Ogilvie et al. BMC Public Health 2010, 10:703 http://www.biomedcentral.com/1471-2458/10/703 England [8]. Greater public health benefits may there- fore accrue from a population strategy to shift the distri- bution of physical activity in the population as a whole than from further efforts to target high-risk groups [9]. This is likely to require a combination of interventions at multiple levels [10] to address different domains of physical activity (domestic, occupational, recreational and transport) and the behaviour- and context-specific determinants of different forms of activity such as walk- ing or cycling [11,12]. One reason for the limited success of previous inter- ventions may be that people’s capacity to respond to public health advice to take more exercise is limited by their surroundings and the opportunities open to them. Programme guidance on physical activity and the envir- onment issued by the National Institute for Health and Clinical Excellence (NICE) has drawn attention to the lack of studies examining whether changing the physical environment leads to changes in physical activity. The guidance specifically identified a need for more, and more rigorous, studies of this kind involving longitudinal designs, comparisons with control groups, and robust measures of physical activity [13]. These features are important to allow the effectiveness, cost-effectiveness and distributional impacts of this type of intervention to be meaningfully compared with those of other approaches to promoting physical activity, for example through consultations in primary care. The lack of credible evidence from intervention studies reflects the challenges of researching the effects of this type of intervention, and public health researchers are increas- ingly attempting to address this lack of evidence by studying the effects of ‘natural experiments’ [14-16]. These include situations in which the environment is modified, as in the M74 study of urban motorway con- struction in Glasgow [17,18] or the iConnect study of walking and cycling infrastructure projects around the UK [19], and others in which people move to a comple- tely new environment, as in the RESIDE study in Wes- tern Australia [20]. Modifying transport infrastructure to support active travel (walking and cycling), for example by constructing cycle routes or redesigning roads to discourage car use, is one way of modifying the physical environment that was identified in the recent Foresight report as one of the top five recommendations for tackling obesity in the UK [21]. The considerable potential for people to incor- porate walking or cycling into their daily routines makes this an attractive strategy for increasing population levels of physical activity. However, several reviews have highlighted the limited quantity and quality of existing studies of the effects of this type of intervention, the very limited evidence that such interventions have been effective in promoting physical activity, and the missed Page 2 of 13 opportunities for rigorous health-oriented studies of the effects of recent major innovations such as the impact of congestion charging in London on physical activity [13,22-26]. It cannot be assumed that people who take up more active travel will become more physically active overall, because the increase in energy expenditure while travelling may be counterbalanced - or even outweighed - by a compensatory decrease in leisure- time physical activity [14]. There is a particular lack of evidence on the relationships between public transport and active travel and on the effects of interventions to improve public transport infrastructure, which may be especially important in a country such as the UK where many people live too far from their workplace to walk or cycle the entire journey. Several studies have shown that using public transport can involve a substantial daily quantity of walking and that commuters who use public transport tend to walk more than those who tra- vel by car [27-30]. One study has also reported a corre- spondence between an increase in cycling and a positive shift in the distribution of overall physical activity in the targeted local populations following a multifaceted inter- vention including some infrastructural changes [31]. To the best of our knowledge, however, no study has yet convincingly demonstrated that investing in new trans- port infrastructure has led to an increase in physical activity in the local population directly attributable to the intervention [13,26]. We have therefore taken the opportunity presented by a ‘natural experiment’ in Cambridgeshire, UK to estab- lish a quasi-experimental study of the effects of a major transport infrastructural intervention on travel beha- viour, physical activity and related wider health impacts. This study is designed to contribute new evidence relat- ing to several unresolved research and policy questions in this area, including (a) the effectiveness of interven- tions in promoting a shift from car use towards more sustainable modes of transport; (b) whether walking and cycling can be promoted as part of longer public trans- port journeys by improving public transport provision; (c) the contribution of active travel to overall physical activity; and (d) the wider health impacts of changes in travel behaviour. Aims and objectives The aim of the study is to address the following primary research question: 1. Is investment in new high-quality transport infra- structure associated with an increase in the use of active modes of travel (walking and cycling)? and the following secondary research questions: 2. What are the wider health impacts of changes in travel behaviour in terms of overall physical activity, wellbeing, sickness absence and carbon emissions?