Protected Left-turn Phases Research Articles

Safety Evaluation of Dual Left-Turn Lane Installations in North Carolina

Crash modification factors (CMFs) were calculated for the conversion of a single left-turn lane to a dual or double left-turn lane (DLTL). Despite their proliferation throughout urban and suburban traffic networks, the safety performance of intersection approaches with DLTLs has not been thoroughly investigated and documented within the library of transportation safety research. To date, no published research has been completed that recommends a CMF for DLTL installations. This research effort lays a foundation for the understanding of the before-after safety effect of this countermeasure. A sample of 36 signalized intersections in North Carolina were investigated that received DLTLs between 2004 and 2021. Of these intersections 18 received their DLTLs with no other accompanying geometric changes and were operating with protected left-turn phasing along the treated approaches before and after their DLTL installations. A before-after evaluation of these 18 sites using an empirical bayes methodology yielded the following CMFs: 0.974 (total crashes), 0.844 (fatal-and-injury crashes), 1.010 (property damage only crashes), 0.831 (frontal impact crashes), 0.951 (rear end crashes), and 1.241 (sideswipe crashes). This research recommends that the CMFs for fatal-and-injury crashes and property damage only crashes be used in future cost–benefit calculations when planning DLTL installations.

Evaluation of Protected Left-Turn Phasing and Leading Pedestrian Intervals Effects on Pedestrian Safety

Pedestrian safety is an important public health issue for the United States, with pedestrian fatalities representing approximately 16% of all traffic-related fatalities in 2016. Nationwide, transportation agencies are increasing their efforts to implement engineering-based improvements that increase pedestrian safety. These agencies need statistically rigorous crash modification factors (CMFs) to demonstrate the safety effectiveness of such countermeasures, and to apply in benefit–cost analyses to justify their implementation. This study focused on developing CMFs for two countermeasures that show promise for improving pedestrian safety: protected or protected/permissive left-turn phasing, and leading pedestrian intervals (LPIs). Data were acquired from four North American cities that had installed one or both of the countermeasures of interest: Chicago, IL; New York City, NY; Charlotte, NC; and Toronto, ON. The empirical Bayes before–after study design was applied to estimate the change in expected crash frequency for crashes following treatment. The protected left-turn phasing evaluation showed a benefit in reducing vehicle–vehicle injury crashes, but did not produce statistically significant results for vehicle–pedestrian crashes. For those crashes a disaggregate analysis did reveal that this treatment could be especially beneficial where pedestrian volumes exceed 5,500 per day. The LPI evaluation showed a statistically significant reduction in vehicle–pedestrian crashes with an estimated CMF of 0.87.

Signal control method and performance evaluation of an improved displaced left-turn intersection design in unsaturated traffic conditions

ABSTRACT This paper proposes an improved layout for the Displaced Left-turn (DLT) intersection by combining the conventional DLT intersection with protected left-turn phases, based on the safe-crossing requirements of pedestrians and non-motorized vehicles. The delay and stops are weighted to form an integrated performance index (PI) in a real-time vehicle-to-infrastructure communication environment. The PI models, pertaining to all vehicles in non-DLT lanes, DLT lanes and shared through-right lanes are established based on the improved DLT intersection in unsaturated traffic conditions. In addition, a dynamic optimized method of traffic signal timing parameters is constructed based on minimizing average PI per vehicle and considering the traffic signals’ coordination at the main intersection and left-turn crossovers. The operational performance of the optimized method is validated using data collected at an intersection in Harbin, China.

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.

