In China vehicle conversion factors are usually obtained from vehicle type classification. This commonly used method results in problems in traffic control. By studying the different impacts of standard and non-standard vehicles with various lengths on the headways of standard vehicles, a conversion factor measurement model in terms of vehicle length is established. Through traffic surveys at typical intersections during peak periods, the headway of a standard vehicle following a standard vehicle, a standard vehicle following a non-standard vehicle, or a non-standard vehicle following a standard vehicle, as well as the length of a non-standard vehicle, have been calibrated. Therefore, the parameters in the model are calibrated by using the data obtained in the mentioned surveys. As a result, in accordance with the vehicle length, the vehicle conversion factor, which is applicable in traffic control, is determined.