Few studies have focused on the potential effects of an increase in the share of aged population on the environmental impacts of road transportation. In order to fill this gap in the literature, this paper empirically analyzes whether there is a relationship between the share of aged population and carbon dioxide (CO2) emissions from road transportation by applying a quadratic function. Using international panel data, it also addresses the level of the turning point in the relationships between the share of aged population and CO2 emissions. The analysis in this paper uses a first-order differential equation to estimate an inverted U-shaped relationship between them in order to alleviate the unit roots issue. The results from 25 OECD countries, consisting mainly of European countries and Japan, indicate that there is a quadratic relationship between CO2 emissions per capita and the share of aged population, and that the turning point is around 16 percent. The results also imply that a relative increase in the number of elderly people is associated with a decrease in CO2 emissions per capita from the road sector when the share of aged population reaches more than 16 percent. (C) 2012 Elsevier Ltd. All rights reserved.