Protected Left Turn Research Articles

Hierarchical Driving Strategy for Connected and Autonomous Vehicles Making a Protected Left Turn at Signalized Intersections

Left-turn execution in autonomous driving at urban intersections is often complex and characterized by unpredicted events, such as vehicles speeding and running a red light. Despite these hazards, autonomous vehicles must drive through intersections safely and efficiently. To solve this problem, a new hierarchical driving strategy (HDS) is proposed for connected and autonomous vehicles making a protected left turn at signalized intersections, which combines the rule-based method and deep reinforcement learning (DRL). The high level of the HDS is the rule-based decision-making module, and the low level is the driving skill, which is dependent on the status. Specifically, the vehicle status when turning left is divided into safe and alert statuses. Further, DRL is used to train the driving skill of the vehicle for each status. The HDS design effectively combines the advantages of end-to-end driving. Moreover, the algorithm was experimentally evaluated in multiple protected left-turn scenarios. Compared with the pure rule-based method, the HDS achieved a 34% reduction in failure rate in the test scenario, and the driving behavior of the autonomous vehicle employing HDS was more intelligent. Moreover, the HDS is highly robust to complex scenarios.

Development and Evaluation of the Dynamic Left Turn Intersection

Dual left turn lanes are typically operated with protected left turn signals, which means extra delay during non-peak hours compared to permissive operation. To reduce this delay, the authors designed, installed, and tested a “dynamic left turn intersection” (DLTi), which is a new way to operate dual left turn lanes. With DLTi, the both lanes are operated with protected phasing during the peak hours when higher capacity is needed, and only the leftmost left turn lane remains open for protected-permissive operation during off-peak hours. The test showed substantial delay savings, the crash experience has been minimal, and the public comments received have been predominantly negative but not overwhelming. Around 85 percent of left-turning motorists complied when the rightmost left turn lane was closed. The team believes that the DLTi test has been a success, and that agencies should begin searching for other suitable locations for DLTi installation.

Engineering Countermeasures for Left Turns at Signalized Intersections: A Review

Left turn crashes can impact the safety of the drivers due to the speed and angle at which they occur. Left turns are specifically reported to affect older drivers more than the other types of crashes. This paper provides a review of the existing engineering countermeasures that have been evaluated to improve driver safety at left turns. Twentyeight studies on left turn signal displays (protected left turns, flashing yellow arrow, and digital countdown timers), intersection geometry (offset left turn lanes, diverging diamond interchange, roundabouts, exit lanes for left turn, left turn bay extension, and contraflow left turn lanes), and driver warning systems (infrastructure warning systems, and in-vehicle warning systems) are reviewed. Eighteen studies were evaluated in the field, nine in laboratory environments, and one online. All countermeasures demonstrated varying levels of effectiveness. We found protected left turns, roundabouts, and warning systems to be the most effective engineering countermeasures. Advantages and disadvantages of each countermeasure and research shortcomings of the evaluation studies are discussed. Review findings may help practitioners and researchers guide more effective countermeasures for left turns for older drivers.

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.

Assessing the Effects of Indirect Left Turn on a Signalized Intersection Performance: A Case Study for the Tehran Metropolitan Area

Many signalized intersections are characterized with frequent left-turn moves. Vehicles waiting for a protected left turn may form long queues, which will increase the intersection delay and negatively impact the network performance. Researchers and practitioners across various countries underline that access management leads to a smoother traffic flow. One way of access management at intersections is to eliminate the direct left-turn maneuver. This study aims to evaluate how the traffic conditions will be affected from replacing the direct left turn with the right-turn U-turn maneuver at intersections. In case of the right-turn U-turn maneuver, a vehicle turns right instead of making the left turn and travels either to the median opening or to the next intersection to make a U-turn. Two simulation models are built using the Synchro Studio and Aimsun simulation software packages based on the data, collected from one of the busiest intersections in Tehran (Iran), to quantify the effects of replacing the direct left turn with the right-turn U-turn maneuver on the intersection and network performance. Results of a comprehensive simulation analysis indicate that the proposed access management treatment not only significantly reduces the total vehicle queue length and the total delay at the considered intersection, but also decreases the total network delay and the total travel time. Furthermore, elimination of the direct left turn increases the number of vehicles entering the network.

