On-road turbulences caused by sources such as atmospheric wind and other vehicles influence the flow field and increases the drag in a vehicle. In this study, we focused on a scenario involving a passing vehicle and investigated its effect on the physical mechanism of the drag increase in order to establish a technique for reducing this drag. Firstly, we conducted on-road measurements of two sedan-type vehicles passed by a truck. Their aerodynamic drag estimated from the base pressure measurements showed different increment when passed by the truck. This result raised the possibility of reducing the drag increase by a modification of the local geometry. Then, we conducted wind tunnel measurements of a simplified one-fifth scale vehicle model in quasi-steady state, in order to understand the flow mechanism of the drag increase systematically. Two main factors of the drag increase, namely the pressure distribution and the change in the wind direction generated by the passing vehicle, were identified from the measurement results. Finally, we conducted the wind tunnel measurements of a scale model modified to mainly reduce the latter factor. As the result of the measurement, we verified the reduction technique for the drag increase.