Increasing the capacity of signalized intersections with left-turn waiting areas

One of the most complex issues for the design of at-grade signalized intersections is accommodating left-turn (LT) movements, especially when approaches have insufficient available spatial resources. In this study, we mitigated this problem by reorganizing left-turning traffic flows within intersections through the use of a left-turn waiting area (LTWA). We proposed a series of design pattern left-turn waiting areas for different combinations of spatial and temporal treatments of left-turn movements: exclusive left-turn lanes with protected left-turn phasing, exclusive left-turn lanes with permitted left-turn phasing, and shared left-turn lanes with permitted left-turn phasing. Based on probability theory, an analytical procedure is developed for estimating the capacity of shared and short lanes. Explicit VISSIM simulations are conducted to validate the accuracy of the proposed capacity models, and the impact of design parameters for the proposed system on the left-turn capacity are studied. On the basis of the analyses, benefits of the proposed system are identified, and the domain of application where these benefits are most significant is identified. In addition, optimal LTWA design scheme and critical LT volumes of exclusive LT lane and protected LT phase with different LTWA schemes are presented from the operation efficiency perspective.

Optimizing signal phase plan, green splits and lane length for isolated signalized intersections

At an isolated signalized intersection, short left-turn lanes may be provided to enhance intersection capacity and level of service. The capacity of an entire intersection depends not only on the effective green time per phase and the length of each short left-turn lane but also on the selected signal phase plan. This paper enumerates 16 typical signal phase plans for a four-leg intersection with protected left-turn phases. Given the existence of short left-turn lanes and the freedom to select signal phase plans, two optimization models are formulated. Numerical examples are carried out to illustrate the application of these models and show the sensitivity of the outcomes to impact factors. The orthogonal experiments show that the primary factor is whether a short left-turn lane is added on an approach and the demand distribution is the secondary factor in determining an optimal allocation of the time-space resources of an intersection. To analyse traffic flow operations under different signal phase sequences, the simulation tests are fulfilled using VISSIM under the assumption that the components of a signal phase plan and green splits are all identical. The simulated results indicate that the signal phase sequence for a specified approach may greatly affect traffic movements at an approach, and the leading green phasing is better than the lagging one for a specified approach when the left-turn bay length is short at the approach. Finally, two variations of the developed models are recommended for use and the procedure for the model application is provided in practice.

A simulation-based approach in determining permitted left-turn capacities

A fundamental objective of traffic signal operations is the development of phasing plans that reduce delays while maintaining a high level of safety. One issue of concern is the treatment of left-turn phasing, which can operate as a protected movement, a permitted movement yielding to conflicting traffic, a combination protected–permitted movement or as a split-phase intersection. While protected-only movements can improve safety for the turning movement, they can also increase delays and congestion at the intersection. Most states maintain independent guidance for determining left-turn phasing; however, the most common identified guidance for protected left-turn phases is using a threshold based on the cross product of the left-turn volume and opposing through movements. The use of the cross product has been questioned recently as an indicator for determining phase selection. Based on simulation analysis within this research, the cross product is shown to be a poor indicator of left-turn capacity and congestion at the intersection.This research proposes a simplified single variable exponential model to determine left-turn capacity based on opposing volume and percent green time to determine left-turn capacity thresholds for protected left-turn phasing. The model is developed based on observed capacity from 450 VISSIM microsimulation scenarios which evaluated varying opposing volume, opposing number of lanes, cycle lengths and green time splits. Validation of the model based on complex Highway Capacity Manual procedures, indicates that the proposed model provides similar correlation to observed capacities. Finally, a nomograph is developed which presents the model in a simple form for interpretation and application by practicing traffic engineers, when required to determine left-turn phasing options. This procedure allows simple determination based on minimum input data needs similar to the cross product determination, without the need for complex hand calculations or computing requirements of the Highway Capacity Manual.

Capacity Matching Based Model for Protected Left Turn Phases Design of Adjacent Signalized Intersections Along Arterial Roads

A protected left turn phase is often used at intersections with heavy left turns. This may induce a capacity gap between adjacent intersections along the arterial road among which only parts of intersection are with protected left turn phase. A model for integrated optimization of protected left turn phases for adjacent intersections along the arterial road is developed to solve this problem. Two objectives are considered: capacity gap minimization and capacity maximization. The problems are formulated as Binary-Integer-Linear-Programs, which are solvable by standard branch-and-bound routine. A set of constraints have been set up to ensure the feasibility of the resulting optimal left turn phase type and signal settings. A field intersections group of the Wei-er Road of Ji’nan city is used to test the proposed model. The results show that the method can decrease the capacity gap between adjacent intersections, reduce the delay as well as increase the capacity in comparison with the field signal plan and signal plan optimized by Synchro. The sensitivity analysis has further demonstrated the potential of the proposed approach to be applied in coordinated design of left turn phases between adjacent intersections along the arterial road under different traffic demandpatterns.

