Health impacts of the M74 urban motorway extension: a mixed-method natural experimental study
Authors: Ogilvie David , Foley Louise , Nimegeer Amy , Olsen Jonathan R , Mitchell Richard , Thomson Hilary , Crawford Fiona , Prins Richard , Hilton Shona , Jones Andy , Humphreys David , Sahlqvist Shannon , Mutrie Nanette 
Published: June 2017

Background
Making travel easier can improve people’s access to opportunities, but motor transport also incurs substantial undesirable health and social impacts.

Aims
To assess how a new urban motorway affected travel and activity patterns, road accidents and well-being in local communities, and how these impacts were experienced and brought about.

Design
The Traffic and Health in Glasgow study, a mixed-method controlled before-and-after study.

Setting
Glasgow, UK.

Participants
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 substudy using accelerometers and global positioning system receivers and 30, living within 400 m of the new motorway, took part in a qualitative substudy along with 12 other informants. Complementary analyses used police STATS19 road traffic accident data (1997–2014) and Scottish Household Survey travel diaries (2009–13).

Intervention
A new 5-mile, six-lane section of the M74 motorway, opened in 2011 and running through predominantly deprived neighbourhoods in south-east Glasgow, with associated changes to the urban landscape.

Main outcome measures
Differences in self-reported travel behaviour (1-day travel record), physical activity (short International Physical Activity Questionnaire) and well-being [Short Form 8 Health Survey (SF-8) and a short version of the Warwick–Edinburgh Mental Well-being Scale], and in the incidence of road traffic accidents.

Methods
A combination of multivariable cohort, cross-sectional, repeat cross-sectional and interrupted time series regression analyses comparing residents of the ‘M74 corridor’ intervention area and two matched control areas, complemented by novel qualitative spatial methods. Graded measures of the proximity of the motorway to each participant’s home served as a further basis for controlled comparisons.

Results
Both benefits and harms were identified. Cohort participants living closer to the new motorway experienced significantly reduced mental well-being (mental component summary of the SF-8 scale) over time compared with those living further away [linear regression coefficient –3.6, 95% confidence interval (CI) –6.6 to –0.7]. In the area surrounding an existing motorway, this association was concentrated among those with chronic conditions. 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, although 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 composition and cohesion, and perceptions of personal safety, were widely perceived as more important to local people.

Limitations
A key limitation of natural experimental studies is that the risk of residual confounding cannot be eliminated.

Conclusions
Overall, these findings highlight the potential for urban infrastructural projects of this kind to add further burdens to already disadvantaged communities, exacerbating inequalities and contributing to poorer health outcomes. The health and social impacts of such initiatives should be more fully taken into account in planning and research.

Funding
The National Institute for Health Research Public Health Research programme.