Analysis of Downstream Queues on Upstream Capacity Expansion of Urban Signalized Intersection

Signalized intersection is a fundamental component of an urban transportation system and appropriate treatments for intersection related congestion and safety issues are increasingly growing in importance. The most direct and intuitive approach to alleviate the recurring congestion is to cope with the peak hour disparity between travel demand and supply at the bottleneck intersections by expanding intersection capacity. However, intersection treatments such as adding lanes, turning movement restrictions, and grade separation that were traditionally applied to improve intersection capacity may not realize the expected benefits of relieving congestion and reducing delay as the traffic conditions at the downstream intersections can be greatly deteriorated by increased upstream arrivals. Additionally, the extended queue generated from downstream intersections can spill back into the upstream intersection and diminish the performance of the upstream treatment. This phenomenon is frequently observed in large urban areas where the traffic volume is heavy, intersection spacing is short and cycle length is long. This study was conducted to provide traffic engineers with a simple, practical and step-by-step analysis method to identify the occurrence and the type of queue spillback (Cyclic and Sustained), to determine the effects of downstream queues on upstream capacity, and to select the best capacity expansion treatment. The theory and methods for measuring the consequence of downstream queue effects and quantifying the potential capacity cutoff are developed based on the updated queue size and intersection capacity estimation methodologies for signalized intersections in the Highway Capacity Manual 2010 (HCM 2010). A spreadsheet-based computational tool was developed to assist in the process of capacity constraint identification and calculation. A case study is presented to demonstrate the practical use of the analysis method.

摘要信号交叉口是城市交通系统的重要组成部分，制定合理的交叉口拥堵和安全治理方案尤为重要。交叉口扩容是缓解交叉口拥堵最直接有效的措施,但是传统的扩容方案（如增加车道，禁止左转和立体交叉等），可能并不能达到预期的效果。由于上游的扩容，下游交叉口的交通状况可能会急剧恶化，导致更长的车辆排队，这些排队车辆可能蔓延至上游交叉口，引起堵塞。此情形在车流量大、交叉口之间距离小和信号灯周期长的情况下表现尤为显著。本文提出了一个简单，准确且实用的方法，用于判定这种下游排队蔓延现象的发生及其种类（循环性或持续性），并且评估这种排队蔓延对上游信号交叉口通行能力的影响。此方法基于最新的美国交通运输署的高速公路通行能力手册。本文还介绍了该方法相应的计算程序表的开发和运用，并给出运用实例。