Left-turn Bay Research Articles

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.

Characterizing the dynamics and uncertainty of queues at signalized intersections with left-turn bay

In most signalized intersections, in order to solve the problem of insufficient road space, left-turn bays are commonly set up at intersections with a shared through and left-turn lane. However, this design tends to result in the potential queue overflow and blockage problems, which further make the discharge process of either left-turn vehicles or through vehicles at the intersections to be interrupted unexpectedly during their respective green phase. As a result, the queueing processes of vehicles present high nonlinearity and uncertainty during a signal cycle. In this paper, a stochastic queueing model is proposed to characterize the dynamics and uncertainty of queues at signalized intersections with a left-turn bay. The proposed model takes full account of the random interaction between the through movement and the left-turn movement under the influence of overflow and blockage of queues. We obtain some average performance indicators as well as the signal-state-dependent dynamic performance measure of queues and its variance at any given time point within a signal cycle. The proposed analytical model is validated against the VISSIM simulation model under different scenarios; the results show a good degree of agreement. In addition, in order to systematically determine the queue dynamics and uncertainty, we also conduct a series of numerical simulations to quantitatively capture the queue characteristics of vehicles at signalized intersections with left-turn bay by considering possible factors.

Modeling Capacity of Through Movement at Signalized Intersection Affected by Short Left-Turn Bay under Different Signal Settings

Through movement capacity is an essential factor used to reflect intersection performance, especially for signalized intersections, where a large proportion of vehicle demand is making through movements. Generally, left-turn spillback is considered a key contributor to affect through movement capacity, and blockage to the left-turn bay is known to decrease left-turn capacity. Previous studies have focused primarily on estimating the through movement capacity under a lagging protected only left-turn (lagging POLT) signal setting, as a left-turn spillback is more likely to happen under such a condition. However, previous studies contained assumptions (e.g., omit spillback), or were dedicated to one specific signal setting. Therefore, in this study, through movement capacity models based on probabilistic modeling of spillback and blockage scenarios are established under four different signal settings (i.e., leading protected only left-turn [leading POLT], lagging left-turn, protected plus permitted left-turn, and permitted plus protected left-turn). Through microscopic simulations, the proposed models are validated, and compared with existing capacity models and the one in the Highway Capacity Manual (HCM). The results of the comparisons demonstrate that the proposed models achieved significant advantages over all the other models and obtained high accuracies in all signal settings. Each proposed model for a given signal setting maintains consistent accuracy across various left-turn bay lengths. The proposed models of this study have the potential to serve as useful tools, for practicing transportation engineers, when determining the appropriate length of a left-turn bay with the consideration of spillback and blockage, and the adequate cycle length with a given bay length.

MODELLING AND SENSITIVITY ANALYSIS OF UNCOORDINATED PAIRED INTERSECTIONS WITH LEFT-TURN BAYS

Left-turn bays are often installed on the road segment between paired intersections. Such left-turn bays may reduce the approach capacities and impact on one another. Four optimization models are put forward for uncoordinated paired intersections with left-turn bays. The phase effective green times and the left-turn bay lengths are the decision variables, maximizing the intersection capacities, minimizing the intersection delays and both of them are respectively regarded as different objective functions, and minimizing the total delay for paired intersections is viewed as another objective function. The total capacity-to-delay ratio is defined to evaluate the operations of paired intersections as a whole. Using the field data, the sensitivities of the optimized outcomes to the weighting factors of the objective functions are analysed. To clarify the influences of different scenarios on traffic stream operations, seven scenarios are tested using VISSIM. The interval estimation and hypothesis testing are used to analyse the simulated data. Three concrete models are recommended to apply in practice with the procedure of model application being provided. The achievements can be applied to optimally assign the temporal-spacial resources for paired intersections when left-turn bays need to be installed and coordinated signals do not need to be considered.

