Drivers’ reading and reaction process was analyzed and divided into four stages,including sensory,perception,judgment and execution.An analysis was also made of the necessary time and distance for lane change under uninterrupted traffic flow on expressway,and a method was proposed to determine visual disappearing site of traffic sign.Critical crossing time gap in lane change was established with a consideration of weaving safety.By introducing success probability of lane change,minimum driving distance for lane change was formulated.Then a theoretical model was established to describe minimum required frontal distance in traffic sign installation.The outputs of the model show that the weaving distance becomes longer when traffic volume is larger.The minimum frontal distance of traffic sign increases evidently as traffic volume becomes larger.The proposed model was tested in a section of TongSan freeway in the city of Yantai to verify its rationality.Finally,the paper presents the minimum frontal distance of traffic sign under different sucess probabilities of lane change.