Protected-permissive Left-turn Research Articles

Safety Evaluation for Conversion From Protected-Only Left-Turn Phasing to Time-of-Day Protected-Permissive Left-Turn Phasing Using Flashing Yellow Arrows

This study investigated the safety effects of the conversion from a protected-only left turn to protected-permissive left turn with flashing yellow arrow (FYA−PPLT) with time-of-day operation. The observational before–after study with the comparison group method was used to develop crash modification factors (CMFs) for the total crashes and two types of target crashes that involve left-turning vehicles on treated approaches (left-turn−same roadway and rear-end crashes). For all potential crashes, crash reports, crash diagrams, and narratives were manually inspected to identify the actual target crashes. The CMFs were separately developed for a full 24-h day and specific time-of-day based on FYA-PPLT operation, and for two severity categories (all severities and fatal & injury). The results showed that the total crashes for 24 h had a slight increase, whereas the target crashes had substantially larger changes (increase in left-turn−same roadway crashes and decrease in rear-end crashes). When looking at the specific time-of-day use of FYA-PPLT, the increase in left-turn−same roadway crashes was significantly higher than the full 24-h period. This implies that engineers could extend protected-only use more on the fringes of the peak periods to mitigate the increase in left-turn-related crashes. The analysis results also showed a trade-off between left-turn–same roadway and rear-end crashes after the signal conversion. The findings reveal how important it is for engineers to properly understand the safety effects on the different categories of target crashes and time periods when considering the signal conversion from a protected-only left turn to FYA-PPLT.

Protected–permissive left turn phasing with flashing yellow arrow signal: study of red intervals for an effective phase transition

Protected–permissive left turn (PPLT) phasing has been popular and widely used in many urban intersections in North America because of its operational benefits. A significant number of intersections have recently been upgraded with four-section signal heads with flashing yellow arrow (FYA) indication for an effective protected–permissive left turn operation. The present study seeks to find appropriate length of two red intervals whose roles are important, but different during the transition period of FYA-PPLT phasing. One is a red interval for delayed-start of permissive left turn movements; the other is additional red interval for delayed-start of opposing through movements. Micro-traffic simulation and conflict analysis are explored to assess the effects of the red intervals on intersection efficiency and safety. A useful reference, which describes the balanced length of the two red intervals under varying traffic levels, is developed as a result.

Impacts of Changing from Permissive/Protected Left-Turn to Protected-Only Phasing: Case Study in the City of Tucson, Arizona

Recent research has focused on the safety or mobility impacts of signal timing. Several studies have compared the choice between a protected-only left turn (PO) and a protected-permissive left turn (PPLT). However, few have compared both the safety and mobility impacts, and their tradeoffs. This study proposed data-driven methods to conduct a pilot study at an intersection in Tucson, Arizona. This study evaluated the impacts on vehicular mobility and multi-modal safety when changing from a PPLT to a PO. First, the daily and annual delay for the through and left-turn movements for the intersection was evaluated using a calibrated delay model and year-long 15-min traffic sensor data. Then, real-world near misses between cyclists, pedestrians, and vehicles were manually collected and analyzed using 48 h of videos. Last, both mobility and safety measures were converted into an annual cost to determine the trade-off between the before (PPLT) and the after (PO) situations. The results of this study demonstrate the feasibility of the proposed methods, providing practitioners with different options to evaluate left-turn phasing strategies effectively and efficiently.

Safety Effects of Flashing Yellow Arrows Used in Protected Permitted Phasing: Comparison of Full Bayes and Empirical Bayes Results

Using the flashing yellow arrow (FYA) signal indication for the permissive portion of protected-permissive left-turn (PPLT) phasing has become an increasingly popular treatment for left-turn signals as drivers are believed to understand the FYA better than the traditional circular green indication. A before-and-after safety evaluation of deploying FYA at PPLT signals at 28 intersections in Virginia was conducted. Each of the study intersections had FYA for the permitted portion of the phase on at least one left-turn approach. The focus was on left-turns that operated in the protected-permissive mode (with circular green indication for the permissive portion) before being converted to PPLT operations with the FYA indication for the permissive portion (PPLT-FYA). Crash records from before and after the activation of FYA were compared using both the full Bayes and empirical Bayes approaches. The results indicate that using the FYA signal indication instead of the circular green indication had a statistically significant effect in reducing overall frequency and severity of crashes. For the intersections studied in this research, total crashes reduced by 12% following conversion from PPLT to PPLT–FYA. The results also indicated that the full Bayes approach to safety effectiveness evaluation can, at a minimum, provide similar results to the well-established empirical Bayes approach. The 95% credible intervals for the expected crash reduction rates estimated with the full Bayes method were generally narrow, suggesting a good degree of confidence in the estimates.