Comparative Analysis of Optimization Models for Paired Intersections with Left-Turn Bays

Left-turn bays may be provided at paired intersections when heavy left-turn demands exist on some approaches. Such left-turn bays may have significant effects on intersection operations. Considering the settings of left-turn bays, two single-objective optimization models are developed for paired intersections with uncoordinated and coordinated signals, respectively. Numerical examples are fulfilled to demonstrate and compare the formulated models, and the orthogonal simulation experiments are carried out. Assuming that the relevance of traffic flow exists, the results show that the optimized effective green times and left-turn bay lengths depend on traffic demand, whereas the optimized offset between paired intersections is sensitive to intersection spacing; the optimization model with coordinated signals outstrips that with uncoordinated signals at 99% confidence level when the distance between paired intersections is not too far and traffic demand is not too low, whereas the former may be inferior to the latter when traffic demand is low enough and intersection spacing is far enough.

A comparison of collision-based and conflict-based safety evaluation of left-turn bay extension

ABSTRACT There is a need to validate the results of conflict-based before-after (BA) evaluations in terms of their collision reduction estimation. This study compares the results of a collision-based and a conflict-based BA safety evaluation of left-turn bay extension. Both collision and conflict data were collected and used in the evaluation. The collision-based BA evaluation was conducted by developing a Poisson–Lognormal intervention (PLNI) model which was estimated by the full Bayesian approach. The conflict-based BA analysis was performed by an extreme value theory (EVT) model developed with the generalized extreme value (GEV) distribution. The comparison showed consistent results from the two approaches. The results showed a reduction of 56.2% in actual collisions from the PLNI model and a reduction of 63.9% in estimated collisions from the EVT model. These results provide support for the validity and reliability of using traffic conflicts in BA studies.

A full Bayes approach for traffic conflict-based before–after safety evaluation using extreme value theory

A full Bayes approach is proposed for traffic conflict-based before-after safety evaluations using extreme value theory. The approach combines traffic conflicts of different sites and periods and develops a uniform generalized extreme value (GEV) model for the treatment effect estimation. Moreover, a hierarchical Bayesian structure is used to link possible covariates to GEV parameters and to account for unobserved heterogeneity among different sites. The proposed approach was applied to evaluate the safety benefits of a left-turn bay extension project in the City of Surrey, Canada, in which traffic conflicts were collected from 3 treatment sites and 3 matched control sites before and after the treatment. A series of models were developed considering different combinations of covariates and their link to different GEV model parameters. Based on the best fitted model, the treatment effects were analyzed quantitatively using the odds ratio (OR) method as well as qualitatively by comparing the shapes of GEV distributions. The results show that there are significant reduction in the expected number of crashes (i.e., OR = 0.409). In addition, there are apparent changes in the shape of GEV distributions for the treatment sites, where GEV distributions shift further away from the risk of crash area after the treatment. Both of these results indicate significant safety improvements after the left-turn bay extension.

Left-Turn Spillback Probability Estimation in a Connected Vehicle Environment

At intersections in which the left-turn bay does not have sufficient length or the left-turn volume is relatively high, left-turn vehicles may spill back and block the adjacent through traffic. This paper aims to develop quantitative measures of the left-turn spillback, and by using the results on spillback probability, develop a suitable signal control strategy. We first develop an improved queue length estimation method for vehicles in the left-turn bay based on Comert and Cetin’s general queue length estimation method with connected vehicles, after which we propose a probabilistic model to measure the left-turn spillback probability at an intersection in a connected environment. The model accuracy is validated with results from microscopic traffic simulation. The effect of bay length is also studied. In the end, a signal control demonstration is presented to show the efficiency of the proposed method in signal control.

Before-after safety analysis using extreme value theory: A case of left-turn bay extension

There is growing interest in the use of traffic conflicts in before and after safety evaluations because of well-recognized quality and quantity problems associated with historical crash records. Most of these studies apply statistical techniques to compare the number of conflicts before and after the implementation of safety countermeasures. However, to identify the number of conflicts, a specific threshold for various conflict indicators needs to be used and the results of the evaluation can vary significantly depending on the selection of this threshold. As well, there is an issue with how to account for conflict severity in the evaluation. This study proposes adopting the extreme value theory approach to overcome these two issues. The approach was applied to a case of left-turn bay extension at three signalized intersections, and the automated traffic conflict technique was used to identify conflicts with TTC values from the video data collected from treatment sites and matching control sites. Generalized extreme value (GEV) models with different covariates were developed and compared. The results show that there are apparent shape change in the GEV distribution (i.e., from narrow peak up to high severities to wide spread with fewer conflicts at high severity levels) after the treatment, indicating reduction in conflict severity. The safety improvement is further confirmed by the total reduction of 63.9% in estimated crashes. Moreover, with the aid of GEV model, the most severe conflicts that are also rare and random are included into the OR calculation, and a significant reduction of 73.2% is found in the estimated most severe conflicts.

