There is compelling evidence that regular physical activity is effective in the prevention of chronic diseases (including cardiovascular disease, type 2 diabetes, some cancers, hypertension, obesity, depression and osteoporosis) and premature death, with the greatest improvements in health status seen when people who are least active become physically active [1, 2]. In the United Kingdom (UK) it is currently recommended that adults should aim to undertake at least 150 minutes of moderate intensity physical activity in bouts of 10 minutes or more throughout the week [3, 4] but many adults in the United Kingdom and other high-income countries do not achieve this [1, 4–6]. Increasing physical activity levels, particularly among the most inactive, is an important aim of current public health policy in the UK [1, 7–9].
The benefits of active travel
One approach to increasing physical activity levels is to promote active travel i.e. walking and cycling. There is increasing evidence of the link between adult obesity levels and travel behaviour, one indicator of which is that countries with highest levels of active travel generally have the lowest obesity rates . Experts in many World Health Organisation (WHO) countries agree that significant public health benefits can be realised through greater use of active transport modes . For example, a systematic review of trials and cohort studies found modest but consistent support for the positive health effects of active travel, including a suggested positive effect on diabetes . Other studies have shown a protective association between active travel and cardiovascular risk [13, 14] and perceived health status . Furthermore, cost benefit analysis for the UK Department for Transport suggests the ratio of benefits to costs were high . The suggested benefits to employers of promoting active travel schemes include: increased productivity, a reduction in sick leave, improved public image as a result of lowering the workplace’s carbon footprint, and savings in providing car parking facilities [14, 17–20].
Walking as active travel
Walking has been described as near perfect exercise . It is a popular, familiar, convenient and free form of exercise that can be incorporated into everyday life and sustained into older age. It is also a carbon neutral mode of transport that has declined in recent decades in parallel with the growth in car use . Even walking at a moderate pace of three miles/hour (five km/hour) expends sufficient energy to meet the definition of moderate intensity physical activity . Hence there are compelling reasons to encourage people to walk more, not only to improve their own health but also to address the problems of climate change [23–26].
In the UK, there are substantial opportunities to increase walking by replacing short journeys undertaken by car. For example, the 2011 National Travel Survey showed 22% of all car trips were shorter than two miles (3.2 km) in length, while 18% of trips of less than one mile were made by car . An opportunity for working adults, especially those who live relatively close to their workplace, to accumulate the recommended moderate activity levels may be through the daily commute.
Although cycling is also an important mode of active transportation, walking may be perceived as a cheaper and safer option for those who are currently inactive: it requires no special equipment and is less likely to involve direct competition with motorised traffic for road space. In addition, for longer journeys, walking can more easily be combined with other transport modes such as buses and trains. In their study promoting active travel to work, Mutrie et al. (2002)  found the intervention group almost twice as likely to report an increase in walking during their journey to work as the control group at six months (odds ratio of 1.93, 95% confidence intervals 1.06 to 3.52) but there was no increase in cycling.
Measuring active travel
Active travel has been associated with increased physical activity in studies using self report . However, a systematic review comparing direct versus self-report measures for assessing physical activity in adults found self-report measures were both higher and lower than directly measured levels . This questions the validity and reliability of self-report measures, and also undermines efforts to correct for self-report differences. However, very few studies have objectively measured the contribution of walking, particularly walking to work, to adult physical activity levels . In the US, a cross-sectional study included 2,364 participants enrolled in the Coronary Artery Risk Development in Young Adults (CARDIA) study who worked outside the home during year 20 of the study (2005–2006) . Associations were examined between walking or cycling to work and objective MVPA using accelerometers and active commuting was found to be positively associated with fitness in men and women, and inversely associated with BMI, obesity, triglyceride levels, blood pressure and insulin level in men. The authors concluded that active commuting should be investigated as a means of maintaining or improving health.
Objective measures of physical activity are more common in studies examining children’s commute to school. Studies investigating differences in physical activity between children who walk to school and those who travel by car have shown that children who walk to school have substantially higher physical activity than car travellers . More recently, longitudinal studies have shown that a change of travel mode from passive (car/bus) to active (walking) is associated with an increase in overall daily physical activity, whilst physical activity declines if children adopt car travel instead of walking to school [34, 35]. Spatial segmentation studies have confirmed the importance of walking to school to children’s overall physical activity, showing that approximately a third of daily MVPA is acquired in the school journey .
The walk to work study
In the UK, public health guidance on workplace health promotion from the National Institute for Health and Clinical Excellence (NICE) has asserted that, although a range of schemes exist to encourage employees to walk or cycle to work, little is known about their impact and the measures of physical activity used are often based on self-report . In this context, the Walk to Work feasibility study  was developed in the south-west of England using objective measurements of physical activity. The main study is examining the feasibility of implementing and evaluating an intervention through which Walk to Work promoters are recruited and trained to encourage fellow employees, who do not currently walk or cycle to work, to increase the amount of walking they undertake during the daily commute. This paper focuses on baseline data to examine the association between travel mode to work and objectively measured physical activity in adults.