Abstract: With an analysis of the insufficient accuracy of the video based vehicle speed measurement based on virtual lines, a method was proposed to improve the measurement accuracy. A rectangle detection area was defined on a road lane, but it was displayed as a trapezoid on the images captured by the video camera. By using twodimensional image calibration transformation, the image coordinates of any point within the trapezoid area could be accurately mapped to the ground coordinates of corresponding point within the rectangle area. By selecting adequate image frames captured during the vehicle running across the detection area, the displacements of the vehicle and corresponding time intervals could be obtained by the proposed algorithm and thus the vehicle speed could be achieved with better accuracy.