Traffic engineers typically operate under the credence "the best traffic control is the least restrictive." That being said, traffic engineers will deploy, for example, a simple two-phase traffic signal operation before considering multiple complex signal phasing. Two-phase signal control is an efficient way to operate a traffic signal. However, the credence is all-so-often challenged as traffic volumes increase such that left-turning motorist experience few available gaps to make a safe maneuver. Traffic engineers are now faced with balancing safety with efficiency. One corresponding mitigation action is the incorporation of a protected left-turn phase, which provides a safe maneuver, but the green time to serve this protected phase must come from other movements. The resulting increase in safety came at the expense of operational efficiency. To regain some of the lost efficiency, traffic signals are designed to protect the left-turn movement during a portion of the signal cycle and allow a permissive movement during the remainder of the signal cycle, resulting in left-turn control that is commonly known as protected permissive left-turn control, or simply PPLT control (or phasing). If a protected movement is warranted, PPLT control has been shown to increase left-turn capacity and reduce delay at intersections (as compared to protected-only control). Traffic engineers typically determine when to use various left-turn phasing and how to implement the chosen left-turn phase treatment after careful consideration of site-specific issues and facts. The National Cooperative Highway Research Program (NCHRP) 3-54 research project has recently completed a comprehensive study on the type of display that should be used for the permissive left turn movement when used in the PPLT control. Findings from the NCHRP 3-54 research included the recommendation that the flashing yellow arrow indication should be included in the MUTCD as an alternative to the circular green indication for the permissive left turn movement when used with PPLT control. This recommendation came as a direct result of studying driver's comprehension and behavioral characteristics along with the operational benefits associated with the flashing yellow arrow indication. This paper has been prepared to provide an informational overview of some of the key elements of the NCHRP 3-54 research including: PPLT control and the display used within the United States, Canada, and Europe; How the MUTCD circular green indication is problematic when used for protected permitted left turn operations; Why a comprehensive research study was conducted to identify a display that would reduce safety issues and improve signal operations; Recommendations made in the NCHRP 3-54 report, which includes the use of the flashing yellow arrow display; and. A description of the operational advantages of the proposed flashing yellow display.
Samenvatting