Health impacts of the M74 urban motorway extension: a natural experimental study

Background In 2011, a five-mile extension to the M74 motorway was opened in Glasgow, United Kingdom, constructed through a predominantly urban, deprived area. We assessed how this new motorway affected travel and activity patterns, road accidents and wellbeing in local communities, and explored how these impacts were experienced and brought about. Methods We conducted a mixed-method controlled before-and-after study. Data were collected from repeat cross-sectional survey samples of 1345 and 1343 adults, recruited in 2005 and 2013 respectively. Of these, 365 formed a longitudinal cohort; 196 took part in a quantitative sub-study using accelerometers and global positioning system receivers; and 30, living within 400m of the new motorway, took part in a qualitative sub-study along with 12 other informants. Complementary analyses used police STATS19 road traffic accident data (1997-2014) and Scottish Household Survey travel diaries (2009-2013). We defined three study areas - the first containing the new M74 motorway extension, the second containing an established motorway and a third control area with no motorway. Within each area, we calculated individual measures of motorway proximity. Using regression analyses, we explored the relationship between living near a motorway and travel behaviour, physical activity, wellbeing and the incidence of road traffic accidents, complemented by novel qualitative methods. Results Both benefits and harms were identified. Cohort participants living closer to the new motorway experienced reduced mental wellbeing over time compared with those living further away (SF8 mental component summary -3.6, 95% CI -6.6 to -0.7). In repeat cross-sectional analyses, participants living closer to a new motorway junction were more likely to report using a car at follow-up than those living further away (odds ratio 3.4, 95% CI 1.1 to 10.7). We found weaker quantitative evidence of a decline in physical activity participation, and no quantitative evidence of an overall change in either active travel or accidents, associated with motorway exposure. Qualitative evidence suggested that while the new motorway improved connectivity for those with dispersed social networks and access to motor vehicles, the impacts were more complex for others, some of whom found the motorway to be a cause of severance. Changes in community cohesion and perceptions of personal safety were widely perceived as more important to local people. Conclusions While we identified both benefits and harms, overall these findings highlight the potential for new major road infrastructure to add further burdens to already disadvantaged communities, exacerbating inequalities and contributing to poorer health outcomes.