Abstract: To explore the influence of upstream transition length on expressway main lane traffic volume, the headway characteristics in main lanes of upstream transition area were analyzed based on actual data, showing the headway distribution in accord with Erlang distribution. Considering the vehicles' changeable critical headway when driving along upstream transition area, a vehicle merging model was proposed by gap acceptance theory and differential methods, thus a new optimal design method of the upstream transition length was presented. This method reveales the effect of upstream transition length, traffic flow of main lane and merging speed on merging probability. Results show that the merging model can well consider the effect of expressway geometry characteristic and traffic conditions on driver's merging behavior. The upstream transition length provides a new method for the design of work zones.