Dynamic Modeling And Traffic Control Mechanism Analysis Of Capacity Drop At Expressway Bottleneck

The growth of road mileage can never catch up with the growth of traffic demand.The traffic congestion caused by this problem is very obvious in the domestic and foreign metropolitan expressways and suburban freeways,which seriously affects the traffic efficiency,traffic safety and traffic environment.Ramp metering(RM)and variable speed limit control(VSLC)are two of the most commonly used measures to alleviate traffic congestion on expressways,which can avoid bottleneck congestion and capacity drop(CD)by adjusting the total flow coming to merging bottleneck.Capacity drop caused by expressway bottleneck congestion is an inherent attribute of“human-driven” traffic flow.This paper studies coordinated ramp metering and variable speed limit control for expressway traffic at presence of capacity drop,and pays special attention to the effect mechanism of RM and VSLC in relation to capacity drop.In addition,macroscopic traffic flow model is the core foundation and important means to evaluate the effectiveness of traffic flow control strategies.This paper proposes a general macroscopic modelling method and a research framework for complex traffic flow on expressway,and constructs a traffic flow model that can fully reproduces CD,so as to improve the accuracy and credibility of the modeling.Firstly,this paper proposes a systematic macroscopic modelling research framework containing more than 20 elements,and constructs a modelling method based on the second-order model METANET.This method can accurately modelling complex traffic flow phenomena such as traffic congestion,capacity drop,traffic flow inhomogeneity,stop-and-go waves and scattering under normal and abnormal environment conditions.At the same time,this modelling method can be used to construct a macroscopic traffic flow model which can fully reproduce CD.Secondly,this paper proves the general property of network traffic flow control based on mathematical analysis,and analyzes the relationship between RM and VSLC and CD.Thirdly,based on nonlinear optimal control theory,this paper develops coordinated ramp metering,variable speed limit control and their joint control strategies to avoid bottleneck congestion and capacity drop on expressway.Finally,this paper uses large-scale real expressway network and simulated expressway network data to verify the feasibility and reliability of the aforementioned general modelling method and research framework,the effectiveness of the control strategies and the rationality of the theoretical analysis conclusions,and analyzes the equivalence of RM and VSLC.The following conclusions are reached in this paper:(1)The proposed macroscopic traffic flow modelling research framework and the METANET-based modelling method are effective and feasible,which is suitable for complex traffic flow modeling of large-scale expressway network under different environmental conditions;(2)The constructed model for CD is reliable and can be used to verify the developed traffic control strategies effectively;(3)The minimization of the total travel time of all vehicles in the network is equivalent to the maximization of the total network outflow.Any expressway strategy that benefits the majority of drivers must be committed to maximizing the cumulative total outflow of the expressway network;(4)The effect mechanism of RM and VSLC are closely related to capacity drop.RM and VSLC pay off because real traffic bottleneck are always accompanied by CD.It is assumed that there is no CD when the bottleneck of an expressway is overloaded,simulation results shows that VSLC does not generate any benefits,while RM can still obtain limited benefits by increasing the off-ramp flow and reducing the total travel time of the expressway network;(5)Off-ramp blockage is a typical long-distance congestion phenomenon on expressway.Any RM or VSLC measures that can avoid CD are also conducive to eliminating off-ramp blockage;(6)VSLC and RM are equivalent in eliminating bottleneck congestion and capacity drop.When both of them can exert the best control performance,RM+VSLC will not gain additional benefits,but can optimize the allocation of road resources.In addition,it is a misconception that RM simply shifts traffic delays from the mainstream of expressway to the onramp.The mainly contributions of this paper are as follows:(1)a general method of macroscopic traffic flow modelling and a systematic research framework are proposed;(2)The macroscopic traffic flow model that can accurately reproduce CD is constructed;(3)The effect mechanism of RM and VSLC is revealed;(4)A variety of expressway traffic flow control strategies optimization methods are proposed;(5)The equivalence of RM and VSLC is revealed.