Does urban form affect travel choices and thus CO2 emissions by individuals? If this is the case, then urban form and policies that influence it deserve serious attention in the context of long-term climate policy. To address this issue. we examine the impact of urban density on commuting behavior, and the consequences for CO2 emissions. The empirical investigation is based on an instrumental variable approach (IV), so as to take account of endogeneity of residence location. We decompose travel demand into components related to modal split and commuting distance by each mode.