Speed limits can reduce air pollution

Levels of air pollution can be significantly reduced in urban areas by introducing traffic speed limits, according to a new study.

The researchers studied air quality along a section of the Amsterdam ring road, where the speed limit was lowered from 100 km/h to 80 km/h, and found that levels of some pollutants were reduced by up to 15 per cent. EU standards for air quality1 are exceeded at many locations in Amsterdam, especially along busy roads.

Traffic related air pollution is known to result in reduced lung growth in children and increase chronic respiratory problems in both adults and children. Although measures have been taken to reduce emissions from traffic across Europe, the effectiveness of these measures has rarely been quantified. There are concerns that reducing urban speed limits can in fact lead to more traffic congestion and therefore increase, rather than lower, levels of air pollution.

The Amsterdam ring road is one of the busiest in the Netherlands, consisting of six lanes. The speed limit has been introduced on a residential stretch of the road, which is used by 92,000 vehicles each day. Monitoring stations measured levels of PM10 and PM1, nitrogen oxides (NOx) and soot or black smoke (BS), both at the site with the reduced speed limit and at a separate section of the ring road, where speed limits had not been reduced.

Taking into account variations in daily traffic flow, congestion and wind direction, the results revealed that roadside concentrations of BS and PM10 decreased significantly in the section with reduced speed limits. The reductions were of 15 per cent for BS, 7 per cent for PM10 and 2 per cent for PM1.

The authors comment that particulate air pollution was significantly reduced during the period of the speed restriction, although no significant effect on NOx was observed. The study also found that congestion levels did not increase, contrary to concerns expressed before the restriction was put in place. The study claims to be the first to clearly demonstrate that reduced speed limits can improve air quality.