Ogilvie et al. BMC Public Health 2010, 10:703 http://www.biomedcentral.com/1471-2458/10/703 possible public health impacts [66], notably on overall physical activity and carbon emissions. These outcomes are closely related to obesity and climate change - two of the most important contemporary public health chal- lenges which are likely to benefit from interdisciplinary research and intersectoral approaches to policymaking [67]. However, the development of common methodolo- gical ground in both measurement and evaluation in this field is still at a relatively early stage [19]. There is also considerable scope for developing new approaches to integrating quantitative and qualitative data and ana- lyses in understanding the contexts, mechanisms and effects of interventions of this kind. The development of such ‘mixed method’ approaches to research will be the subject of a further paper. A second challenge is that of implementing robust quantitative measurement of travel and physical activity behaviour in the context of an unpredictable and fre- quently changing intervention timetable which is com- pletely outside the researchers’ control [68]. Few previous studies of new transport infrastructure have included any measure of physical activity, let alone the combination of self-reported and objective measures used in this study [13,22,26]. The challenges of rapidly recruiting a sample and collecting baseline objective physical activity data before the scheduled opening of the new transport infra- structure will also be the subject of a further paper. A third challenge is that of analysing a cohort dataset with repeated and varying measures of both exposures and outcomes. The busway is being constructed in a shifting political, fiscal and operational climate, and it forms only one element of a continually evolving local transport infrastructure. The study has therefore been designed as a cohort study within which it will be possi- ble to examine the influence of environmental changes (including, but not limited to, those embodied in the busway itself) on patterns of behaviour using quasi- experimental methods within the cohort dataset. This flexible approach to study design may be essential in complex and unpredictable intervention situations of this kind, but it poses undoubted challenges for analysis: not only do different participants take part in different combinations of survey waves and optional additional measurements, but they are also exposed to different ‘doses’ and types of environmental change in different time periods. Although methods exist for analysing observational datasets of this kind, they have less often been applied to the evaluation of interventions. This study will therefore contribute to the development and demonstration of a growing body of methods for under- standing the influence of the physical environment, and changes in the environment, on travel behaviour, physi- cal activity and wider health impacts. Page 11 of 13 Acknowledgements This study was developed and initially funded under the auspices of the Centre for Diet and Activity Research (CEDAR), a UKCRC Public Health Research Centre of Excellence. Funding from the British Heart Foundation, Economic and Social Research Council, Medical Research Council, National Institute for Health Research and the Wellcome Trust, under the auspices of the UK Clinical Research Collaboration, is gratefully acknowledged http://www.esrc.ac.uk/publichealthresearchcentres. The study is now funded by the National Institute for Health Research Public Health Research programme (project number 09/3001/06: see http://www.phr.nihr.ac.uk/ funded_projects). The views and opinions expressed herein are those of the authors and do not necessarily reflect those of the NIHR PHR programme or the Department of Health. The funding bodies had no part in the study design; in the collection, analysis or interpretation of data; in the writing of the manuscript; or in the decision to submit the manuscript for publication. The study was approved by the Hertfordshire Research Ethics Committee (reference numbers 08/H0311/208 and 09/H0311/116). We thank Cambridgeshire County Council for the information and encouragement needed to establish the study, and our colleagues in the study coordination, physical activity epidemiology and physical activity technical groups at the MRC Epidemiology Unit for their advice on study design and assistance with the original application for ethical approval. Author details 1Medical Research Council Epidemiology Unit and UKCRC Centre for Diet and Activity Research (CEDAR), Institute of Public Health, Cambridge, UK. 2School of Environmental Sciences, University of East Anglia and UKCRC Centre for Diet and Activity Research (CEDAR), Norwich, UK. 3Centre for Transport Studies, University College London, London, UK. 4General Practice and Primary Care Research Unit, Institute of Public Health, Cambridge, UK. Authors’ contributions DO (principal investigator) had the original idea for the study and led the design of the study and the writing of the original protocol, the applications for grant funding and ethical approval, and this manuscript. SG, AJ and RM (co-investigators) contributed to the design and the writing of the original protocol and grant application. CG devised the photo-elicitation element of the study. JP and LY contributed to the design of parts of the study through their initial work on the collection or analysis of baseline data and JP prepared Figure 1. NJ devised the spatial sampling strategy for the study and conducted an initial set of qualitative interviews that informed the design of the main series of interviews. SC contributed to the design of the qualitative elements of the study. CC (study coordinator) contributed to the design, protocol and applications for ethical approval for parts of the study. All authors contributed to the critical revision of the paper and approved the final version. Competing interests The authors declare that they have no competing interests. Received: 29 October 2010 Accepted: 16 November 2010 Published: 16 November 2010 References 1. 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