In the situation that a vehicle is driving along the lane and there is a camera above the lane. The relationship between the vertical coordinates of the reference point on car body on the image plane of the camera and the driving distance of the vehicle are used to obtain the driving distance of the vehicle indirectlyso as to identify the vehicle speed. Under the condition of general lane cameras’ parameters (focal lengthangleheight)through theoretical analysis and experimental verificationit is concluded that the vertical coordinate of the reference point on car body on the image plane of the camera has an approximately linear relationship with the reciprocal of vehicle driving distance. By selecting a reference point at the front or rear of the car body and calibrating the lane camera on site a linear fitting formula of the vertical coordinate of the reference point on car body on the image plane and the reciprocal of the vehicle driving distance can be obtainedand then the vehicle speed can be measured. This method with easily operation and good measurement results can be used in the judicial authentication of vehicle speed for traffic accident.