Left-turn Phasing Research Articles

Developing Decision Boundaries for Left-Turn Treatments

The purpose of this research was to evaluate the interaction of left-turn and opposing through traffic volumes for permitted and protected left-turn phasing at intersections and develop boundaries that help identify when to switch from permitted to protected phasing at signalized intersections. Permitted phasing allows vehicles to turn left after yielding to opposing vehicles; protected phasing provides an exclusive phase for vehicles to turn left without opposition; and protected-permitted phasing combines these phasing alternatives, allowing both permitted and protected turning movements. Intersections with 1, 2, and 3 opposing-lane configurations with permitted and protected-permitted models (split into green times of 10, 15, and 20 s) were analyzed for a total of 12 simulation models. Each model was divided into 100–225 different volume scenarios, with incremental increases in left-turn and opposing volumes. By exporting trajectory files from VISSIM and importing these into the Surrogate Safety Assessment Model, crossing conflicts for each volume combination in each model were extracted. MATLAB was then used to create contour maps representing the number of crossing conflicts per hour associated with different combinations of left-turn and opposing volume. Basic decision boundaries were examined in each contour map. Statistical analysis software was used to perform a linear regression analysis on transformed data and to develop natural log-based equations that form the decision boundaries for each configuration and phase alternative. These equations were graphed and final decision boundaries developed for the 1-, 2-, and 3-lane configurations between permitted and protected-permitted phasing as well as between protected-permitted and protected phasing.

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.

Signalization options to improve capacity and delay at roundabouts through microsimulation approach: A case study on arterial roadways in Ghana

Roundabouts are very useful when flows are balanced but can have operational challenges when flows increase. Most roundabouts on arterial roadways in Ghana experience recurring congestion during peak traffic. The four roundabouts between Ejisu Municipality and Asokwa Municipality on the N6 highway are typical of the situation found on arterials countrywide. They experience recurring long queues and delays during the peak hours and over-representation of truck rollover accidents. In Ghana, historically, most roundabouts with capacity problems have been developed into interchanges at very high cost without consideration of other effective at-grade configurations. When roundabouts are signalized, various configurations can be explored to enhance capacity while preserving the safety benefits. Using simulation techniques, various configurations and design options were tested for their effectiveness at two of the roundabouts, located at KNUST police station and Ejisu, with similar demand and high left turning volumes. The objectives of this study were to a) evaluate the performance of the selected roundabouts on the corridor; b) explore alternative intersection configurations to improve capacity and delay. Using video data collection techniques, 2-h time stamped footages of AM and PM peak traffic flows were captured for two days. Geometric design parameters of the existing roundabouts, queue lengths, travel times, and approach flows were also captured. VISSIM models of the selected roundabouts were developed and calibrated for the baseline conditions. Signalized unconventional roundabout configuration, with a phase for major road through traffic were simulated and assessed for capacity, delay and queue length. The results showed that signalized roundabout intersections effectively increased capacity by as much as 50% in some cases, reduced delay and queue length significantly, and maintained the benefits of roundabouts including safety. The performance could be further enhanced by introducing separate left turn phases to reduce conflicts or through a right turn U-turn (RTUT) concept. • The study explored the operational performances of a proposed signalised unconventional roundabout through microsimulation. • The proposed intersection layout provided two lanes through the central island for the through traffic on the major road. • The signalised roundabout concept was comparatively a better improvement option to alleviate congestion. • A right turn U-turn (RTUT) concept was recommended to further enhance the performance of the signalised roundabout option.

