It is often said that a high-speed limit allows fast-moving vehicles to stir up the air and disperse exhaust pollution.  This doesn’t work in urban areas.  Wherever roads are congested, traffic movement is continually interrupted by vehicles turning in and out of side roads, stopping at pedestrian crossings, or parking at the side of the road.  If they can, drivers will move quickly from one obstruction to the next, and stop abruptly when forced to do so.  Each stop is followed by a period of acceleration, which injects a burst of fuel into the engine that results in additional pollution.

It’s all about smoothing the traffic flow.  If the speed limit is lowered to 20 mph, drivers don’t need to accelerate as much after each blockage to restore their original cruising speed, so the fuel emissions are reduced.  In addition, when moving at 20 mph, a driver can more easily anticipate blockages and slow down in advance so the deceleration-acceleration cycle is less pronounced and wastes less fuel.  This was confirmed many years ago by research in Germany that showed that when the speed limit in a built-up area was reduced from 50kmh (31mph) to 30kmh (19mph), drivers saved fuel and emissions fell (Newman and Kenworthy 1992, 39 –40).

By contrast, traffic calming measures can increase pollution.  For example, road humps force drivers to slow down to a very low speed.  Afterwards, they accelerate again, and in the process, generate emissions that could have been avoided if they had continued at a more modest but steady speed.  So it’s important to distinguish between physical measures on the one hand (which don’t help in terms of exhaust emissions) and 20 mph speed limits (which do).

For these reasons, in its policy document NG70, the UK National Institute for Public Health and Care Excellence NICE recommends 20 mph limits to reduce speeds in urban areas where average speeds are already low (below around 24 mph).

References

NEWMAN, P AND KENWORTHY, J, with ROBINSON, L (1992) Winning back the cities.  Pluto Press.

NATIONAL INSTITUTE FOR HEALTH AND CARE EXCELLENCE (NICE) (2017) Air pollution: outdoor air quality and health. NICE guideline NG70. Available for downloading  from https://www.nice.org.uk/guidance/ng70

Chris Wright on behalf of the Faversham Society

04 August 2018

There is additional information online from other organizations

 20's Plenty National

 Medical

Home Office

Rod King (20’s Plenty Chair and founder) the Independent Daily Edition, citing research by NGO Global Action Plan.