Abstract: Direct left turn from driveways is considered as a contributor to accident and delay in at-grade intersections. Safety benefits of prohibiting direct left turn from driveways have been identified in many studies. Several engineering alternatives have been developed to accommodate prohibited driveway left turns. These alternatives are (1) right-turn followed by U-turn at the down stream intersection; (2) right-turn followed by median U-turn before the intersection; (3) right-turn followed by median U-turn after the intersection; (4) upstream Jug Handle, and (5) downstream Jug Handle. Each of the five treatments reroutes the left-turn vehicles at intersections, mid-block segments or side streets. Consequently, the alternatives of the indirect left-turning measures give rise to changes in drivers’ driving behaviors and safety. Based on the practices and experiences from several states as well as the authors’ studies, this paper presents the results from reviewing impacts of the indirect left-turning measures on driving behaviors and safety of the affected drivers. As a result of implementing the alternatives, changes in turning maneuvers and driving tasks, travel speed, speed disturbance, travel time, and drivers’ perceptions of acceptable gaps in diverted turning maneuvers are addressed in the discussions of the impact on driving behaviors. And then, changes in conflict areas, conflict counts, and crash types are addressed in the discussions of impact on drivers’ safety. In conclusions, major characteristics of the indirect left-turning measures are summarized and some key issues are also discussed.

Key words: direct left turn, indirect left turn, Jug Handle, median U-turn, driving behavior, safety, driveway

摘要： 鉴于直接左转机动车交通常是路口事故和延误的诱因之一，左转禁驶在许多研究中都作为一项安全措施处理，直接左转处理方式有多种，大致包括：(1) 下游路口右转后U形调头直行；(2) 当前路口右转后在下一路口前的隔离带打U形调头直行；(3) 当前路口右转后在下一路口后的隔离带打U形调头直行；(4) 在当前路口上游打“壶柄状(Jug Handle)”转行；(5) 在当前路口下游打“壶柄状”转行。这五种措施的基本原则是将直接的左转行交通转换成发生在其它地点（比如下游路口，隔离带或横向街道等）的不同形态的转向交通，可称作“非直接左转交通”。而这些“非直接左转交通”不同程度地改变了驾驶人员的驾驶行为和安全因素。根据一些州的实际例子以及作者的实践经验，本文就“非直接左转交通”措施对行驶行为和安全的影响进行研究，包括转向操作，驾驶目标，行驶速度，车速干扰，行驶时间，转向中对可接受的车流空隙感觉判断。另外，“非直接左转交通”引起冲突区域，冲突点分布和碰撞类型等方面的变化对驾驶人员的安全影响也进行了分析。最后，本文总结了“非直接左转交通”措施的主要特点以及所涉及的一些尚待解决的难点问题。

关键词: 直接左转, 非直接左转;“壶柄状”转行, 隔离带U形调头, 驾驶行为, 安全, 机动车道