NEW APPROACH TO ADDITIONAL LEFT-TURN SECTIONS OF THE TRAFFIC SIGNAL REGULATION

Introduction. The paper describes methods of using the additional left-turn phase of regulation. The authors view the main foreign and domestic methods used in the specialization of the left turn in a separate phase of regulation. To propose a new approach to the specialization of the left-turn direction in a separate phase of regulation the authors investigate one of the most loaded intersections in Voronezh and determine the main characteristics of the traffic flow. Therefore, the authors suggest new parameters in choosing of the necessary method of traffic organization at controlled intersections.Methods and materials. The paper analyses the main methods, which were conditionally related to two approaches: the accident rate of the site and the characteristics of the traffic flow (speed and intensity of the left-turn flow). The authors explained that the new approach, which took into account such parameters as speed, vehicle delays and queue length, allowed effectively controlling the intersection due to the optimal choice of the necessary traffic organization method and due to the result of changes in the parameters by combining the intensity of the left-turn flow and forward flow.Results. The authors developed the approach to the additional left-turn section introduction at the controlled intersection based on a comparison of the left-turn intensity and forward flow and on the analysis of changes in the main characteristics of the traffic flow (speed, vehicle delays and queue length).Discussion and conclusions. The authors concluded that it is necessary to use the proposed approach at the stage of modeling, design and reorganization of the regulated area on the basis of changes in the main characteristics of the traffic flow.

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.

Investigation on Two-Stop-Line Signalized Roundabout: Capacity and Optimal Cycle Length

Two-stop-line signalized roundabouts (TSLSR) are widely utilized in China. To calculate the capacity and optimal cycle length accurately, a model considering internal space constraint of the roundabout is developed in this paper. Firstly, the operational principle of TSLSR is analyzed. TSLSR is modeled as being equivalent to a four-phase intersection with left-turn protection phases, and the left-turn lane is further simplified as the “short-lane model”. Secondly, based on the above analysis, the capacity of TSLSR is modeled and additionally, the accuracy and sensitivity of the model are also analyzed. Finally, the optimal cycle length of TSLSR which maximizes the capacity is put forward. Results show that the accuracy of the capacity model is low without considering the internal space constraint of the roundabout. However, the model developed in this paper is shown to improve the accuracy (about 20%) and control the relative error to be within 10%. The parameter sensitivity analysis demonstrates that the cycle length and radius of the central island have a significant influence on the capacity of the left-turn lane and results in an optimal value of signal cycle length maximizing the capacity.

Intersection Operation with Non-Traditional Dynamic Lane Scheme through Vehicle-to-Signal Connection

This paper proposes an information-based dynamic lane (IDYL) scheme for signalized intersections with exclusive left-turn phases. Similar to the tandem design, the proposed scheme aims to increase the capacity of an isolated intersection by sorting incoming vehicles based on their turning movements. Its novelty is to guide vehicles of different movements into pre-designated dynamic lanes without stopping them via pre-signal. The assumption is that vehicles themselves or their drivers have access to, and can act on, real-time signal timing information through vehicle-to-signal connection to select the correct lane to enter as they approach the intersection. A mixed integer program is proposed to optimize jointly the lane configuration, timing plan, and dynamic lane utilization for an intersection. Results from numerical and simulation experiments show that IDYL can increase the reserve capacity by more than 25% when implemented on all legs of a standard four-leg intersection, and reduce the delay by around 15% when implemented on two opposing legs. The results from this study could help traffic engineers to operate signalized intersections with dynamic lanes when vehicle-to-signal connection becomes widely available in the near future.

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.

Modeling the permissive-only left-turn queue at signals

The traffic performance of signalized intersections can be evaluated by the average queue length of vehicles. However, the queue formation and dissipation of vehicles at a signalized intersection is a very complicated stochastic process; thus, it can hardly be explicitly described by the average index. Instead, the dynamic behavior of vehicles queueing at signalized intersections tends to be covered up by the average value of queue length, especially when a permissive-only left-turn phasing is used. To this end, a stochastic queueing model is developed to analytically investigate the evolution of queue length at a shared lane under the permissive-only phasing. In this paper, the queue length distribution of vehicles on a shared lane with permissive-only left-turn phasing is obtained, which can be used to analytically describe the dynamic characteristics of queue length of permissive-only left-turn vehicles. In addition, the factors that could affect the dynamic behavior of permissive-only left-turn queue are also experimentally determined, which will help to determine the selection of left-turn operational phasing and the type of left-turn lane (i.e., shared or exclusive left-turn lane) for a specific traffic condition.

