Exploring the influences of density on travel behavior using propensity score matching

The causality issue has become one of the key questions in the debate over the relationship between the built environment and travel behavior. Since a residential self-selection effect exists, it is important to know if the observed influence of the built environment on travel behavior diminishes substantially once we control for self-selection. Using 5537 adult respondents to the 2006 Great Triangle Travel Survey in North Carolina, this study applied the propensity score matching approach to identify the causal effect of density on travel behavior and the relative contribution of self-selection to travel behavior. The results showed that, after removing self-selection bias, residents living in high-density neighborhoods travel, on average, 3.31 fewer miles per person per day than those who live in low-density neighborhoods. Self-selection effects account for 28%, 64%, and 49% of the observed influences of density on personal miles travelled, driving duration, and transit duration, respectively. We also found that different modeling approaches produce different point estimates, and that interval estimates of treatment effects tend to have a large variation. This points to a caveat of using point estimates to evaluate the impacts of the built environment on travel behavior.