Assessing Safety Improvements to Pedestrian Crossings Using Automated Conflict Analysis

Surrogate safety measures, such as the traffic conflict technique, have been promoted as an alternative or complementary approach to evaluate road safety from a broader perspective than collision statistics alone. The before-and-after evaluation of safety treatments is one application that can significantly benefit from the use of surrogate safety measures. This study demonstrated the use of automated traffic conflict analysis to conduct before-and-after safety evaluations. The objective was to conduct a time-series (before-to-after) safety evaluation for an intersection in Surrey, British Columbia, Canada, where several pedestrian-related countermeasures were implemented. Treatments included one or more of the following: use of protected-only turns for left-turning vehicles, installation of pedestrian countdown timers, crosswalk realignment, and use of drop-down sidewalks. Results indicated a significant decrease in both pedestrian conflict frequency and severity at the intersection following the treatments. A greater reduction of conflicts was exhibited on the western portion of the intersection where several countermeasures were implemented, including a protected left-turn phase, dual drop-down ramps, and a shift in the crosswalk further west. This variation in conflict reduction along different segments of the same intersection gives a particularized view of the impact of the adopted countermeasures. The outcome of this research provides evidence that surrogate safety indicators can be used effectively to diagnose safety problems and evaluate countermeasures at intersections.

Improving Intersection Behavior through Delay-Based Left-Turn Phase Initiation

Serving protected left-turn phases for one or two vehicles can often be an inefficient use of cycle green time when the opposing through movements are over capacity. This paper assesses the performance of an intersection at which controller logic is applied to delay the call for a protected left-turn phase on the basis of vehicle wait times. During four weeks of evaluation, the delay on left-turn phase calls was varied in 25-s increments from 0 to 75 s. The results indicate that delaying left-turn phase initiation substantially increases the amount of green time for saturated through movements while minimally increasing the travel delay for left-turning drivers. The recommendation is made for agencies to consider using a delay in the range of 25 to 50 s for calling protected phases at intersections at which the opposing through movement is oversaturated and could benefit from additional green time. This paper presents one of the first quantitative studies evaluating potential intersection capacity and performance improvements with respect to left-turn detector delay.

Effects of Left-Turn Waiting Areas on Capacity and Level of Service of Signalized Intersections

Transportation professionals in China have started using an unconventional left-turn waiting area design as an innovative approach to mitigate traffic congestion at signalized intersections. The waiting area is set up beyond the stop bar at single or dual left-turn lanes at signalized intersections with lagging protected left-turn phases. This paper evaluated the effects of left-turn waiting areas on capacity and level of service of exclusive left-turn lanes at signalized intersections using empirical data. Capacity and delay models were developed by analyzing the arrival and discharge patterns of left-turning vehicles at single and dual left-turn lanes with differently sized left-turn waiting areas. The saturation headway, start-up lost time, and clearance time for left-turning passenger cars were estimated with field data measured from 22 sites. The capacity and delay models were calibrated and validated against both field-measured and simulated data. The results of sensitivity analyses showed that left-turn waiting areas increase the capacity of left-turn lanes at signalized intersections, and the capacity gains will increase with an increase in the storage capacity of the left-turn waiting areas. A procedure was proposed to determine the level of service of left-turn lanes with left-turn waiting areas at signalized intersections.

