Ogilvie, David Bruce
Shifting towards healthier transport? From systematic review to primary research.
PhD thesis, University of Glasgow.
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Promoting a shift from using cars towards walking and cycling (a modal shift) has the potential to improve population health by reducing the adverse health effects associated with exposure to motor traffic and increasing the population level of physical activity through active travel. However, little is known about the effects of interventions which might achieve this by changing urban design, transport infrastructure or other putative determinants of population travel behaviour.
I conducted a systematic review of the best available evidence about the effects of interventions to promote a modal shift. I searched twenty electronic literature databases as well as websites, bibliographies and reference lists and invited experts to contribute additional references. I identified 69 relevant studies and devised a two-dimensional hierarchy of study utility based on study design and study population with which I selected a subset of studies for inclusion. I appraised the quality of these studies; extracted data on the effects of interventions on choice of mode of transport, how these effects were distributed in the population, and associated effects on measures of individual and population health and wellbeing; and produced a narrative synthesis of the findings.
Twenty-two studies were included. These comprised three randomised controlled trials, seven non-randomised controlled prospective studies, 11 uncontrolled prospective studies, and one controlled retrospective study of interventions applied to urban populations or areas in which outcomes were assessed in a sample of local people. I found some evidence that targeted behaviour change programmes could change the behaviour of motivated subgroups, resulting (in the largest study) in a modal shift of around 5% of all trips at a population level. Single studies of commuter subsidies and a new railway station also showed positive effects. The balance of best available evidence about other types of intervention such as publicity campaigns, traffic calming and cycling infrastructure suggested that they had not been effective. Participants in trials of active commuting experienced short term improvements in certain measures of health and fitness, but I found no good evidence about health effects associated with any effective intervention at population level.
Most relevant studies were not found in mainstream health or social science literature databases. Further analysis of the 47 excluded studies did not change the overall conclusions about effectiveness, but did identify additional categories of intervention that merit further research and provided evidence to challenge assumptions about the actual effects of progressive urban transport policies. The contributions of internet publications, serendipitous discoveries and the initially-excluded studies to the total set of relevant evidence suggested that undertaking a comprehensive search may have provided unique evidence and insights that would not have been obtained using a more focused search.
I identified an evaluative bias whereby the effects of population-level interventions were less likely than those of individual-level interventions to have been studied using the most rigorous study designs. Understanding of how environmental and policy factors may influence active travel and physical activity currently relies heavily on evidence from cross-sectional studies of correlates rather than intervention studies. I therefore took advantage of the opportunity presented by a local ‘natural experiment’ — the construction of a new urban section of the M74 motorway in Glasgow — to design, develop and complete the cross-sectional (baseline) phase of a new primary study of the effects of a major environmental intervention.
Using a combination of census data, geographical data and field visits, I delineated an intervention study area close to the proposed route of the new motorway and two matched control areas elsewhere in Glasgow. I collected and described data from residents in the three study areas (n=1322) on socioeconomic status, the local environment, travel behaviour, physical activity and general health and wellbeing using a postal questionnaire incorporating two established instruments (the SF-8 and the short-form International Physical Activity Questionnaire), a travel diary and a new 14-item neighbourhood rating scale whose test–retest reliability I established in a subset of respondents (n=125). I then analysed the correlates of active travel and physical activity using logistic regression. Using travel diary data from Scottish Household Survey respondents (n=39067), I also compared the characteristics and travel behaviour of residents living close to the proposed route with those living in the rest of Scotland and analysed the correlates of active travel using logistic regression.
Overall data quality and the test–retest reliability of the new neighbourhood scale appeared acceptable. Local residents reported less car travel than expected from national data. In the local study area, active travel was associated with being younger, being an owner-occupier, not having to travel a long distance to work and not having access to a car, whereas overall physical activity was associated with living in social-rented accommodation and not being overweight. After adjusting for individual and household characteristics, neither perceptions of the local environment nor the objective proximity of respondents’ homes to motorway or major road infrastructure appeared to explain much of the variance in active travel or overall physical activity, although I did find a significant positive association between active travel and perceived proximity to shops. Apart from access to local amenities, therefore, environmental characteristics may be of limited relevance as explanatory factors for active travel in this comparatively deprived urban population which has a low level of car ownership and may therefore have less capacity for making discretionary travel choices than the populations studied in most published research on the environmental correlates of physical activity.
The design and baseline data for the M74 study now provide the basis for a controlled longitudinal study, which could not otherwise have been carried out, of changes in perceptions of the local environment, active travel, physical activity, and general health and wellbeing associated with a major intervention in the built environment. This will, in time, contribute to addressing calls to produce better evidence about the health impacts of natural experiments in public policy.
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