A before-after study of the effects on safety of environmental speed limits in the city of Oslo, Norway

Starting in the winter of 2004-2005, a temporary speed limit of 60. km/h (ordinary speed limit: 80. km/h) was introduced on one of the major arterial roads in the city of Oslo, Norway as a measure to reduce air pollution, in particular the spread of micro-particles torn from the road surface by studded tyres. The speed limit, referred to as an environmental speed limit, was in force from November 1 to March 31. Similar speed limits were later introduced on other arterial roads in Oslo. This paper presents a before-and-after study of the effects of these speed limits on accidents. Four study designs were employed: (1) A simple before-after study; (2) A before-after study using the rest of Oslo as comparison group; (3) A before-after study based on accident rates; (4) An empirical Bayes before-after study. The latter design is widely regarded as the best, but its implementation in the current study was not straightforward. The number of injury accidents was reduced by about 25-35% according to all study designs. The estimate of effect did not differ much between the different study designs. It is reasonable to rule out confounding by chance variation, long-term trends, changes in traffic volume and regression-to-the-mean. It cannot be claimed, however, that the entire accident reduction was attributable to the environmental speed limits exclusively.