Research On The Impact Of Connected And Autonomous Vehicle Platoons Length On Highway Traffic Flow Characteristics

With the rapid development of the connected vehicle and autonomous drive,nowadays,the Connected and Autonomous Vehicles(CAV)technology,as a hot topic in global intelligent transportation technology research,has become one of the important technical means for solving transportation problems in the academic field.Before completely replacing conventional vehicles,the status will stay for a long term that the CAVs will be mixed with such vehicles in traffic.This thesis mainly studies the length of the CAV platoons in the mixed traffic flow environment where the CAV platoons participate.By employing MATLAB to simulate the mixed traffic flow on highways,this study focuses on the maximum platoon length of the CAV platoons and explores their impact on the traffic flow characteristics of highways.The main research work is as follows:(1)This study establishes a cellular automata model for mixed traffic flow on highways involving the CAV platoons.First,in terms of the four different vehicle-following modes of mixed traffic flow and the F-STCA lane-changing model,the Na Sch model based on the Gipps safety distance is refined and improved according to the acceleration attenuation characteristics of vehicles driving on highways.Finally,a cellular automata model for mixed traffic flow on highways with multiple lanes is proposed with the maximum platoon length,by considering the lane-changing behavior characteristics and decelerated randomization probability of different types of vehicles,analyzing the behavior of the CAV platoons and their configurations,and distinguishing the reaction time under the four different vehicle-following modes.(2)This study investigates the evolution of the impacts of maximum platoon length on traffic flow characteristics from multiple perspectives such as the CAV penetration rate and density change.In line with the established model,based on the current status of highways in China and the basic assumptions of the CAV platoons,a large number of simulation experiments are conducted to select representative data to explore traffic flow characteristics by combining various vehicle and road conditions: the road congestion situations are compared under different maximum platoons lengths through spatio-temporal maps;the influence and degree of the maximum platoons length change on the variation of traffic flow velocity are analyzed under different penetration rate and density conditions through the thermodynamic diagram of average speed and velocity density,especially the nonlinear law of the average speed being affected by the maximum platoons length in the medium to the high density traffic flow;the impact of maximum platoons length on road capacity under different the CAV penetration rate and the magnitude of the change are investigated by considering the evolution law of the fundamental diagram with the change of the maximum platoons length in the three-dimensional coordinate system.The results demonstrate: when the length of the maximum platoon is 5 vehicles,the traffic flow congestion phenomenon faster propagates upstream in the reverse wave.The average speed change of formation vehicles is more regular in medium to high-density traffic flow while the speed of non-formation vehicles is more stable;When the penetration rate of CAV is high,as the maximum platoons length increases,the average speed of vehicles at equal density increases,but such phenomenon slows down after the length of the maximum platoon increases to 5vehicles;The mixed use of the CAVs and the platoons can both improve the capacity of mixed traffic flow,and this improvement often reaches the most significant state at critical densities;Under the same CAV penetration rate,the effect of maximum platoon length on traffic flow is more salient at medium to high densities;Under the medium to the high density of vehicles,the longer the maximum platoon length,the greater the flow,yet,this improvement is only noticeable when the CAV penetration rate increases.(3)This study explores the changes in the stability and safety of the mixed traffic flow under multiple influencing factors such as maximum platoon length.Grounded on the above-mentioned research,this study selects two indicators,namely,the Speed Standard Deviation(SD)and Time Exposed Rear-End Crash Risk Index(TERCRI),in order to further prob the stability and safety of two-lane mixed traffic flow on highways with the CAV platoons.The results demonstrate: when the CAV penetration rate is high,the peak index of the SD characterizing the stability of the mixed traffic flow moves towards the direction of longer maximum platoon length;When the CAV penetration rate reaches 100%,the peak index of the SD spreads towards the directions of medium to the high density of the platoon length;Under the medium to the high density of the platoon length,the increase in the CAV penetration rate and the increase in maximum platoon length will not make the traffic flow unstable;Excessive length of the maximum platoon has a minor impact on the safety of mixed traffic flow.Meanwhile,as the CAV penetration rate increases,such an impact gradually apparently appears but disappears when the CAV penetration rate reaches 100%.In summary,considering the average speed of the overall traffic flow,as the CAV penetration rate does not reach a high level,the maximum platoon length on high-volume highways should not be set too long,and it is appropriate to keep about 5 vehicles.However,as the CAV penetration rate reaches 100%,this restriction will not come.In this sense,it can be seen that in the historical stage in the future where the penetration rate of CAVs cannot reach a high level,the threshold limit for the maximum CAV platoon length on the road in China should still be around 4-6 vehicles.