The Tongji University Driving Simulator was used to conduct experiment and collect speed data. Speed change was divided into three intervals: substantial speed decrease, steady speed (minimal change) and substantial speed increase. Multinomial Logistic models were used to investigate the relationships between geometric design characteristics of combined curves and adjacent segments and speed changes. The results show that: for combined curves, it is more difficult to maintain speed as the lengths of combined curves and the mean grades of combined curves increase; the probability to increase speed is higher when combined curves are downslopes; the probability to maintain speed is higher when combined curves turn left; it is more difficult to maintain speed as the grade changes between the preceding segments and the combined curves increase; the probability to decrease speed is higher when the curvatures of the following segments increase.