Start-Up and Acceleration Profile for Protected Left-Turn Vehicles at Signalized Intersections

AbstractThe law of start-up, acceleration profile and leave-out for queuing vehicles at signal intersections is a fundamental question for the queuing theory, traffic simulation and etc in traffic engineering. This paper investigates the vehicular flow at two protected left-turns by video cameras installed at signal intersections, and checks the accuracy of speed processing software. Based on the statistical analysis and the method of curve fitting, the research team establishes the model of start-up and speed-time profile for the queuing vehicles in the protected left-turns at signal intersections. Then the leave-out time of vehicles is calculated based on the established formulas in these models, which is consistent with the observed leave-out time of vehicles by the video. The developed models proved to be accurate. The paper concludes that the queuing vehicles follow by a linear start after the green light, and the speed-time profile indicates an “S” shape from vehicles starting to reach the stable saturation flow. The speed and the queue sequence of first vehicle of saturation flow are different at different intersections, but the law of start-up of vehicles is the same: The same reaction time for the first vehicle and the same linear start-time interval for the adjacent vehicle in the queue. The relationship of leave-out time and the queue sequence of vehicle is nonlinear before the saturation flow. According to the models developed in this paper, the queue sequence of first vehicle of saturation flow and the law of leave-out time of vehicles can be calculated accurately, which is the fundamental for the queuing theory and the traffic simulation.

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.

Critical older driver errors in a national sample of serious U.S. crashes

ObjectiveOlder drivers are at increased risk of crash involvement per mile traveled. The purpose of this study was to examine older driver errors in serious crashes to determine which errors are most prevalent. MethodsThe National Highway Traffic Safety Administration’s National Motor Vehicle Crash Causation Survey collected in-depth, on-scene data for a nationally representative sample of 5470 U.S. police-reported passenger vehicle crashes during 2005–2007 for which emergency medical services were dispatched. There were 620 crashes involving 647 drivers aged 70 and older, representing 250,504 crash-involved older drivers. The proportion of various critical errors made by drivers aged 70 and older were compared with those made by drivers aged 35–54. ResultsDriver error was the critical reason for 97% of crashes involving older drivers. Among older drivers who made critical errors, the most common were inadequate surveillance (33%) and misjudgment of the length of a gap between vehicles or of another vehicle’s speed, illegal maneuvers, medical events, and daydreaming (6% each). Inadequate surveillance (33% vs. 22%) and gap or speed misjudgment errors (6% vs. 3%) were more prevalent among older drivers than middle-aged drivers. Seventy-one percent of older drivers’ inadequate surveillance errors were due to looking and not seeing another vehicle or failing to see a traffic control rather than failing to look, compared with 40% of inadequate surveillance errors among middle-aged drivers. About two-thirds (66%) of older drivers’ inadequate surveillance errors and 77% of their gap or speed misjudgment errors were made when turning left at intersections. When older drivers traveled off the edge of the road or traveled over the lane line, this was most commonly due to non-performance errors such as medical events (51% and 44%, respectively), whereas middle-aged drivers were involved in these crash types for other reasons. Gap or speed misjudgment errors and inadequate surveillance errors were significantly more prevalent among female older drivers than among female middle-aged drivers, but the prevalence of these errors did not differ significantly between older and middle-aged male drivers. These errors comprised 51% of errors among older female drivers but only 31% among older male drivers. ConclusionsEfforts to reduce older driver crash involvements should focus on diminishing the likelihood of the most common driver errors. Countermeasures that simplify or remove the need to make left turns across traffic such as roundabouts, protected left turn signals, and diverging diamond intersection designs could decrease the frequency of inadequate surveillance and gap or speed misjudgment errors. In the future, vehicle-to-vehicle and vehicle-to-infrastructure communications may also help protect older drivers from these errors.

