An experimental study of the aerodynamic interference between road vehicles during the passing process was performed. 1/10 scale models for a truck and a semi-trailer were employed. Static type testing was followed. Thus, eleven relative longitudinal positions between both vehicles were examined in a wind tunnel. Moreover, to investigate the severity of close proximity of vehicles, four lateral spacings between both vehicles were considered.The pressure distribution over horizontal planes of the circumference of both vehicles were measured separately at first for each vehicle. Next, these measurements were repeated forty four times for each vehicle to account for both longitudinal and lateral relative positions between both vehicles. The difference in the pressure coefficient Cp over both sides of each vehicle furnishes the side force and yawing moment on each vehicle. The sensitivity of the small vehicles to both influences, leads to a drift from its path. Moreover, the corresponding side force and yawing moment of the trailer about the hitch point of the semi-trailer, leads to its turn around the hitch point. Consequently the safety and stability of the nearby vehicle is critically influenced.