Theoretically, then, the free fare scheme for young people may have had a range of effects on the public health of Londoners. Evidence to date suggests that effects on‘processes’ such as travel mode choice and travel mode distribution are likely to affect health behaviours and outcomes such as active travel and injury rates. Reducing fares (in this case to‘no cost’) is likely to increase use of bus travel in the target
population74displacing other modes of transport and/or creating additional journeys.
To summarise, the risks and benefits to health from the provision of free public transport are likely to accrue from the increased availability of transport and changes in the share of modes of transport used (e.g. switching from walking to bus travel). In the USA, where public transport is more likely to replace car use than walking and cycling, there is some evidence that increasing access to public transport can increase activity levels. Increased walking to public transport is enough to have had a public health impact on obesity, particularly for men.55–57In England, free bus travel has been identified as providing a benefit
private car use than other parts of the country,77improving access to affordable public transport may
reduce the amounts of active travel undertaken by replacing walking or cycling rather than car use. As the risk of RTI varies by transport mode,44any modal shift is likely to have implications for injury rates. Young
people are at particular risk of assault,78and greater access to public transport potentially increases this
risk. More tangential benefits which may be associated with young people’s increased access to public transport include increased social inclusion, and decreased future reliance on private car travel. They are also likely to have effects on broader determinants of health, such as social inclusion and independent travel. These broader health implications of well-being that arise from social inclusion or the ability to make independent choices about travel are important, but there is in general less evidence on the wider social and health effects of different transportation choices79and real challenges in operationalising
concepts such as‘inclusion’ for research.80Clearly, an evaluation of the public health effects of a transport
intervention needs to account for potential effects on social inclusion and well-being, but these are difficult to measure. Finally, in addition to benefits or costs for the target group (12- to 17-year-olds), there may be effects on other transport users if they are displaced from buses.
The free bus pass scheme is a‘natural experiment’ which allows us to evaluate these potential pathways linking a large-scale transport intervention to health determinants, behaviours and outcomes. Given the range of health benefits and disbenefits that theoretically arise from the intervention; the complexity of the system in which this intervention was introduced and the difficulties in operationalising distal health outcomes which are important but (to date) under-researched, any evaluation will require a mix of methodological strategies. This study therefore aimed to evaluate the impact of free bus travel on the public health by identifying the best available strategies to explore these pathways, using a mixed-method design. Our specific aims were to:
1. provide empirical evidence for the impact of this intervention on key health behaviours and outcomes (e.g. injuries, active travel) for young people
2. explore the effects on the determinants of health (e.g. access to education and training) 3. identify the effects of increased young people’s access to bus travel on older citizens 4. develop and apply methods for economic assessment, and
5. contribute to the development of methods to strengthen causal inference in non-randomised designs. The approach we took to meeting these aims is outlined in the next chapter.
Chapter 2 Methodology: evaluating
‘natural
experiments’ using mixed methods
Introduction
Drawing on existing research, Chapter 1 outlined the range of health impacts free bus travel may have had on the public health, and summarised the aims of an evaluation of these impacts. Like many other policy interventions with potential impacts on the determinants of health, it is impossible to generate‘best evidence’ such as that from a randomised controlled trial (RCT) on the effects of the intervention. The scheme has already happened, there are no obvious control groups, and no opportunities for the research team to control exposure to the intervention. The intervention could, however, be considered a‘natural experiment’, in that although not under the control of the research team, it is amenable to research using natural variations in exposure,81such as between the target group (young people) and others in the
population. Given the urgent need to improve the public health evidence base in general,45and for
evidence on transport interventions in particular, there have been calls to exploit‘natural experiments’ to contribute‘good enough evidence’ to inform policy decisions.9,82The free bus scheme is one such
opportunity. This intervention has a number of advantages which make it suitable as a potential natural experiment:81(1) a RCT is not possible; (2) we already have (from research evidence) some reasonable
expectation that health impacts will accrue from changes in transport behaviour; (3) there are available secondary data sources on some of these impacts that cover both pre- and post-intervention periods; and (4) there are potential‘exposed’ and ‘non-exposed’ individuals. Finally, there is a policy incentive, given that there is little evidence to inform current debate, and London’s transport interventions are of wider interest in the context of policy drivers to increase the sustainability of transport systems.
This chapter describes the design and approach we took to evaluating the free bus travel scheme as a natural experiment. We discuss the overall design and aims of the study, the methodological challenges in evaluating‘natural experiments’ of this type and how we addressed them, and the specific methods used for the quantitative and qualitative components of the study.