Walking and cycling to school provide a convenient opportunity to incorporate physical activity into the daily routine of children and adolescents. Children [10-13] and adolescents [14] who actively commute to school attain more minutes of daily physical activity than those who use motorized transport. Only 30% of Irish adolescents have reported that they actively commute to school [15].

Attempts to increase active travel and improve the walking environment for young people have resulted in a surge of resources and campaigns to develop safe walking and cycling routes to school [16,17]. School proximity to residential homes has been identified as an important determinant of active commuting among children [18]. More children walk or cycle to school as distance decreases [19-21]. Similar studies among adolescents are scarce [22].

Despite the fact that parents consistently cite distance as the number one barrier to their children actively commuting to school, [11,23,24] only 31% of US children, who live within 1 mile of their school choose to walk, and only 2% who live within 2 miles choose to cycle [23]. Among Irish adolescents, 22% of car users live within 1 mile, and 39% live within 2 miles of their school [9]. Where distance is not a barrier to active commuting, other factors such as convenient access to foot or cycle paths may inhibit walking or cycling.

The distance (miles) of the actual route travelled to school, and the usual mode of travel were assessed using a self-report questionnaire that was completed under supervision. Similar questions have previously been used in this age group [15]. Mode of travel responses were categorised as active commuting by foot or bicycle, or inactive commuting by car, bus or train. Adolescents who used mixed mode trips (for example walk/cycle to bus/train) responded based on the longest portion of their journey only. All adolescents who travelled by bus or train are assumed to undertake some walking or cycling to get to public transport but the amount of physical activity undertaken is unknown and this is a limitation of this study. Bus travel could have been public or private school buses. It is recognised that the mode of travel and determinants of trips to and from school may differ. To delimit this study, the return journey was not reported and this manuscript focuses on the journey to school.

A subset of participants (N = 272, mean age 15.93 ± 0.63 years, 51.6% male, 62.5% active commuters) self-reported distance travelled to school and drew their actual route on a detailed street level map (scale of 1:2500). The actual distance was measured using a map wheel (Scalex Corporation, California, U.S.A). Perceived and actual measurements were correlated (0.22, p < 0.001) and there was no significant difference between perceived and actual distance travelled (1.26 vs.1.23 miles, p = 0.774), indicating that perceived distance is a valid measurement tool. This was true for males (1.19 vs. 1.25 miles, p = 0.631) and females (1.32 vs. 1.22 miles, p = 0.356).

Barriers to active travel were assessed through an open response question. Individuals who travelled by car, bus or train were asked "Why do you choose not to walk or cycle?" Parental occupation was obtained to determine socio-economic status [25]. Participants were asked if they had a disability that restricted their participation in physical activity. Area of residence was classified as i) large city (>500,000 inhabitants), ii) suburbs or outskirts of a city (<500,000 but > 50,000), iii) town (<50,000) or iv) village (<5,000) [15].

Being female reduces the odds of active commuting by 36%. McMillan and colleagues (2006) reported a slightly higher value of 41.5% in 8–11 yr old girls indicating a reduced gender effect on mode choice among older youth [37]. Factors other than distance explain gender differences in mode; males and females travelled similar distances by foot, car and bus. Observed difference in distance travelled by bicycle and train are tentative due to small numbers of females using these modes. Many other factors might explain gender differences in mode, for example, perceptions of personal safety from real or perceived crime are predictors of recreational physical activity among adolescents, [38] especially females [39] and further research is required to identify if these factors also influence utilitarian activities such as active commuting to school. Though fewer females cycled to school, the distance they covered was further than males. This may reflect a high level of motivation among this minority. Research into the reasons for such low levels of cycling among female cyclists is required.

The further an adolescent lives from school, the less likely they are to walk or cycle. This extends previous findings in children [18-20] and signifies the importance of locating schools in or near residential communities. With the advancing sprawl around major cities in Ireland, and increasing evidence of the completion of new developments without the provision of schools and local amenities, such evidence is timely and should be considered in policy guidelines for urban planning and development. Among Irish adolescents the criterion distance for walking and cycling to school was ≤ 1.5 miles (2.4 km) and ≤ 2.5 miles (4.0 km) respectively. This indicates that 2.5 miles could be used as a general cut-off within which both walking and cycling to school are achievable. This criterion is greater than previously suggested adult guidelines [40] but lower than the 3.0-mile criterion required for government-subsidised transport to school for post-primary pupils in Ireland [41] and the U.K [42]. In Denmark, where rates of active commuting are 75%, 14–15 y old secondary school students must live a distance of ≥ 5 miles from school to avail of free transport [42].

The Healthy People 2010 initiative in the US seeks to increase the proportion of trips made by walking to school to 50% and by cycling to 5%, for children and adolescents living within one mile of their school [43]. This study provides evidence for the use of distance-related goals for promotion of active commuting, and reveals the need for population specific targets. Irish adolescents are already meeting U.S targets for 2010: approximately three quarters of Irish teenagers who live within one mile walk to school, and 8% within 2 miles cycle. The potential for modal shift in Ireland lies among the adolescents who live between 1.0 and 2.5 miles, and specifically in increasing the proportion who cycle to school. The 39% of car users, and 11% of bus users who live within 2.5 miles of their school are legitimate targets for change to active modes of travel. Among adolescents who reported distance as a barrier to active commuting, over 92% lived ≥ 2.5 miles from school and only 7% perceived 2.5 miles as too far to walk or cycle to school, indicating the acceptability of this criterion distance. Further research is required into the determinants of travel behaviours among adolescents who travel short distances by motorised means, and adolescents who perceive short distances as too far.

Not surprisingly we found that as population density decreases, the travel distance to school increases, resulting in fewer adolescents actively commuting. Since fewer adolescents in areas of low density live within the proposed 2.5-mile criterion, this reduces the likelihood of active commuting making a contribution to daily minutes of physical activity, except among the highly motivated. Health promotion initiatives for low-density areas should focus on alternative strategies for increasing physical activity. In areas where transit supply is adequate, previously suggested promotion efforts could be applied to target these individuals including mixed mode travel [44] and "walk a stop" campaigns [45]. Mixed mode trips and using different modes for journeys to and from school were not examined in this study, but should be examined in future research as those who travel by bus or train may undertake more physical activity than those who walk or cycle the full distance to school.

Self reported barriers to active commuting were explored in this study. Similar to research among children, [11,23,24] distance was established as the most important perceived barrier among adolescents. In addition, new previously unconsidered reasons emerged. Lack of time, intrinsic factors such as laziness and tiredness, and convenience were more important than weather, traffic related danger or heavy bags. Two potential reasons for the difference from previous research are considered. Firstly, this study measured only the journey to school, for which issues like time and convenience may be considerably more important than they are for the return journey. Similarly, issues surrounding tiredness and laziness may be related to the adolescent's motivation to go to school, and these are unlikely to affect the return trip. Secondly, previous research in this area was conducted with younger children, and was based on parental report of barriers. It is hardly surprising then that traffic related danger or heavy bags were more commonly cited. This research suggests that the determinants of active travel differ from childhood to adolescence and highlights the need for adolescent-specific research. Adolescents who cited distance as a barrier lived further from school than those who gave other reasons for inactive commuting. Objective measurements of distance travelled are required to identify if distance is a real or a perceived barrier to active travel.