The causality issue has become one of the key questions in the debate over the relationship between the built environment and travel behavior. To ascertain whether changes to the built environment are a cost-effective way to change travel behavior, it is necessary to determine the magnitude of the effect. Further, it is important to understand whether the observed influence of the built environment on travel behavior diminishes substantially once we control for self-selection. Using 1553 residents living in four traditional and four suburban neighborhoods in Northern California, this study explores the causal effect of neighborhood type on walking behavior and the relationship between this effect and the observed influence of neighborhood type on walking behavior. Specifically, propensity score stratification, which has been widely used to reduce selection bias, was applied. The results showed that, on average, the causal influences of neighborhood type are likely to be overstated by 64% for utilitarian walking frequency and 16% for recreational walking frequency, if residential self-selection is not controlled for. However, neighborhood type still plays a more important role in affecting walking behavior than does self-selection. This study also offers a basic tutorial for the propensity score stratification approach and discusses its strengths and weaknesses for applications in the field of land use and travel behavior.