Crash Modification Factors for the Flashing Yellow Arrow Treatment at Signalized Intersections

This paper presents the results of an evaluation of the flashing yellow arrow (FYA) treatment using data from signalized intersections in Nevada, North Carolina, Oklahoma, and Oregon. The evaluation method was an empirical Bayes before–after analysis. The treatments were divided into seven categories depending on the phasing system in the before period (permissive, protected–permissive, or protected), phasing system in the after period (FYA permissive or FYA protected–permissive), the number of roads where the FYA was implemented (one road or both roads), and the number of legs at the intersections (three or four). The first five treatment categories involved permissive or protected–permissive phasing in the before period. Intersections in these five treatment categories experienced a reduction in the primary target crashes under consideration: left turn crashes and left turn with opposing through crashes. The reduction ranged from 15% to 50%, depending on the treatment category. Intersections that had at least one protected left turn phase in the before period and had FYA protected–permissive left turn phase in the after period experienced an increase in left turn crashes and left turn with opposing through crashes, indicating that replacing a fully protected left turn with FYA will likely cause an increase in left turn crashes.

Aggressive Driving Behavior in the Protected/Permissive Left Turn(PPLT) Intersections

Aggressive Driving Behavior in the Protected/Permissive Left Turn(PPLT) Intersections

Safety Performance of Flashing Yellow Arrow for Protected–Permissive Left-Turn Signal Control in Central Illinois

In 2010, the Illinois Department of Transportation began implementing the flashing yellow arrow (FYA) at intersections operating with protected–permissive left-turn (PPLT) control. Research was conducted to evaluate the safety-effectiveness of FYAs at 86 intersections and 164 approaches in central Illinois. The effectiveness evaluation was performed with 3 years of before-and-after FYA installation crash data and the empirical Bayes method. In the before condition, the left-turn signals operated with a circular green display indicating the permissive interval of PPLT control using a five-section signal head. In the after condition, the FYA replaced the circular green display for the permissive interval of PPLT with a four-section signal head. Supplemental traffic signs were mounted on the mast arm adjacent to the left-turn signal at over half of the FYA installations. The results of the comprehensive safety evaluation of the FYA for PPLT control are presented. Analyses were also performed to assess the effects of the FYA supplemental signs and the effects of the FYA overall on two subsets of at-fault drivers: older drivers (age 65+) and younger drivers (age 16 to 21). The resulting mean crash modification factors for the targeted crash types ranged from 0.589 to 0.714. The findings of this research support the continued use of FYAs for PPLT control to improve safety at signalized intersections in central Illinois.

Study of Feasibility Analysis for the Protected-Permissive Left-Turn Signal Control in Three-Leg Signalized Intersections Using a Microscopic Traffic Simulation Model

PURPOSES : This study evaluated the feasibility of implementing protected-permissive left-turn (PPLT) signals at three-leg signalized intersections. METHODS: A three-leg signalized intersection with permissive left-turn was first selected. A VISSIM simulation model was constructed using data collected from the test site. The VISSIM network was calibrated by adjusting related parameter values in order to minimize the difference between the simulated and surveyed critical gap. The calibrated network was validated by the number of waiting left-turning vehicles per cycle. Finally, the mobility and safety measures were extracted from simulation runs in which permissive, protected left turns as well as PPLTs were realized based on diverse traffic volume scenarios. RESULTS : The mobility-related measures of effectiveness (MOEs) of the case with PPLT outperformed the other two left-turn treatment scenarios. In particular, the average waiting time per cycle for the left-turn vehicles in the case with PPLT was reduced by 30 s. The safety-related MOEs of the case with PPLT were somewhat higher than those in the case with protected left-turns and much higher than those in the case with permissive left-turns. CONCLUSIONS : Based on the mobility- and safety-related MOEs generated from the VISSIM simulation runs, the use of PPLT seems to be feasible at three-leg signalized intersections where the left-turn is permissive and a pedestrian signal exists at the conflicting approach. However, in order to use the PPLT in earnest, it is necessary to revise the road traffic act, traffic signs, and related manuals.

Safety and Operational Impacts of Optional Flashing Yellow Arrow Delay

Considerable debate exists over whether to delay the start of the flashing yellow arrow (FYA) signal indication (e.g., with a red arrow signal indication) during the transition from a protected movement to a permissive movement in a leading left protected–permissive left turn. The 2009 Manual on Uniform Traffic Control Devices is silent on this topic. This paper reviews the state of the practice on the use of the optional FYA delay through a literature review, a survey of state departments of transportation, and consultations with practitioners and national experts. A simulation study was also used to assess the potential impacts of the optional FYA delay on traffic safety and operations. The state-of-the-practice review indicated some preference for the use of the red arrow signal indication because of the perceived safety benefit. For example, 71% of the responding state departments of transportation that use the FYA also delayed the start of the FYA. The simulation analysis showed significant safety benefits in a delay to the start of the FYA signal indication for all scenarios except the scenario with a low opposing through traffic volume, with no significant negative impacts on average delay, average queue length, or average stopped delay for either left-turning traffic or the intersection as a whole being found.