Signal optimization for an isolated intersection with illegal permissive left-turning movement

Existing two-phase signal optimization models neglect the unyielding behavior of illegal permissive left-turners at intersections in many developing countries. At permitted left-turn phase warranted intersections, left-turn traffic usually needs to yield to the opposing through during permitted phasing; however, in certain circumstances such an explicit permissive left-turn yielding rule may be violated when aggressive left-turners compete with the opposing through vehicles to take the conflict point and pass through a two-phase signal-controlled intersection without yielding. Such driving behaviors, very commonly observed at urban intersections in many developing countries, have been demonstrated to have significant impact on the intersection operational performance, which has not been taken into consideration in the existing traffic signal design paradigms. This paper presents a cell-based signal optimization model for intersections without the explicit left-turn yielding rule during permitted phases. The model features its effectiveness to capture traffic dynamics and interactions between conflicting movements during permitted phases and to design appropriate signal timing plans to improve the operational performance of such intersections. Extensive experimental analyses and field validation in comparison with results from TRANSYT-7F demonstrate the promising property of the proposed model.

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.

Approach-level real-time crash risk analysis for signalized intersections

Intersections are among the most dangerous roadway facilities due to the complex traffic conflicting movements and frequent stop-and-go traffic. However, previous intersection safety analyses were conducted based on static and highly aggregated data (e.g., annual average daily traffic (AADT), annual crash frequency). These aggregated data may result in unreliable findings simply because they are averages and cannot represent the real conditions at the time of crash occurrence. This study attempts to investigate the relationship between crash occurrence at signalized intersections and real-time traffic, signal timing, and weather characteristics based on 23 signalized intersections in Central Florida. The intersection and intersection-related crashes were collected and then divided into two types, i.e., within intersection crashes and intersection entrance crashes. Bayesian conditional logistic models were developed for these two kinds of crashes, respectively. For the within intersection models, the model results showed that the through volume from “A” approach (the traveling approach of at-fault vehicle), the left turn volume from “B” approach (near-side crossing approach), and the overall average flow ratio (OAFR) from “D” approach (far-side crossing approach), were found to have significant positive effects on the odds of crash occurrence. Moreover, the increased adaptability for the left turn signal timing of “B” approach and more priority for “A” approach could significantly decrease the odds of crash occurrence. For the intersection entrance models, average speed was found to have significant negative effect on the odds of crash occurrence. The longer average green time and longer average waiting time for the left turn phase, higher green ratio for the through phase, and higher adaptability for the through phase can significantly improve the safety performance of intersection entrance area. In addition, the average queue length on the through lanes was found to have positive effect on the odds of crash occurrence. These results are important in real-time safety applications at signalized intersections in the context of proactive traffic management.

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.

A measurement to driving situation awareness in signalized intersections

Drivers get involved easily in Left-Turn Across Path with Opposite Direction Traffic (LTAP-OD) conflicts at signalized intersection with unprotected left-turn phasing. This study classified the left-turn drivers’ performance into four types: (1) the correct acceptance of safe lags/gaps, (2) the correct rejection of dangerous lags/gaps, (3) the false rejection of safe lags/gaps and (4) the false acceptance of dangerous lags/gaps. Based on the high mounted video camera data, the logistic regression model was used to obtain the critical gaps for estimating whether the lags/gaps were safe or not. The results show that the cognition behavior of the left-turn drivers should be improved while the gap/lag is drawn near the critical gap. Furthermore, the conservative drivers are more likely to reject the larger lags/gaps and yield the right-of-way to the opposing vehicles. Simultaneously the poor response execution may cause drivers to have difficulties during the turning performance when the lags/gaps are large enough. These results could be used as the basis for a discussion of the right time to support the drivers.

