Abstract: The average-speed control is a measure for measuring and controlling the spead of motor vehicles based on measurement of distance and time. Its key components consist of cameras triggered by monitoring terminals, recognition of vehicle information, and data processing. The shortest distance of road sections is obtained by a motor vehicle driving in fixed lanes in China at present. This method ignores lane changes of the vehicle and the distance obtained is not the shortest distance the motor vehicle can travel. This paper proposes a new method to calculate the shortest distance that a motor vehicle can travel with an interval-speed measurement system. The principles and key points of the method are provided.