This paper investigates the ride comfort provided by cars equipped with passive and semi-active suspension systems. This investigation is part of a feasibility study for a high speed freeway system for the State of Texas that is proposed for the year 2020.The investigation described in this paper tested two automobiles, one with a passive suspension and one with a semi-active suspension at speeds from 30 mph to 80 mph. The two cars were identical in all other operational aspects except their suspensions. The tests determined the magnitudes of the linear accelerations of the automobiles' front passenger seat for the various testing speeds. These accelerations were converted into a form which could be compared to the International Organization for Standardization (ISO) standards pertaining to the relationship between these accelerations and rider comfort. The ride comfort at 150 mph was predicted from extrapolation of the test data.The results of the investigation showed that the semi-active suspension system did decrease the accelerations transmitted to the passenger, thus providing increased rider comfort. However, this increased rider comfort was modest and it was determined that the ride-comfort-limiting accelerations occurred in the frequency band corresponding to the natural frequency of the automobile body including suspensions, and the natural frequency of the automobile's tires. The investigation also determined that road conditions are an important factor affecting rider comfort even when the automobile is equipped with a semi-active suspension.