Kent C. Kacir

Traffic characteristics of protected/permitted left-turn signal displays

At least four variations of the permitted indication in protected/permitted left-turn (PPLT) control have been developed in an attempt to improve the level of driver understanding and safety. These variations replace the green ball permitted indication with a flashing red ball, a flashing yellow ball, a flashing red arrow, or a flashing yellow arrow indication. In addition, the Manual on Uniform Traffic Control Devices allows several PPLT signal display arrangements. The variability in indication and arrangement has led to a myriad of PPLT displays throughout the United States. The level of driver understanding related to each PPLT display type, and the associated impact on traffic operations and safety, has not been quantified. A study was conducted to evaluate the operational characteristics associated with different PPLT signal displays. Specifically, the study quantified saturation flow rate, start-up lost time, response time, and follow-up headway associated with selected PPLT displays. No differences in saturation flow rate and start-up lost time were found due to the type of PPLT signal display. Saturation flow rates ranged from 1,770 to 2,400 vehicles per hour of green per lane and were related to differences in driver behavior between geographic locations. The variation in start-up lost time and response time between locations was primarily related to differences in phase sequence. The flashing red permitted indications were associated with the longest follow-up headway times, since drivers are required to stop before turning left with a flashing red permitted indication. The shortest follow-up headway was associated with the five-section cluster display using a green ball indication.

Potential application of flashing yellow arrow permissive indication in separated left-turn lanes

The consideration being given to the adoption of the flashing yellow arrow (FYA) permissive indication has led to a number of additional studies evaluating potential scenarios in which FYA may be effectively used. One potential scenario is in intersections with wide medians where the left-turn lane and the corresponding signals are separated from the adjacent through and right-turn lanes. Left-turn maneuvers at locations with wide medians typically operate with protected-only left-turn phasing and separate signal displays, as drivers cannot see the adjacent through movement signals; however, some transportation professionals have implemented a flashing red arrow (FRA) that requires drivers to first stop before accepting a gap in the opposing traffic stream. This research quantified driver comprehension of the FYA permissive indications compared with that of the FRA indication for use at locations with wide medians. The research used both driving simulator and static experiments with 264 drivers responding...

Driver Understanding of Simultaneous Traffic Signal Indications in Protected Left Turns

A comprehensive assessment of protected and permitted left-turn (PPLT) signal displays was performed considering safety, operational performance, and driver-understanding measures. The research focused on a study of driver understanding of protected left-turn indications. All currently used PPLT display arrangements and protected indication combinations were evaluated, including those with simultaneous green-arrow and red- or green-ball indications and those with the green-arrow indication only. Driver understanding was evaluated through a computer-based driver survey completed by 2,465 drivers. In total, 73,950 survey responses were received pertaining to the 200 different survey scenarios evaluated, 24,863 pertaining to protected left-turn indications. Findings show that the simultaneous illumination of the green-arrow and red-ball indications in a five-section PPLT signal display during a protected left-turn phase significantly reduces driver understanding and increases driver error. This finding is especially true for drivers over the age of 65. Simultaneous illumination of the green-arrow and green-ball indications also resulted in levels of driver understanding lower than the green-arrow-only indication; however, these differences were not statistically significant.

Drivers' understanding of protected-permitted left-turn signal displays

A comprehensive assessment of protected–permitted left-turn (PPLT) signal displays was performed considering safety, operational performance, and driver understanding measures. The research focuses on a study of driver understanding of permitted left-turn indications. All currently used PPLT display arrangements and permitted indication combinations were evaluated, including those with flashing red and yellow permitted indications. Driver understanding was evaluated through a computer-based driver survey completed by 2,465 drivers. A total of 73,950 survey responses were received pertaining to the 200 different survey scenarios evaluated. The study results indicate that yellow or red flashing permitted indications may lead to higher levels of driver comprehension. Both the flashing red and yellow permitted indications had a significantly higher correct response rate than did the green ball permitted indication. Drivers over the age of 65 found the flashing ball permitted indications easier to comprehend and responded more quickly with fewer fail critical (turning left without the right-of-way) errors. Higher correct response rates with flashing permitted indications were also found in other important demographic groups, including inexperienced drivers and drivers with limited education.

Evaluation of Flashing Yellow Arrow in Traffic Signal Displays with Simultaneous Permissive Indications

The results of a comprehensive study on protected and permissive leftturn (PPLT) operations showed that the flashing yellow arrow (FYA) indication was an acceptable and recommended application for permissive left turns. As documented in NCHRP Report 493, an FYA permissive indication was recommended for implementation in the FHWA Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD). Research findings suggested that the FYA be implemented in an exclusive four-section vertical signal display centered over the left-turn lane, which differs from the most common application ofPPLT signal displays. Many traffic engineers currently use a five-section clustered-arrangement shared signal head to meet MUTCD requirements of two signal heads per major approach. FYA implementation in the five-section display would require an interim retrofit requiring the FYA to be displayed simultaneously with the through-movement circular green (CG), circular yellow, or circular red indication. This research eva...