Ogilvie et al. BMC Public Health 2010, 10:703 http://www.biomedcentral.com/1471-2458/10/703 viewed as a form of sensitivity analysis of the effects of the provisional decisions made in defining the ‘intervention’ and ‘control’ areas. They may reveal unexpected findings about the distances participants are prepared to walk, cycle or drive to gain access to the guided busway, thereby contributing to more accurate modelling of the transport impacts of future infrastructure projects of this kind. Variables and measurement instruments Core questionnaire survey Core data are collected from each participant using a questionnaire that incorporates the Recent Physical Activity Questionnaire (RPAQ). RPAQ is designed to ascertain domestic, occupational, recreational and transport-related physical activity, from which physical activity energy expenditure (PAEE) and total energy expenditure (TEE) can be estimated using an estab- lished algorithm. RPAQ is closely based on the pre- viously-validated EPAQ2 questionnaire [39] developed for the EPIC-Norfolk cohort study and subsequently also used in intervention studies, but takes the past four weeks rather than the past year as its reference period. A validation study of RPAQ using healthy Cambridge volunteers aged 21-57 has shown the esti- mated PAEE to have good test-retest reliability (ICC = 0.76) and unusually strong criterion validity (r = 0.39) against PAEE objectively assessed using the doubly labelled water technique [40]. The questionnaire also includes: A seven-day retrospective travel record focusing on the journey to and from work, based on an instrument used (and shown to have acceptable test-retest reliabil- ity) in a previous study of active commuting [41] A one-day record of all journeys made on the pre- vious day, adapted and simplified from the travel diary used in the UK National Travel Survey [42] and pre- viously used in the M74 study in Glasgow [17] Items on perceived characteristics of the environ- ment, previously used (and shown to have acceptable test-retest reliability) in the M74 study in Glasgow [17,43] but applied in this study to participants’ routes to and from work [44,45] rather than to their residential environments Items on the mediators of changing travel mode choice predicted by the Theory of Planned Behaviour, adapted from those used in a previous intervention study and applied to car use [46] Items from the Self-Report Index of Habit Strength applied to car use [47] An item on self-reported sickness absence in the past year, previously shown to be strongly correlated with sickness absence objectively verified from employ- ment records in the Whitehall study [48] Page 7 of 13 The SF-8 scale for assessing general physical and mental health as a measure of wellbeing [49] Miscellaneous items to capture key demographic, socioeconomic and other health-related characteristics including age, gender, level of educational attainment, access to cars and bicycles, possession of a driving licence, car fuel type and engine size, presence of long- term limiting illness or disability, difficulty walking and self-reported height and weight. The questionnaire survey is repeated annually throughout the duration of the study at the same time of year to minimise the influence of seasonal variations in travel behaviour. From 2010 onwards, the question- naire includes items on awareness and use of the guided busway (including walking and cycling along the route, which is possible even before the busway is open to bus traffic) and reasons for using or not using the guided busway. At follow-up, participants are also asked to report any recent changes in household circumstances likely to influence travel behaviour (such as pregnancy, childbirth, children starting or moving school, or adults taking on new responsibilities at work or as carers). Basic activity monitoring A random sample of participants who indicate their will- ingness in principle to undertake basic activity monitor- ing are selected to receive an Actigraph activity monitor as well as a questionnaire in their initial survey pack. Because of the limited pool of monitors available, partici- pants in the ‘intervention’ and ‘control’ groups are priori- tised for receipt of monitors over those in the ‘reserve’ group. The Actigraph is a small, lightweight acceler- ometer that provides detailed information about the intensity, frequency and duration of physical activity, has been extensively validated in both laboratory and free-liv- ing conditions [50], and has been successfully used with high response and completion rates in previous popula- tion-based observational studies [51]. Monitors are drawn from a pool of Actigraph GT1M and GT3X mod- els, both set to record activity counts in five-second epochs in the biaxial mode. Participants are asked to wear their monitor on an elastic waistband on the right hip during waking hours for seven days and to record on a log sheet the times at which the monitor was removed and reattached and the reasons for removal. Those who complete activity monitoring are offered simple indivi- dualised feedback on their activity data in the form of a bar chart comparing their recorded daily minutes of moderate-to-vigorous physical activity (MVPA) to the average for their age/sex group and to current public health recommendations. They are then asked to repeat the activity monitoring with their annual follow-up ques- tionnaire surveys in order to provide longitudinal within- subject objective physical activity data.