Model calculations suggest that the emissions by road traffic (passenger vehicles and trucks) of ozone precursors (NOx, CO, hydrocarbons) have a substantial impact on the concentration of tropospheric ozone at the regional scale in the boundary layer and at the hemispheric scale in the free troposphere. Increases in the surface ozone concentration resulting from road traffic are typically 5--15 % at mid-latitudes in the Northern hemisphere during summer. Similar levels of ozone changes are calculated in the Southern hemisphere during austral summertime, but with perturbations less uniformly distributed than in the Northern hemisphere. Ozone changes produced in the upper troposphere as a result of road traffic are of the same magnitude (5--8% in July) as the changes generated by commercial aircraft operations. A traffic-induced reduction of 3% is estimated for the globally and annually averaged lifetime of methane.