Implications of Distracted Driving on Start-Up Lost Time for Dual Left-Turn Lanes

Previous research has found that distracted driving measurably increases driver response times to unanticipated roadway hazards. These instances are of particular consequence as they tend to be highly correlated with vehicle crashes resulting in property damage and/or injury. However, comparatively little attention has been allocated to quantifying the negative impacts of distracted driving on driver reaction to anticipated stimuli. This study empirically determined the impact of distracted driving on queue discharge rates at signalized intersections with protected left turn phases for dual left turn lanes. Observational studies were conducted at 11 intersection approaches at six signalized intersections in three states resulting in the observation of 844 distracted and 3,726 undistracted left turning drivers. A statistical model was developed for the startup lost time resulting from the average headways of the samples in each state. The difference in startup lost times between queues comprised entirely of distracted or undistracted drivers

Coordinated Optimization of Signal Timings for Intersection Approach with Presignals

Many congested intersections have heavy traffic volume on movements for which there is insufficient capacity because of geometric limitations. Installing presignals at midblock locations and reorganizing traffic upstream of the approach of an intersection combine to be a promising and cost-effective method for addressing these capacity limitations. A coordinated optimization model was developed for an isolated intersection approach with presignals to increase the protected left-turn phase capacity. The presignal model was based on two principles: (a) explicitly capture the interaction between the presignal and the main signal by modeling the queuing process and capacity constraints of temporal and spatial limitations of the intersection and (b) optimize the signal timings of both the presignal and the main signal as well as the offset between them to produce the best operational strategy for the approach. The minimum green time required and the delay-minimization problems are considered. Extensive experimental analysis has shown that the presignal model outperforms the conventional control method (without presignal). Sensitivity analysis of the signal timing method that will assist traffic engineers with selecting the appropriate length of the sorting area, phase sequence, and early starting time of presignals was conducted. The results from the study offer a basis for traffic practitioners, researchers, and authorities on which to design and utilize presignals in locations where there is a need to increase intersection capacity for congested movements.

Safety Effectiveness of Selected Treatments at Urban Signalized Intersections

This study conducted a before–after evaluation by means of the empirical Bayes methodology for four types of treatments at signalized intersections with data from Winston-Salem, North Carolina. The results indicated that changing to protected left-turn phasing from permissive or permissive–protected phasing could lead to a virtual elimination of left-turn crashes but other crashes, which were likely to be less severe, could increase. Conversion from nighttime flashback to regular phasing seemed effective in reducing nighttime crashes. Replacing 8-in. signal heads with 12-in. heads seemed effective in reducing right-angle crashes, but this measure could increase other, less-severe crashes. Adding another red-signal lens to an existing one or changing from permissive to permissive–protected left-turn phasing did not seem particularly effective in reducing crashes, but these results were not definitive because they were based on a limited number of sites. Further research using data from other jurisdictions is needed, so that more definitive conclusions can be made about the safety effectiveness of these treatments.

Ordered Probit Modeling of User Perceptions of Protected Left-Turn Signals

There is an absence of nationally accepted warrants for left-turn phasing at signalized intersections. Some guidelines recommend a protected left-turn phasing for turns with three or more opposing through lanes. However, there is no research that justifies this recommendation. A Web-based motorist survey was conducted to find how motorists perceive difficulty and their preference for protected left-turn signalization vis-a-vis the number of opposing lanes. The ordered probit modeling of 2,017 responses suggests that driver perception of difficulty to make turns without protected left-turn signals increases with intersection size. As high as 91% of the respondents stated that they much or extremely prefer a protected left-turn signal at an intersection where left-turn vehicles have to cross three lanes of opposing through traffic. The ordered probit models also provide indications of the effects of various sociological variables on the probabilities of driver perceptions of difficulty and preference. The estimated models provide a sensitive tool for making decisions about the left turn treatment at signalized intersections.

