Abstract: Under the influence of earthquake, natural disaster, traffic accident and imbalance between the daily traffic supply and demand, the road network is in instability condition.The capacity random variation leads to the randomness of road network traffic time.Starting with the issues linked with traffic time, the author analyzes the mechanism of each factor on road network traffic time, defines the reliability of rout travel time and solves the problem by using Monte Carlo.At last, a simple example is provided.