Physical and psychological health benefits of physical activity (PA) for children and adolescents are well-documented (Janssen and LeBlanc, 2010; Poitras et al., 2016). Nevertheless, a majority of children and adolescents are not sufficiently active (Guthold et al., 2020), and PA levels decline from childhood to adolescence (Farooq et al., 2020). In Germany, only 4% of children and adolescents in Germany meet the daily PA guidelines of the World Health Organization based on device-based measures (Burchartz et al., 2021).

Together with sports activities, physical education, and housework, active transport (AT), such as walking or cycling, can contribute to sufficient PA levels (Denstel et al., 2015; Martin et al., 2016). AT may further increase cardiovascular fitness (Larouche et al., 2014b; Henriques-Neto et al., 2020), and improve psychosomatic symptoms (Kleszczewska et al., 2020). Approximately 48% of children and adolescents in Germany use active school transport (AST), with declining rates during adolescence, especially among girls (Reimers et al., 2021). Promoting AST could therefore be a valuable component to increase health-promoting PA for youth in Germany. Transitions from primary to secondary school and from secondary school to early adulthood are important developmental phases as multiple major changes occur simultaneously within a very short time. Children and adolescents experience multiple transitions simultaneously across different domains (e. g., social) and contexts (e. g., home, school) as shown in the Multiple and Multi-dimensional Transitions Theory (Jindal-Snape, 2016; Jindal-Snape et al., 2020). Those transitions are influenced by interactions with peers, parents, and teachers, leading to a series of dynamic, continuously evolving changes during this period which influence each child differently. For example, during the primary-secondary school transition, not only the educational and social structures change substantially, but also the organizational structure and the physical school environment as well as the distance from home to school which affect not only educational outcomes and well-being (Jindal-Snape et al., 2020) but also influence PA behavior in and after school (Meester et al., 2014).

In the context of school transport, AST is a relatively stable behavior during childhood (Carver et al., 2011) and adolescence (Martinez-Gomez et al., 2014; Falconer et al., 2015; Lau et al., 2017). Maintaining active (i. e., walking or cycling) or passive transport modes (i. e., public and car transport) is more likely than switching from active to passive or passive to active transport modes during these stages of life. However, AST behavior is not stable during the transition between these stages, especially from childhood to adolescence (Carver et al., 2011). During this period, the switch from primary to secondary school is a key event, often leading to a change in school location (Vanwolleghem et al., 2016) and peer groups (Carver et al., 2011), which may result in a discontinuation of AST behavior. Nevertheless, starting AST at an early age can predict AST later in adolescence. For example, Cardon et al. (2012) found that 72% of children living in Flanders, Belgium, who had cycled to school at age ten still did so at age 16, showing strong stability over six years despite a school transition. In addition, early cyclists in this study are four times more likely to be cycling in adolescence at 16 years of age. In a study by Falconer et al. (2015) conducted in the UK, 12-year-old children using AST were more likely to use active transport modes at the ages of 14 and 16. Changes in AT during the transition from adolescence to early adulthood have mainly been examined in university students, finding that AT rates decrease after high school (Molina-García et al., 2015, van Dyck et al., 2015).

The decision whether to use AT is influenced by different physical environmental, parental, and individual factors, as described in the Conceptual Framework for Youth's Active Travel by Panter et al. (2008). Main moderators such as age and gender can alter the strength of these relationships (Panter et al., 2008). Empirical evidence suggests that a low socioeconomic status (SES) including a lower parental education and income is often associated with higher AST (Pont et al., 2009; Rothman et al., 2018). In Germany, adolescents with a low SES and those with a migration background were more likely to walk to school (Reimers et al., 2013), however, mixed results are reported for cycling (Reimers et al., 2013, Reimers et al., 2021). Concerning the physical environment, children and adolescents from urban areas have higher AST rates than their rural counterparts (Johansson et al., 2012; Reimers et al., 2013; Yang et al., 2016; Christiana et al., 2021). However, Reimers et al. (2021) only found an association in boys but not in girls while others report no association between AST and urban status (Mandic et al., 2015; Delisle Nyström et al., 2019).

Longitudinal studies focusing on factors influencing the maintenance or change in AST behavior from childhood to adolescence have found that a low SES and living in urban areas are associated with maintenance or uptake of AST (Pabayo et al., 2011; Trang et al., 2012; Murtagh et al., 2016; Vanwolleghem et al., 2016). However, there is a lack of studies addressing transport modes in more detail than “active” and “passive”, especially in longitudinal studies. This classification, specifically the role of public transport, is currently being debated as walking or cycling to and from the bus stop or train station may accumulate a substantial amount of PA compared to passive transport modes such as car transport (Voss et al., 2015; Durand et al., 2016; Ermagun and Levinson, 2017) and should therefore be taken into account for when studying AST behavior (Reimers et al., 2021). Due to the high effort, there is a lack of longitudinal studies on AST focusing on school transition periods, especially from primary school into early adulthood, which also take public transport as a possible semi-active transport mode into account.

Therefore, the aims of this study are (1) to examine the role of socio-structural factors on changes in school transport mode over six years during the transition from primary to secondary school and from primary school into early adulthood and (2) to describe the stability and change of school transport modes during these transition periods according to their strongest predictors in Germany.