The symmetric intersection design and traffic control optimization

It is well recognized that the left-turning movement reduces the intersection capacity significantly, because exclusive left turn phases are needed to discharge left turn vehicles only. This paper proposes the concept of Left-Hand Traffic (LHT) arterial, on where vehicles follow left-hand traffic rules as in England and India. The unconventional intersection where a LHT arterial intersects with a Right-Hand Traffic (RHT) arterial is named as symmetric intersection. It is only need three basic signal phases to separate all conflicts at symmetric intersection, while it at least need four signal phases at a conventional intersection. So, compared with the conventional intersection, the symmetric intersection can provide longer green time for the left-turning and the through movement, which can increase the capacity significantly. Through-movement waiting areas (TWAs) can be set at the symmetric intersection effectively, which can increase the capacity and short the cycle length furthermore. And the symmetric intersection is Channelized to improve the safety of TWAs. The Binary-Mixed-Integer-Linear-Programming (BMILP) model is employed to formulate the capacity maximization problem and signal cycle length minimization problem of the symmetric intersection. The BMILP model can be solved by standard branch-and-bound algorithms efficiently and outputs the lane allocation, signal timing decisions, and other decisions. Experiments analysis shows that the symmetric intersection with TWAs can increase the capacity and short the signal cycle length.

Analytical evaluation of the use of left-turn phasing for single left-turn lane only

At most traditional signalized intersections, three types of phasing can be provided to left turns: protected only phasing, permitted only phasing, and a combination of protected and permitted phasing (protected/permitted phasing). While numerous guidelines for the selection of left-turn phasing have been developed, there is no widely recognized guideline or criterion for the use of left-turn phasing under specific traffic conditions especially for a single left-turn lane. Focusing on the inherent mechanism of the dynamic nature and uncertainty of left-turn queues under different left-turn phasing, this paper develops a left-turn queueing model with uncertain second vacation to analytically evaluate the performance of left turns for different left-turn phasing, which considers various factors including the type of left-turn phasing, signal timing, left-turn volume, opposing through volume, type of left-turn lane, number of opposing through lanes, and number of sneakers, etc. For different left-turn phasing, the left-turn queues formation and dissipation with different characteristics of vacations (i.e., server absences during the red time and uncertain blocked time caused by the opposing through flow during the permitted green time) are formulated, and the queue length distributions of left turns along with the time within one cycle and at an arbitrary time are derived. Furthermore, three sets of left-turn performance measures are obtained: primary queueing measures, fuel-consumption (or emissions) related measures, and safety related measure. On the basis of these performance measures, an analytical evaluation framework for left-turn queues is established, which can provide a more accurate and detailed basis for evaluating and improving the use of left-turn phasing. Model validation indicates that the proposed model can be an effective tool to evaluate the use of left-turn phasing under different traffic conditions. In addition, numerical experiments are also performed to theoretically identify the factors that could affect the performance of left turns and thus help to determine the left-turn phasing selection under different conditions.

A General Maximum Progression Model to Concurrently Synchronize Left-Turn and through Traffic Flows on an Arterial

In the existing bandwidth-based methods, through traffic flows are considered as the coordination objects and offered progression bands accordingly. However, at certain times or nodes in the road network, when the left-turn traffic flows have a higher priority than the through traffic flows, it would be inappropriate to still provide the progression bands to the through traffic flows; the left-turn traffic flows should instead be considered as the coordination objects to potentially achieve better control. Considering this, a general maximum progression model to concurrently synchronize left-turn and through traffic flows is established by using a time-space diagram. The general model can deal with all the patterns of the left-turn phases by introducing two new binary variables into the constraints; that is, these variables allow all the patterns of the left-turn phases to deal with a single formulation. By using the measures of effectiveness (average delay time, average vehicle stops, and average travel time) acquired by a traffic simulation software, VISSIM, the validity of the general model is verified. The results show that, compared with the MULTIBAND, the proposed general model can effectively reduce the delay time, vehicle stops, and travel time and, thus, achieve better traffic control.

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.

Pedestrian safety under permissive left-turn signal control

Abstract At intersections with permissive only signal control, pedestrians will move at the phase with the parallel through vehicular movement and the permissive left-turn movement. The left-turn vehicles have to yield to both opposing vehicles and pedestrians at the crosswalk. Under such complicated driving conditions, collision risks rise if left-turn vehicles make misjudgments and fail to yield to the pedestrians. In this research, driving-simulation based experiments were conducted for identifying and assessing the impacts of the factors that contribute to the collision between the left-turn vehicles and the pedestrians during the permissive left-turn phase. The results of this study showed that the percentage of left-turn trucks and the pedestrian volume has significant impacts on the pedestrian safety under permissive left-turn signal control.

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.