Research On Time-Space Priority Strategy And Modeling For Bus Rapid Transit

As traffic congestion in urban areas is getting worse, Bus Rapid Transit (BRT) has been significantly developed in many cities. By the early 2009, BRT system has been opened and operated in eleven cities of China, and more cities are actively planning, designing and constructing BRT system.While BRT system can achieve "space priority" by setting bus only lane in the section and "time priority" by implementation of signal priority control in the intersection, it can not avoid the negative impacts on other traffic in the competition direction. Therefore, study on the BRT priority to realize win-win in access priority and less negative impacts on other traffic, will not only have significant academic value in conducing to a profound understanding of "people-oriented", "efficiency priority" and "the right way priority" in bus priority concept, deepening and raising priority access theoretical system, but also has important practical significance in promoting the development of modern intelligent transportation system and relieving urban traffic congestion.The paper focuses on signal priority control strategies and methods, bus-only lane planning methods, benefit assessment of priority access and other aspects about BRT have been researched, based on the analysis of domestic and foreign research, the main contents and conclusions are as follows:①It is discussed that planning techniques and adaptability of different types of bus-only lane,signal priority control mode and strategies and features of vehicle detection/ positioning technology, based on the analysis of characteristics of BRT and structure and function of priority control system, and the corresponding development strategies have been proposed. At last, the effectiveness of BRT priority access measures has been verified by case study of the south axis of Beijing BRT.②Signal priority strategy simulation model is constructed based on cellular automata in the paper, and some evaluation indicators such as the average speed, traffic density and traffic flow have been proposed to assess the changes of the traffic flow characteristics in the crossing, and BRT signal priority parameters (the maximum time of green extension gbrtmax and the biggest time of red truncation rbrtmax),the signal priority detector distance L from the intersection and the impact of the generation probabilityαbrt to the intersection have been simulated. Case studies show that, on the one hand, the signal priority control need to give priority to increase gbrtmax and decrease rbrtmax so that can reduce the impact on society traffic flow which is in the fixed signal cycle condition; on the other hand, the reasonable range need to be controlled when given the gbrtmax and rbrtmax.As it is too short, it will make signal priority green time extension very short and the benefit of BRT vehicles less while significantly affected by random factors, volatility increases. When it is too long, it makes the green extended time requested by BRT more than the limited of gbrtmax, signal priority will lapse; Secondly, under the condition of given gbrtmax, rbrtmax and L, excessively increasingαbrtmax, the traffic density of BRT is sharply increase and traffic speed reduced. Especially with the increase ofαbrt, the frequency of BRT signal priority implementation is more and more, and request for time extension is gradually closed to gbrtmax, until the green light extend priority does not meet the conditions, the equivalent of fixed signal timing.③Two level fuzzy control method of BRT signal priority is constructed considering the conditions of signal priority, then the process of control and concrete realization of the algorithm have been detailed. With the single-stage fuzzy control, the proposed method has the following characteristics:signal priority conditions are set and non-priority phase of traffic flow efficiency is taken into account. Signal priority control access conditions have been constructed from deviation of the BRT arriving time and the schedule,load factor, and system comprehensive benefits, so that the relatively low efficiency of signal priority control have been shielded, and also reduce the ineffective control and improve the control efficiency. Taking into account of BRT signal priority phase and social traffic flow, which means both considering the busy degree of the green light of signal priority phase and also non-priority phase urgency degree between the red light phase of the social movement and the next number of vehicles queued to wait for the green light.④Considering the induced traffic flow generated under the premise of bus-only lane planning, traffic time model of traffic flow has been constructed before and after the planning of BRT lane, and travel time change has been analyzed in some cases of increasing bus lane such as compressing existing lanes, widening roads, etc, all base on impedance function and the calculation of road capacity. Case studies show that as the degree of bus lane compression, systems, and the negative impact of bus traffic will be dramatically increased. Road widening to increase BRT lane, the integration of the existing bus routes, improve the social traffic flow environment. On this base, from the macro, macro and micro three levels, BRT planning framework has been proposed, and all levels of content and methods have also been elaborated. Compared to traditional method of bus lane's planning, the framework can fully absorb the experience of planners at different levels, easy to grasp the lane planning as the general framework, between the upper and lower planning guidance and feedback role.⑤Based on the intersection through rate, benefit of evaluation model of BRT priority access has been build from the "line" point, and evaluation indicators such as the change rate of traffic overall revenue,rate change of the unit cost of revenue in traffic flow, then sensitivity analysis included benefit of priority access to the road travel speed,non-priority phase of the signal compression ratio coefficient and the average wait time at red traffic light scale factor through simulation. Compared to existing models, the proposed model has been overall considered benefits of BRT priority access control, not only considering the implementation of signal priority control and planning of the effectiveness of lane changes, but also considered benefits of social traffic flow changes along the BRT corridor. It makes up inadequate of existing research which only considering the revenue of signal priority control but lack of the considering on benefits of bus-lane planning and impact on social traffic flow, and benefit assessment methods of BRT priority access have been further improved.⑥Benefit of evaluation model about the green light to extend in a single intersection have been constructed and red light off early priority strategy have been constructed from the "point" perspective under BRT priority, and calculation methods of equivalent car conversion, benefit of average person conversion have also been proposed, then a numerical example for green extension strategy has been analyzed. The results show that when green extension signal priority strategy has been implemented in the intersection, the through rate of BRT vehicles has been increased, which makes the equivalent car conversion delay and average people delay reduce by 11.3% in the intersection. With the growth of green time extension, delay changes of the overall intersection have been at first decreased and then increased. When green time extension was 8s, the entire system's delay is minimum, the equivalent car conversion delay and average people delay are 67.53s and 9.29s.