Ogilvie et al. BMC Public Health 2010, 10:703 http://www.biomedcentral.com/1471-2458/10/703 3. What are the determinants of the use and uptake of active modes of travel? 4. How are any changes in travel behaviour distributed in the population? 5. How are travel behaviour and changes in travel behaviour embedded in and shaped by the wider social context? 6. Are changes in travel behaviour sustained over time? in a quasi-experimental cohort study of adults who travel to work in Cambridge, combined with nested in- depth quantitative and qualitative studies. Design and methods Overall research design The Commuting and Health in Cambridge study com- prises three main elements: 1. A cohort study of travel behaviour, physical activity and wider health impacts in adults who travel to work in Cambridge, using repeated postal questionnaires and basic objective measurement of physical activity using accelerometers 2. In-depth quantitative studies of subgroups of parti- cipants to investigate physical activity energy expendi- ture, travel and movement patterns and estimated carbon emissions using household travel diaries, com- bined heart rate and movement sensors and global posi- tioning system (GPS) receivers 3. A longitudinal qualitative interview study to eluci- date participants’ attitudes, experiences and practices and to understand how environmental and social factors interact to influence travel behaviour, for whom and in what circumstances. The impacts of a specific intervention - the opening of the Cambridgeshire Guided Busway (defined below under ‘Intervention’) - and of other changes in the phy- sical environment will be examined using a controlled quasi-experimental design within the overall cohort dataset. Setting The city of Cambridge lies approximately 80 km north- east of London and has a population of approximately 108,000 [32]. The Cambridge sub-region includes the smaller cathedral city of Ely and market towns such as Huntingdon, Newmarket, Royston and St Neots as well as a large number of small settlements (Figure 1). Unlike most cities in the UK, Cambridge has a distinct cycling culture related to a combination of factors including its flat topography, its large student population and the traf- fic congestion in its historic city centre. This is reflected in the 2008 Cambridgeshire Travel for Work survey, which found that 21.1% of those surveyed cycled to work [33] - substantially higher than the UK average Page 3 of 13 of 3.3% [32]. Conversely, the proportion of employees who travel to work by car is lower in Cambridge (54.9%) than in the UK as a whole (61.5%) [32]. Cam- bridge has good road transport links: primary roads connect it to surrounding towns, the M11 motorway (freeway) links it with London and the south, and the A14 trunk road provides a major east-west route. The city is also well served by frequent train services to London and some other more distant cities, as well as to certain towns and villages in its immediate hinter- land. On the other hand, the cost of housing in the city of Cambridge is high by UK standards and the Cambridge sub-region is a predominantly rural area with limited rural public transport and a relatively high level of rural car ownership [32,34]. Participants Inclusion criteria The study population comprises adults aged 16 and over who work in areas of Cambridge served by the Cam- bridgeshire Guided Busway (CGB) and live within a radius of approximately 30 kilometres of the city centre (Figure 1) but not within the same immediate area of the city as their workplace. Within this definition, parti- cipants are eligible for inclusion irrespective of their employer, workplace, type or grade of occupation, length of employment contract or working hours; whether they also work at other locations; and whether they have any disability that may limit their mobility. Participants are ineligible if they are currently taking part in another research study that involves measuring their physical activity, or if they live in on-site staff accommodation associated with their workplace and therefore do not routinely commute to the site. Recruitment Since the study is focused on travel to work, participants are recruited through workplaces rather than from a general population sampling frame such as an electoral or primary care register. However, in the interests of data protection and to assure participants of the inde- pendence of the study from their employers, participants are not recruited through corporate staff databases. Instead, they are approached using a combination of recruitment stands, newspaper and magazine advertise- ments, posters, fliers, and announcements distributed on the investigators’ behalf by employers through corporate email distribution lists, intranets and staff newsletters. Participants who opt in to the study by responding to any of these recruitment methods are entered into a prize draw to win one of eight £50 gift vouchers. Recruitment commenced in March 2009. The majority of participants were recruited in 2009 and took part in baseline data collection between May and October 2009, with limited additional recruitment to the cohort and