Average weights, controlled for length, the so-called body mass index (BMI) are increasing and a lot of health issues are related with this weight gain. Governments around the world try to turn this trend around. One of the policies that is expected to influence BMI, is influencing travel behavior by making changes in the built environment. However, a lot is still unknown about the relationship between the built environment, travel behavior and BMI and about the causality of this relationship. For assessing whether implementing adaptions in the built environment is an effective policy, it is needed to further research this relationship. Using longitudinal structural equation modeling the short and long term relationships between the factors are defined and the strength of these relationships is assessed. The study finds that making travel behavior strongly influences BMI, and thus that steering travel behavior is a good approach for counteracting weight gain. For influencing travel behavior the study finds that both by making adaptions to the built environment, and by directly influencing the attitudes towards travel modes travel behavior can be influenced. In order to create effective policies research has to be conducted on the influence of detailed design characteristics of the built environment, on the impact of policies on other health indicators than BMI and on the applicability of the found results for younger and less affluent citizens.