Behavior of Left-Turning Drivers during Permissive Interval of Protected–Permissive Operation

A study was conducted to assess the differences in operations and driver behavior in response to three signal displays for the permissive interval of a protected–permissive left-turn (PPLT) phase. A comparative two-phased approach based on field observations and video recordings of driver behavior was used to conduct the research. Phase 1 examined driver behavior at six intersection approaches in Peoria, Illinois, that operated with different permissive left-turn displays: circular green (CG), flashing yellow arrow (FYA), and flashing red arrow (FRA). The CG indication had been the predominant indication used for the left-turn permissive indication in Peoria until 2010, when FYAs were installed throughout the city. FRAs are operated at two adjacent intersections in Peoria and were installed as a site-specific countermeasure. The Phase 1 findings showed that the highest proportion of drivers' safe and efficient actions was in response to the FYA as compared with the CG and FRA displays. In Phase 2, a comparative analysis of four intersection approaches was conducted to determine the impact on driver understanding of the permissive left-turn CG display in two geographic areas: one in Peoria, where multiple permissive left-turn signal indications for the PPLT phase (CG, FYA, and FRA) are currently in use, and one in Blooming-ton, Illinois, where only the CG indication is used. The Phase 2 research found, on the basis of observed safe and efficient driver actions, that there were no differences in driver understanding of the CG permissive left-turn display at the study approaches in the two cities.

Protected–Permissive Left-Turn Signal Control Mode: New Analytical Approach to Estimate Operational Benefit and Safety Cost

Protected–permissive left-turn (PPLT) and protected only (PO) left-turn signal control modes have been widely used at signalized intersections. The selection between these two control modes is a complicated process in which practitioners need to consider safety and operational impacts. In this study researchers developed analytical models for estimating the operational benefits and the safety risks associated with the use of the PPLT control mode. Evaluation results demonstrated that the proposed models could provide accurate and reliable estimates for the delay reduction and the potential conflicts caused by the use of the PPLT mode. Results of this study will be useful for developing quantitative criteria for the selection between the PO and PPLT modes.

Evaluation of Traffic Signal Displays for Protected-Permissive Left-Turn Control Using Driving Simulator Technology

The objective of this research was to determine the safety and effectiveness of selected protected/permissive left-turn (PPLT) signal displays through driver comprehension evaluations. Driver evaluations were conducted using full-scale, fixed-base, fully-interactive driving simulators located at the University of Massachusetts and the Texas Transportation Institute. The PPLT displays were also evaluated in a static environment to provide comparison data. The 12 experimental PPLT signal displays evaluated differed in permissive indication, arrangement, location, and through movement indication. Each PPLT signal display included only the circular green ball and/or flashing yellow arrow permissive indications. Driver comprehension was determined from the distribution of correct and incorrect responses. Findings from the driving simulator study showed a high level of comprehension (91%) with no variation between PPLT displays. The lack of significant differences in driver comprehension is itself a significant finding, indicating the flashing yellow arrow is an alternative to the circular green ball permissive indication.

Driving Simulators for Evaluation of Novel Traffic-Control Devices: Protected–Permissive Left-Turn Signal Display Analysis

A study evaluated drivers’ comprehension of several experimental five-section protected–permissive left-turn (PPLT) signal displays. A full-scale driving simulator and static driver study were used. Study methods were compared while evaluating drivers’ comprehension and response to various combinations of five-section PPLT signal-display arrangements (horizontal, vertical, and cluster) and permissive left-turn indications (green ball, flashing red ball, flashing yellow ball, flashing red arrow, and flashing yellow arrow). The results showed that the type of five-section PPLT signal display arrangement has very little effect on driver comprehension of the permissive left-turn maneuver. The type of permissive indication used in five-section PPLT signal displays had a significant effect on driver comprehension, since the green ball, flashing yellow ball, and flashing yellow arrow were the best understood. When combining five-section PPLT signal-display arrangements and permissive indications, the five-section horizontal arrangement with a flashing-yellow-ball permissive indication had the highest level of driver comprehension. The lack of surrounding driving cues in the static driver study led to significantly higher fail-critical (serious) response rates. The green-ball permissive indication had a driver comprehension rate over 30% lower in the static study, clearly showing that drivers do not correctly comprehend the meaning of the green ball (assume it is protected) and use other information to make left-turn decisions while driving. The findings of this research show that driving simulation provides an effective study method and effectively replicates the actual driving environment. Simulators should be considered when conducting driver comprehension analyses.