This research work is concerned with experimental studies of the aerodynamic interference between passenger cars. Aerodynamic interference covers the two cases of vehicles in a convoy and in parallel passing (overtaking). Vehicles are assumed to be moving in open roads. Flow visualization technique is used to clarify the flow behavior, especially wake structure behind interfering passenger cars.This paper is a trial to further explain the aerodynamic characteristics, usually reported in the form of pressure distributions and drag forces, through visible observations. Visualization images of passenger cars in close interference are presented for the first time. Special attention is paid to the effect of Reynolds number on flow pattern especially in severe interference conditions. Results show that flow structure around interfering cars changes dramatically depending on the gap between the cars and Reynolds number. Interesting flow structures are captured and analyzed.