Abstract: Firstly,experiments are conducted on 108 sections of 3 typical two-lane highways and the continuous speed of cars for experiments are obtained.Then the speed features are studied in these processes,and the conclusion is drawn that the original speed and the transverse space between car and the roadside intruders are the primary factors influencing a car when it passes through.The models of the impact on the speed are also built using regression analysis.On this condition,roadside interference coefficients were put forward based on the theoretic analysis of the relations between the number of roadside interferences and operating speed.Finally,this paper offers a calculated table of the roadside interference coefficient of every kind of transect and varied number of roadside interferences.