This study investigated the perception mechanism of a flat ride during highway driving. A flat ride is treated here as a comprehensive ride comfort perception consisting of a floating feeling, a body tossing feeling and other sensations induced by low-frequency oscillations. In this study, vehicle motion was measured using a GPS receiver and an inertial navigation system. The driver's head motion relative to the vehicle was measured with a motion capture system consisting of cameras attached to the vehicle. Absolute head motion with six degrees of freedom in space was calculated by using the measured motion data. The results of an analysis showed that a flat ride could be quantified based on the amount of absolute head motion. Laboratory tests were also conducted to investigate the effects of motion in the visual field on the perception of ride comfort. The experimental results showed that perceptions of even the same motion varied depending on the visual conditions, such as the location of the hood and instrument cluster. It was found that such differences in motion perception affected the evaluation of a flat ride.