Air quality effects of an urban highway speed limit reduction

A speed limit intervention on part of the Amsterdam ring highway, adjoined with apartment buildings, was implemented. The objective of this study was to assess whether, and to what extent, a lowering of the maximum speed limit from 100 to 80 kph had reduced traffic related air pollution in the direct vicinity of a highway. A monitoring station of the Amsterdam Air Quality Monitoring Network is situated adjacent to the intervened road section. Daily mean concentrations (PM10, PM1, Black Smoke and NOx) in the first year since the intervention were compared with measured concentrations in the prior year. The intervention effect was adjusted for daily traffic flow, congestion and downwind exposure. The concentration changes were compared with those observed at a section of the same ring highway where the speed limit had not been reduced. Since the intervention, the adjusted traffic contribution to PM10 concentrations has decreased by 2.20 μg m−3 (95%-CI: 1.43–2.98), PM1 0.42 μg m−3 (95%-CI: 0.01–0.82) and Black Smoke 3.57 μg m−3 (95%-CI: 1.50–5.65). At the not intervened highway section the adjusted traffic contribution to PM10 and Black Smoke concentrations has also decreased by 0.97 and 2.43 μg m−3 respectively. However, decreases were significantly greater for PM10 and PM1 at the intervention site. In conclusion, this study demonstrates a significant reduction of PM10 and PM1 as a result of reducing the speed limit at an urban ring highway.