Exhaust gas pollution

The impact of 20mph zones on air pollution is hard to analyse because it depends on so many variable, including engine type, driving styles, traffic conditions and vehicle weight. Most research finds little difference in levels of the main pollutant gases before and after the introduction of 20mph.
Individual drivers, however, can make a difference by driving as steadily as possible – it is acceleration and braking that increases fuel consumption and emissions. In that regard, by smoothing the flow of traffic, 20mph zones are better than physical traffic calming measures, particularly speed bumps. In the longer term, the transition to electric vehicles will eliminate almost all exhaust emissions.

Particulates

Particulate matter emissions from tyres and brakes now present a greater threat to health than exhaust gases (because modern engines have much lower exhaust emissions than those of the past). Unfortunately, the shift to electric vehicles may actually make the situation worse due to their being heavier and capable of rapid acceleration (both factors increase tyre and brake wear).
Many studies have found that less particulate matter is produced at 20mph than at 30mph. For example, in Edinburgh one year after switching most roads from 30mph to 20mph, the areas with the reduced speed limit saw an 8% reduction in particulate matter. A similar pattern was seen in London, where an 8–10% decrease in particulate matter was observed after the introduction of 20mph zones.

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