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 |
20mph |
0.0726 |
0.00218 |
271.95 |
30mph |
0.0673 |
0.00237 |
266.35 |
|
Impact of 20mph drive cycle |
+7.9% |
-8.3% |
+2.1% |
|
Diesel 1.4 – 2.0 litre, EURO IV |
20mph |
0.7437 |
0.01758 |
201.58 |
30mph |
0.8104 |
0.01917 |
203.48 |
|
Impact of 20mph drive cycle |
-8.2% |
-8.3% |
-0.9% |
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.
Emission |
Total g/km/100 cars |
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 |
NOx |
34.2247 |
32.0907 |
-2.134 |
0.0637 |
Equal to removing 32 petrol cars |
PM10 |
0.8586 |
0.7878 |
-0.0708 |
0.00237 |
Equal to removing 30 petrol cars |
CO2 |
24308.81 |
24591.31 |
+282.5 |
266.35 |
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]”
[1] https://www.cityoflondon.gov.uk/business/environmental-health/environmental-protection/air-quality/Documents/speed-restriction-air-quality-report-2013-for-web.pdf
[2] Whilst the report only shows results for EURO IV vehicles rather than EURO V and VI, it also excludes commercial vehicles which can be expected to show even larger emission reductions in a 20mph cycle.
[4] http://www.20splenty.org/transitioning_to_national_20
For article on the extent of air quality related deaths in UK see http://www.theguardian.com/environment/2015/apr/02/air-pollution-may-cause-more-uk-deaths-than-previously-thought-say-scientists
Showing 4 reactions
Sign in with
Facebook TwitterOf course where lower speed limits encourage drivers to travel by walking, cycling or public transport then there are additional reductions as well.
So what!