Study On Traffic Flow Model Of Urban Expressway Under Rain Weather Based On Cellular Automaton

Traffic congestion has become a prominent problem in the process of urbanization.When it rains, road surface friction coefficient is decreased and the vehicles speeds are reduced. Not only is the road traffic capacity reduced and traffic congestion exacerbated, but also it is prone to form chain congestion, causing local congestion in the road network. Therefore, based on the feature that once it rains then the cities are blocked, the cellular automata model is established to simulate the transportation system in order to investigate the dynamic evolution of traffic flow on expressways.First of all, according to the literature review, the safe stopping distance experiment data is analyzed and the road environmental characteristics under rain weather are discussed. The maximum braking deceleration, visual range, safe stopping distance and other parameters are determined. Due to the specialty and complexity of the road and traffic environment in the rain, three safe distances are defined in the consideration of vehicle emerging stops, which are the acceleration safe distance, deceleration safe distance and cruise safe distance. And the cellular automaton model of traffic flow is proposed that is based on the safe distances defined under the rain weather.Secondly, based on the proposed cellular automaton model considering the safe distances, a cellular automaton of traffic flow model on single lane with periodic boundary condition is proposed to simulate the traffic flow of expressways. The traffic characteristics are analyzed through three aspects of traffic flow parameters, vehicle speed and traffic capacity. The results tell that impacts of rainfall intensity on traffic free flow is little and the impacts on traffic flow with high-density is heavily that increase with increasing rainfall intensity. As the rainfall intensity increases, the critical density and congestion density are reduced. Compared to the results with fine weather, the maximum flow under the condition of light rain, medium rain and heavy rain are decreased by 2.4%,5.5% and 10.3%, respectively. And the greater the rainfall intensity is, the smaller the speed dispersion is under the high-density areas. As the rainfall intensity increases, the traffic capacity is reduced.Then, based on the cellular automaton model of traffic flow that based on safe distances, a cellular automaton of traffic flow model on single lane with open boundary condition is proposed by setting the traffic bottleneck to simulate and analyze the three-phase traffic flow of expressways through the aspects of phase diagram and flow characteristics of different phases under different rainfall weather. It has been shown that as the rainfall intensity increases, the free flow phase area becomes smaller, and the wide moving jams phase area becomes larger. And rainfall intensity has less affects on traffic flow with the free flow phase, but it influences heavily the narrow jam formation. The greater the rainfall intensity is, the higher the frequency of the narrow jam formation is, and the more slowly it dissipates. For the wide moving jams phase, the greater the intensity of rainfall is, the longer the congestions continue and the more slowly the phase dissipates.Finally, to analyze the characteristics of synchronized flow further, the correlation analysis is carried out on the data of the cellular automaton model of traffic flow with periodic boundary condition and open boundary condition. The results tell that the autocorrelation coefficients of the speed, density and flow flux in the synchronized flow phase are close to zero, and the cross-correlation of flow with the average density is also close to 0 and there is no long-range correlation among different parameters.