Abstract: The influence of guide distance and initial speed on driving behavior is directly related to the traffic safety in expressway exit． In order to analyze the effects of different combinations of the two factors on driving behavior，na- turalistic driving experiments under nine combination conditions were carried out on the exit section of a mountainous expressway service area with the design speed of 100 km/h． The combination conditions contained three kinds of exit guide distances ( 700，500 and 300m) and three initial speeds ( 110，100 and 90 km/h) ． The driving behavior cha- racteristic parameters such as the speed at the portal of the starting of deceleration lane，the maximum deceleration and the starting position of lane-changing behavior，were collected and analyzed． The results show that both the exit guide distance and the initial speed have a significant effect on the speed at the portal of the starting of deceleration lane，which is 8 ～18km/h lower than the design speed under all combination conditions． The maximum deceleration is significantly affected by the initial speed but not by the exit guide distance． When the exit guide distance is 700m and the initial speed is 100km/h or 110km/h，there is a twice deceleration phenomenon which differs from that under other conditions． Exit guide distance is the key factor to influence the starting position of lane-changing behavior． Specifically，when the exit guide distance is 300m，the initial speed could obviously influence the starting position of lane-changing; while the influence became less obvious as the exit guide distance increase． The results can provide a reference for understanding the changing rules of expressway driving behavior and optimizing export design．

Key words: driving behavior, expressway exit, exit guide distance, initial speed, driving safety