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.

Use of Decision Assistance Curves in Advanced Warrant Analysis for Indirect Left-Turn Intersections

This paper develops decision assistance curves (DACs) to compare delay-based performance measures for three indirect left-turn (ILT) intersections—median U-turn, continuous-flow intersection, and jughandle—relative to a conventional signalized intersection. The DACs consist of two graphical tools: (a) a DAC classifier and (b) two sets of DAC contours. DAC classifier plots are used to select the intersection type that produces the minimum system average delay for a specified main and cross-street volume configuration. DAC contour plots are used to estimate the system average delay difference between a chosen ILT and a conventional signalized intersection and to estimate the increase in average delay compared with a conventional signalized intersection for the movement most negatively affected. These tools can be used by planners, engineers, or other decision makers to identify visually the intersection type that provides the least average system delay under given volume conditions and estimated trade-offs for specific intersection types. The conventional signalized intersection, with protected left turns, was never optimal under studied scenarios. This finding implies that, for all the studied conditions, at least one ILT or permitted left-turn alternative produces lower delay than the conventional signalized intersection.

Task analysis of transit bus drivers’ left-turn maneuver: Potential countermeasures for the reduction of collisions with pedestrians

Left-turn maneuvers by transit buses are associated with a particularly high rate of collisions with pedestrians. The primary objective of the current study was, through task analysis, to gather a detailed outline of all mental and perceptual tasks that transit bus drivers complete when making a left turn at a signalized intersection. As a secondary objective, based on the task analysis examination, we proposed several countermeasures for the reduction of collision between transit buses and pedestrians. The task analysis revealed that transit bus drivers engage in various tasks during all stages of a left-turn maneuver. The results also showed that the task demand is at a peak during the performance of one of the most critical tasks – detecting pedestrians. Non-driving tasks that transit bus drivers are required to perform (e.g., collecting fares, providing directions to passengers) further increase the tasks demands. Based on the results of the task analysis, we proposed several potential solutions for the reduction of collisions between transit buses and pedestrians. The proposed countermeasures included a technology-based (collision warning system for both drivers and the pedestrians) and infrastructure-based (protected left turn signal for buses only) solutions, as well as examination of individual factors (e.g., stress coping ability) and the relationship with different aspects of driving performance.

Impact of Intersection Design on Pedestrians’ Choice to Cross on Red

The existing research and modeling applications for predicting pedestrians’ decision to cross on red at signalized intersections have most commonly modeled this decision as a function of pedestrian characteristics (such as age and mobility restrictions), traffic conditions, and signal delay. For this study, field observations suggested that the physical configuration of the crosswalk should also be an important predictor of the decision to cross on red, because pedestrians should be less likely to cross on red at major intersections with longer crosswalks. In addition, the overall configuration and complexity of the signal also have an impact on signal compliance. The study tested these hypotheses by collecting video data of pedestrian behavior at signalized intersections in Washington, D.C., and by building a binary logit model to predict the probability of crossing on red. The study found that certain signal phases, such as protected left turns, were strongly correlated with a higher probability of crossing on red. The study also found that the length of crosswalks was inversely correlated with the probability of crossing on red. Through tests of the model's predictive accuracy under various specifications, the design variables (e.g., geometry and types of signal phases) were found to be key in improving the model's predictive ability. The findings offer insights for the design of signalized intersections and shed light on the complexity of the relationship between crossing on red and pedestrian safety, because some characteristics that are associated with a lower incidence of pedestrian crashes are also associated with more crossing on red.

