Study strengths and limitations
adolescence, weakness
The main strength of the present study is the cluster randomized controlled design, which is to date the best method to establish causal effects. The schools were included in the study before randomization, decreasing the risk of selection bias. Despite this, the groups were not equal at baseline, and we observed higher prevalence of active travel among adolescence attending an intervention school (Fig. Our main outcome was self-reported travel mode, where physical activity during active travel (cycling or walking) implies more activity than motorized travel. In relation to this, a study weakness is the lack of a measure of intensity of physical activity during transport. One may therefore speculate if the extra physical activity could have been compensated for by lesser intensity such as cycling or walking at a slower pace, or even fewer days per week as we only ask for “usual” travel mode in the questionnaire. In addition, the lack of data on travelled distance is another noteworthy limitation. Data on distance would enable us to calculate the amount active travel performed. Also, previous studies consistently illustrated that distance is a significant predictor of active travel [We strengthened the internal validity by establishing convergent validity of the questionnaire items [