Implementing Red Light Camera Programs: Guidance from Economic Analysis of Safety Benefits

Red light camera (RLC) systems are believed to decrease the right-angle crashes that they are targeting but to have the undesirable side effect of increasing rear-end crashes. This belief was confirmed in a before-after study of 132 RLC installations in seven U.S. jurisdictions, as reported in a companion paper. In that research, the extent to which the increase in rear-end crashes negates the benefits for right-angle crashes was unclear, given the perceptions of severity differences in the two crash types. This paper reports on an examination of the changes in crash costs, based on a consideration of rear-end and right-angle unit crash costs for various severity levels, to establish the aggregate effects of the RLC programs evaluated. Part of the project derived the required unit costs by using information from national U.S. databases. The overall results show a modest to moderate economic benefit of between $28,000 and $50,000 per treated site year, depending on assumptions made. The ability to aggregate economic costs across crash types and severity created the opportunity to try to isolate program implementation factors and intersection characteristics that would favor the installation of RLC systems by using the aggregate economic benefit at each RLC site as the outcome variable. This investigation found, for example, that the greatest economic benefits are associated with the highest total entering annual average daily traffic, the largest ratios of right-angle to rear-end crashes, and the presence of protected left-turn phases.

Implementing Red Light Camera Programs

Red light camera (RLC) systems are believed to decrease the right-angle crashes that they are targeting but to have the undesirable side effect of increasing rear-end crashes. This belief was confirmed in a before–after study of 132 RLC installations in seven U.S. jurisdictions, as reported in a companion paper. In that research, the extent to which the increase in rear-end crashes negates the benefits for right-angle crashes was unclear, given the perceptions of severity differences in the two crash types. This paper reports on an examination of the changes in crash costs, based on a consideration of rear-end and right-angle unit crash costs for various severity levels, to establish the aggregate effects of the RLC programs evaluated. Part of the project derived the required unit costs by using information from national U.S. databases. The overall results show a modest to moderate economic benefit of between $28,000 and $50,000 per treated site year, depending on assumptions made. The ability to aggregate economic costs across crash types and severity created the opportunity to try to isolate program implementation factors and intersection characteristics that would favor the installation of RLC systems by using the aggregate economic benefit at each RLC site as the outcome variable. This investigation found, for example, that the greatest economic benefits are associated with the highest total entering annual average daily traffic, the largest ratios of right-angle to rear-end crashes, and the presence of protected left-turn phases.

Signalized Intersection Level of Service Incorporating Safety Risk

A methodology that quantifies potential conflicts between left-turning vehicles and opposing through vehicles and pedestrians is presented. The methodology was based on and designed to be compatible with the Highway Capacity Manual 2000. A model was developed to combine delay and safety to get a comprehensive level-of-service indicator, the delay and safety index (DS). A case study of two intersections shows that if potential conflict is not considered, the signal timing plan with permitted left turns delivers a better level of service than that with protected left turns. However, if potential conflict is considered, the level of service under protected left-turn phasing is better than that under permitted left-turn phasing, according to the DS, when the safety weight factors exceed a certain value. The proposed method models the trade-off between safety and efficiency explicitly and considers both vehicle-to-vehicle and vehicle-to-pedestrian conflicts associated with left turns.

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.

New approach for developing warrants of protected left-turn phase at signalized intersections

The research embodied in this paper presents a new approach for the development of guidelines for the installation of a protected left-turn phase at signalized intersections when permissive-only left-turn operation is present. This approach is based on maintaining intersection traffic operation at optimum efficiency. Three analyses were presented and discussed and they involved the use of the new approach on some hypothetical basic scenarios at a four-legged intersection with single lane in each approach. The first scenario involved exclusive left-turn lane operation while the other two scenarios involved shared-lane operation. Exhaustive signal optimization analyses were conducted using a signal optimization software package called “Signal Expert”. Regression models were developed from optimization results that allow the analyst to make the decision on protected left-turn phase installation using the basic input data of signal timing design without the need to perform field measurements. The regression results showed that the transition from permissive to protected/permissive left-turn operation, based on system optimization, is mainly a function of traffic conditions and that this transition (interface) is predictable. The results also suggested that these warrants are of reasonable accuracy when compared with those in the current practice.