Evaluating the safety and operational impacts of left-turn bay extension at signalized intersections using automated video analysis

Left-turn lanes are commonly introduced to provide space to accommodate comfortable deceleration and adequate storage of turning vehicles. Operational shortcomings may arise due to inadequate length, including overflow and blockage of left-turn entrance by queues on an adjacent through lane. This study investigates the potential safety and operational benefits of treating left-turn lanes by extending the length further upstream a signalized intersection. Video data was collected at three treated left-turn lanes as well as three matched control lanes; all in both before and after treatment conditions. Safety parameters consisted of the counts and severities of traffic conflicts occurring on the left-turn lanes and inside the intersection. There was a marked reduction in traffic conflict counts in all treated sites. The overall treatment effect, which accounts for the simultaneous change in control sites, was 63.2% (p < 0.05). There was a marked reduction in frequency of traffic conflicts at different severity levels. The mobility benefit of the treatment was demonstrated in terms of the reduction in average travel time for left-turn as well as through vehicles. The count of traffic signal cycles with blocked left-turn entrance was considerably reduced after the treatment. The use of collision data gathered from more sites is suggested as potential future work to further evaluate this treatment.

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.

도로안전시설의 사고감소효과 메타분석 : 신호교차로를 대상으로

교차로에서 발생한 교통사고건수는 최근 10년간 지속적으로 증가하였고, 2014년 전체 교통사고의 44%를 차지하면서 교차로는 교통안전에 취약한 도로형태로 주목되고 있다. 특히 선정된 사고다발지점 대다수가 신호교차로를 포함하고 있어, 신호교차로의 안전개선 노력이 우선적으로 필요한 실정이다. 이에 정부는 주로 도로안전시설 개선사업을 시행하여 주어진 예산 내에서 단기간에 높은 효과를 얻을 수 있는 안전개선사업을 수행하고자 한다. 이를 위하여 도로안전시설의 사고감소효과를 확인하는 연구가 지속적으로 등장하고 있으나 독립적으로 수행된 연구별 특성은 종합하기 어렵고, 각 연구결과가 대표성을 갖는데 한계가 있기 때문에 여전히 한정된 예산과 불확실한 도로안전시설 설치효과로 인해 선뜻 안전개선사업을 진행하지 못하고 있다. 따라서 본 연구에서는 메타분석을 통해 기존의 신호교차로 교통사고발생에 영향을 미치는 변인을 활용한 여러 사고예측연구 결과를 종합적으로 검토하고, 각 변인이 대표할 수 있는 개선효과를 도출하고자 하였다. 본 연구에서는 신호교차로 전체 사고건수를 종속변수로 하는 연구 중 19건의 논문을 선정하고 8개 변인에 대한 34개의 개별효과크기(effect size)를 산출하였다. 메타분석 결과, 도로안전시설 설치와 신호교차로 교통사고의 관계는 좌회전 전용차로를 제외하고는 모두 통계적으로 유의하게 감소한 사고영향을 나타내어, 도로안전시설이 사고감소에 영향을 준다는 통계적인 결과자료를 도출하였다. 그 중 도류화설치의 효과크기가 가장 높았으며, 좌회전차로 가속구간, 조명시설, 횡단보도, 우회전 전용차로, 중앙분리대 순으로 효과가 높은 것으로 나타났다. The number of traffic accidents at intersections has been increased over the past decade, and in 2014, accounted for 44% of the total traffic accidents. In addition, since most of road accident black spots include signalized intersections, the improvement of safety on signalized intersection is top priority for national road safety program in Korea. For the decision of most effective action plan for the safety of signalized intersection, this study conducted meta-analyses to assess the effects of various road safety facilities at signalized intersection on traffic accidents. We selected 19 articles regarding the prediction of traffic accidents at signalized intersection and calculated 34 individual effect sizes on 8 variables included in the articles. The results found that there was a statistically significant negative relationship between the occurrence of traffic accident and the presence of all of road safety facilities except an exclusive left turn bay. It shows that most of the road safety facilities could prevent traffic accidents, and intersection channelization has the most effective on the reduction of traffic accidents followed by left-turn acceleration lane, lightning, crosswalk, exclusive right-turn lane and median barrier.

