing 20mph limits. Key figures are:-

Vehicle Type[2]

Drive Cycle speed limit

NOx (g/km)

PM10 (g/km)

CO2 (g/km)

Petrol 1.4 – 2.0 litre, EURO IV









Impact of 20mph drive cycle




Diesel 1.4 – 2.0 litre, EURO IV









Impact of 20mph drive cycle




In 2014, 37% of cars on UK roads were diesels[3]. Where a 20mph limit is implemented the reduction in total NOx and PM10 emissions is large enough to be the equivalent of taking nearly half of the 63% petrol cars off the roads.


Total g/km/100 cars
at 30mph (A)

Total g/km/100 cars at 20mph (B)

Change g/km/100 cars

Std g/km for a petrol car

Effect of implementing a 20mph limit drive cycle expressed as






Equal to removing 32 petrol cars






Equal to removing 30 petrol cars






Equal to 1 extra petrol car

20mph limits therefore provide a significant reduction in the most toxic emissions for built-up roads so assisting local authority compliance with legal air quality requirements and reducing premature air quality related deaths.

Rod King MBE, Founder and Campaign Director of 20’s Plenty for Us said:-

“Many UK cities break legal NOx and PM10 pollution levels, killing people early and risking fines. A 20mph built up limit simply, immediately and effectively reduces dirty fumes. This government should urgently wake up to the air quality gains from 20mph.  Lower speeds give so many road safety, active travel and public health benefits. The government should stop hiding behind the fig-leaf of “localism” on speed limits and immediately cut the requirement for, and costs of, 20mph repeater signs.  Then change the national limit from 30mph to 20mph letting local authorities decide where a higher speed limit is appropriate taking full account of all their public health responsibilities.[4]

Correction. An earlier version of this briefing showed the CO2 emissions for diesel in a 30mph drive cycle at 201.58 g/km (ie the same as petrol) in the first table above. This was a typo and is now corrected at 203.48 g/km. However this correct figure had been used in the calculations for the second table. Hence the conclusions of the briefing still stand.

Showing 4 reactions

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  • Rod King
    commented 2015-10-11 11:41:02 +0100
    See the new report from Brake calling on government to cut the red tape on 20mph limit and implement a national 20mph limit. http://www.brake.org.uk/news/1488-government-red-tape-puts-brakes-on-20mph-limits
  • Rod King
    commented 2015-10-04 17:52:15 +0100
    Thanks for your comment Robert. What we are showing is that with today’s mix of diesel and petrol cars then a reduction in speed limit to 20mph does make a significant difference in in the levels of NOx and PM10 emissions. In order to put that saving into perspective we have identified the equivalent number of petrol cars that this reduction is equal to. You can also reflect that reduction in the equivalent number of diesel cars removed, in which case it is 2.6 for NOx and 3.7 for PM10.

    Of course where lower speed limits encourage drivers to travel by walking, cycling or public transport then there are additional reductions as well.
  • Robert Bull
    commented 2015-10-04 17:19:27 +0100
    This is crazy it shows Diesels are 12 times more polluting (NOx) 8 times worse (PM10) than petrol cars and they focus on another factor which because diesels run lower revs reduces with 20mph running so generating a wild claim that slowing diesels is like getting rid of Petrol Cars!!!
    So what!
  • Astrid Dawe
    commented 2015-10-03 08:52:08 +0100
    Surely this study carries even more weight following the revelations from VW !!!!!