According to the characteristics of traffic jam at intersections and on the basis of qualitative reasoning,a time-related steady-state evaluation index （ SEI） is proposed to describe the traffic situations at the entrance of intersections on urban road. Simulated results indicate that,before the formation of traffic jam at intersections,the SEI of traffic flow abruptly changes and a transition from steady state to unsteady state and finally to steady-state may occur,and that the critical congestion point determined by the SEI basically accords with the formation and dissipation time of traffic jam reflected by the total queue length.