Exploring the Impact of Access Designs on Crash Injury Severity on Multilane Highways

Objective: Access design is a critical factor that influences the safety and mobility of urban/suburban multilane highways due to the interactions between access movements and through traffic. An effective way for improving the safety and mobility of multilane highways is to control access maneuvers by implementing appropriate access designs. Understanding the impact of access designs on crash injury severity is beneficial for implementing effective countermeasures to mitigate crash injury at access points. Thus, the objectives of this article are to investigate the impact of access designs on crash injury severity and identify contributing factors of crash injury severity at access points of multilane highways. Methods: A total of 1830 crash records were collected at 149 access points with different access designs for a period of 3 years (2008–2010) in Florida. A heterogeneous logit model, relaxing the constraint of identical variances across observations in the traditional ordered choice models, was developed to evaluate the impact of access designs on crash injury severity and identify contributing factors. The marginal effects of the developed model were used to interpret the impact of access designs and other contributing factors. Results: At 4-leg access points, given that a crash has occurred, replacing full median openings with closed medians will reduce the probability of severe injury (fatality, incapacitating injury, or nonincapacitating injury) by 9.73 percent; substituting full median openings with directional median openings will reduce the probability of severe injury by 11.02 percent. At 3-leg access points, given that a crash has occurred, closed medians significantly experience a lower risk of severe injury than full median openings; however, there is no evidence that directional median openings are similarly effective. Other contributing factors of crash injury severity at access points were identified as number of lanes, shoulder width, median width, driveway density, left-turn bay on major roads, speed limit, average annual daily traffic (AADT), high-density residential area, daylight, driver age, and truck involvement. Conclusions: Installation of directional median openings is a reasonable safety treatment at 4-leg access points because this access design has better safety performance than full median openings in terms of crash injury severity; in addition, it provides fewer restrictions on accessibility than closed medians. Other effective treatments include installing left-turn storage bays at median openings and increasing the width of shoulders and medians. Supplemental materials are available for this article. Go to the publisher's online edition of Traffic Injury Prevention to view the supplemental file.

Modeling Delay during Heavy Traffic for Signalized Intersections with Short Left-Turn Bays

This paper develops theoretical delay models for protected left-turn operations at a pretimed signalized intersection during heavy traffic. When the through traffic demand is heavy, residual queues from the previous cycle may occur and elevate the probability of blockage to the left-turn bay, leading to increased delay for left turns. A probabilistic left-turn delay model based on the queuing diagram is proposed for a leading left-turn operation; the influence of residual queues and blockage by the through traffic are taken into account. When the left-turn demand becomes heavy, the left turns may spill back to block the through traffic, resulting in through traffic delays. Through traffic delay is modeled probabilistically on the basis of the analysis of left-turn bay spillback for a lagging protected left-turn operation. The left-turn delay models are validated through carefully designed simulation studies using VISSIM and the results are compared with those from the Highway Capacity Manual (HCM) delay model, which does not consider blockage or spillback situations. The proposed delay models can be used to replace the uniform delay term in the HCM model for high-demand situations when the left-turn operation is affected by spillback and blockage.

