Research On The Conventional Bus Signal Priority Strategies

In order to handle the increasing urban traffic congestion,a number of countries have adopted the policy to develop public transit infrastructure with first priorities.From the technical perspective,the commonly used transit priority is to construct various dedicated transit lanes,which has been implemented in many cities in China.The technology of transit priority for travel time,known as transit bus signal priority,has not received a widespread application due to the disconnected relationship between the signal bus priority technology and the traffic operation.The research attempts to summarize the drawbacks of current transit bus signal-priority technology,analyze the contributing factors to the difficulties of practical implementations,and develop feasible transit bus signal-priority strategies in the traffic signal control system.The research targets the conventional bus in the urban traffic system and studies the signal-priority strategies at isolated signalized intersections or coordinated ones along an arterial.The research findings are applicable to roadways with dedicated bus lanes and those without.Note that the research does not touch on the traffic flow detection technologies and control-algorithm design and focus on the bus signal-priority strategies only.The research starts with analyzing how to optimize the minimum green time in the bus signal-priority strategy at an isolated intersection,which is one of the principal components.The research concludes that an optimal transit bus signal-priority can not be reached;even under the setting that transit-bus signal priority phase is designed and switched from that without any override phase in the peak hours,based on the current definition of minimum green time.The research proposes to redefine the concept of minimum green time and establish a model to calculate the minimum green time on the condition that the queued vehicles will not overspill back to the upstream intersection.Second,the research proposes various responsive signal-priority strategies on the basis of the redefined minimum green time and stipulates the guidelines for the strategy implementation.The research finds out that if the requests for transit bus signal are overloaded,the signal-priority strategies may conflict with each other and consequently interfere with the regular operations of transit buses.And thus the “optimal” strategy may not be able to satisfy the traffic demand.The research develops the bus signal priority schemes according to the local optimal strategy at an isolated intersection and the overall performance as well.The study predicts the arrival rate of vehicle-persons at each entering approach;the object function of developing signal-priority strategies is to maximize the number of vehicle-persons getting through the intersection without any delay.Third,the signal delay models proposed in HCM(2010)and Webster method are applicable to compute a relatively long period control delay,and may not be in accordance with the condition when a short-period delay is calculated in signal-priority schemes.In order to resolve this issue,the study utilizes the restricted roadway speed as the measurement of delay,by means of short-period signal delay model.The model attempts to minimize the person-delay and evaluate the const-benefit of the signal priority schemes.In addition,the research proposes a concept of signal priority response threshold and requires that signal priority schemes should generate anticipated benefits and overloaded priority strategies be avoided to maintain the normal operation of the initial signal operations.At last,the research states that in order to achieve a successful transit bus signal priority in the dedicated bus lanes,the design of bus lane should be tightly cooperated with the signal timing-phasing plans,and the original design plan of the dedicated bus lanes should be revised accordingly.The paper also tries to mitigate the interference of bus signal priority plan by introducing a reverse left-turn dedicated bus lane.