A method for detecting a vehicle type, a vehicle speed and width of a detecting area by a vehicle radar sensor is disclosed. A radio wave is transmitted to a tracked vehicle. Subsequently, the reflective radio wave from the vehicle is received. The Doppler frequency versus time distribution is generated from the reflective radio wave. Because the reflective radio wave is influenced by the Doppler Effect, a parallelogram or a shape close to a parallelogram of a consecutive motion diagram is shown in the Doppler frequency versus time distribution of the vehicle. According to the consecutive motion diagram, certain information, such as the length and speed of the tracked vehicle and the width of the detecting area, can be acquired.