Impact of Left-Turn Spillover on through Movement Discharge at Signalized Intersections

While the Highway Capacity Manual (HCM) 2000 does provide a separate procedure to estimate the back of queue for a lane group, its signalized intersection analysis procedure does not explicitly account for the impact to through movement traffic as a result of left-turn bay spillover. If the HCM methodology is used for an analysis of an intersection approach where left-turn spillover interferes with the discharge of through vehicles, the estimated delay results, and corresponding level of service, will be overly optimistic. Thus, the objective of this study was to determine the factors that significantly affect left-turn lane spillover and develop a model, or models, to predict the expected through movement capacity and discharge rate as a function of this spillover. Two approaches were used in this study: an analytical approach that developed a probability of left-turn spillover equation to incorporate into a capacity estimation equation, and an empirical approach incorporating the use of microsimulation to develop equations to estimate through movement discharge as a function of left-turn spillover. The developed probabilistic model was an enhancement to one from the literature. However, its accuracy of estimation was still limited due to the issue of intercycle queue-formation dependencies not being taken into account. The two models developed from regression analysis of simulation data—one for intersection approaches with only a single through lane and one for intersection approaches with multiple through lanes—replicate the simulation results quite reasonably.

Analytical Models for Protected plus Permitted Left-Turn Capacity at Signalized Intersection with Heavy Traffic

This paper presents theoretical analysis of left-turn operations under heavy traffic. When traffic demand is heavy, residual queues from previous cycles may occur and contribute to blockage of the left-turn bay by adjacent through traffic. Such blockage reduces the left-turn capacity and is a random phenomenon. An improved model for protected left-turn capacity is first proposed, taking into account the influence of residual queues and blockage by through traffic. In addition, the classic model always overestimates the potential permitted left-turn capacity when opposing traffic is heavy. The potential permitted left-turn capacity is then modeled in a probabilistic way, and a simple estimation is suggested by investigating its relation to the classic model. Furthermore, because left-turn capacity may vary in response to a blockage, the coefficients of variance for protected left-turn capacity and for queue clearance time are introduced to evaluate the protected left-turn timing plan. The developed analytical models are validated with CORSIM simulations.

Modeling Left-Turn Blockage and Capacity at Signalized Intersection with Short Left-Turn Bay

A left-turn bay that does not have a sufficient length would be blocked by the adjacent through traffic. Left-turn vehicles can also spill back to block the through traffic when the left-turn demand is high. These two types of left-turn-related blockages have adverse effects on the operation of a signalized intersection. Left-turn bay blockage by through traffic will reduce the left-turn capacity, and left-turn bay spillback will affect the adjacent through capacity. Both problems are uncharted in the Highway Capacity Manual (HCM). This research studies these two blockage scenarios associated with left-turn bays under different left-turn signal operations. The capacity of the left-turn and through movements is modeled as a function of the length of the left-turn bay by using a probabilistic approach, taking into account potential blockage due to a short left-turn bay length. The developed capacity model will be validated on the basis of CORSIM simulations of a real-world signalized intersection. The research is aimed at estimating the left-turn and adjacent through capacity when left-turn bay blockage or spillback occurs, to complement the HCM method and highlighting the effect of the length of the left-turn bay and the left-turn signal strategy.

Comparison of Two Unconventional Intersection Schemes

The operational performance of signalized intersections can be significantly affected by heavy left-turn movements. Therefore, several measures to improve the performance of intersections with heavy left-turn movements, including some unconventional schemes, have been investigated. In this paper the performance of two of these unconventional schemes, namely, the crossover displaced left-turn (XDL) intersection and the upstream signalized crossover (USC) intersection, is compared. The XDL intersection eliminates left-turn opposing conflicts by displacing the left-turn lane to the opposing direction and crossing the left-traffic to the left side of the road before the intersection. The USC intersection eliminates left-turn opposing conflicts by crossing both the left and through traffic to the left side of the road before the intersection. The microsimulation software VisSim was used to analyze the two unconventional intersections for comparison. The analysis showed that the XDL outperforms the conventional intersection under various volume scenarios. In contrast, the USC outperforms the conventional intersection under moderate- and high-volume conditions or in the existence of extremely heavy left-turn movements. The results showed that the USC and the XDL exhibit similar average delays in low-, moderate-, and moderately high-volume conditions, with the XDL slightly outperforming the USC. However, the XDL significantly outperforms the USC under high-volume scenarios. The XDL has, as a basic design element, a left-turn bay that extends between the primary and the secondary intersections. The existence of this bay is probably the main reason for the high capacity of the XDL. Such an additional bay requires more right-of-way and higher construction costs than USC.