Abstract: This paper studies the effects of speed differences on the practical capacity of freeway based on the speed dispersion definition of Average Speed Difference. It is found that there is no obvious relationship between speed dispersions and the practical capacity under high level of service,while under medium and low level of service the practical capacity is lower than the theoretical capacity on the effects of speed dispersions. Based on the analysis of safe car-following behaviors,the relations among vehicle speed,space headway and speed dispersion are established,from which the modifying factor model of practical capacity of speed dispersions is derived.