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

Exposure to lateral collision in signalized intersections with protected left turn under different traffic control strategies

This paper proposes an original definition of the exposure to lateral collision in signalized intersections and discusses the results of a real world experiment. This exposure is defined as the duration of situations where the stream that is given the right-of-way goes through the conflict zone while road users are waiting in the cross-traffic approach. This measure, obtained from video sensors, makes it possible to compare different operating conditions such as different traffic signal strategies. The data from a real world experiment is used, where the adaptive real-time strategy CRONOS (ContRol Of Networks by Optimization of Switchovers) and a time-plan strategy with vehicle-actuated ranges alternately controlled an isolated intersection near Paris. Hourly samples with similar traffic volumes are compared and the exposure to lateral collision is different in various areas of the intersection and various traffic conditions for the two strategies. The total exposure under peak hour traffic conditions drops by roughly 5min/h with the CRONOS strategy compared to the time-plan strategy, which occurs mostly on entry streams. The results are analyzed through the decomposition of cycles in phase sequences and recommendations are made for traffic control strategies.

Geometric Models to Calculate Left-Turn Sight Distance for Signalized Intersections on Horizontal Curves

At signalized intersections where protected left turn signals are not provided, simultaneous opposing left-turn vehicles may cause insufficient sight distance for each other. There is a lack of literature presenting left-turn sight distance analysis at intersections located on curved highways. In this paper, sight distance calculation models focusing on curved-approach intersections were developed for left-turn maneuvers toward the outside of the curve and toward the inside of the curve, separately. Using those models, it was found that, for left-turn maneuvers toward the outside of the curve, opposing left-turn vehicles might cause serious sight distance problems even for a 3.7-m median. However, for a left-turn maneuver toward the inside of the curve, the curve may mitigate the sight distance problem and the median width plays a more important role in the available sight distance, especially when the curve radius is relatively large. The models and related analyses can be used to lay out intersection design or evaluate the sight distance problem of an existing intersection located on the horizontal curve to ensure safe left-turn maneuvers by drivers.

INVESTIGATING THE ADJUSTMENT FACTOR FOR PROTECTED LEFT TURNS FROM A SHARED LANE

The purpose of this paper is to propose a more accurate method for the estimation of the left-turn adjustment factor for protected left turns at traffic signals. Research on the left-turn adjustment factor for two-lane approaches with protected left turns from a shared lane, gave results that differ from those of the HCM. Based on extended field measurements, the model proposed in this paper relates the left-turn adjustment factor for protected left turns from a shared lane to a number of traffic parameters. Such as the distribution of vehicles in the lanes, the mean volume per lane and the proportion of the left-turning vehicles.

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.

Fatal crash risk for older drivers at intersections

Risk of fatal crash involvement was calculated for older drivers relative to drivers aged 40–49 in the United States during the years 1994–1995. The results indicated that drivers ages 65–69 were 2.26 times more at risk for multiple-vehicle involvements at intersections compared with 1.29 times more at risk in all other situations. The comparable figures for drivers aged 85 and older were 10.62 for multiple-vehicle involvements at intersections compared with 3.74 for all other situations. The relative crash risk was particularly high for older drivers at uncontrolled and stop sign-controlled locations; when traveling straight or when just starting to enter the intersection; and when the specific behavioral error in the crash was failure to yield. Countermeasures will likely involve reducing or simplifying the need to detect and evaluate moving traffic coming from the left and right when at intersections. This can be accomplished by traffic signals with protected left turns, four-way stop signs, and one-way streets. Whereas such devices involve significant cost in terms of dollars and travel delay, their cost-effectiveness may have to be revisited as the United States population continues to age.

A laboratory investigation of signal indications for protected left turns.

The major objective of this research was to investigate and compare driver comprehension and understanding of different types of left-turn signal clearance indications for protected left turns. Forty subjects, male and female drivers, were tested. The test utilized color slides and color motion-picture film segments to present the subject with both steady and flashing left-turn signal indications. After each presentation of a signal indication, the subject responded by pressing one of three answer buttons. Accuracy and reaction times of each response were recorded. The results of this investigation recommended that three signal sequences be field tested as they conveyed their intended messages to the subjects more efficiently than the remaining eleven. The results also indicated that none of the three flashing signals tested proved to be effective.