After Peter Newman and Jeffrey Kenworthy published ”Cities and Automobile Dependence” in 1989, population density was brought to public attention as an important factor to explain transport mobility and energy use. However, several related issues still remain open: Is an increase in population density more effective than rising gas prices in reducing transport energy use? How much does per capita transport energy use change as population density in cities changes? And what kind of factors influence changes in population density? In this presentation, using city-level data in the US, Japan and other countries, the population-density elasticity of car vehicle miles traveled and/or transport energy use is estimated, taking income, gasoline price, and other factors into consideration. I show that the population-density elasticity tends to be higher than the price elasticity in cities with low population densities.