Left-turn Intersections Research Articles

Optimal operation of displaced left-turn intersections: A lane-based approach

Displaced left-turn (DLT) intersections that resolve the conflict between left-turn and opposing-through movements at the pre-signal are probably the most extensively used innovative intersection designs. The DLT intersection concept can be extended to ten different types according to the location of the left-turn transition area, the number of DLT approaches, and the possible setting of the bypass right-turn lane. This paper presents a generalized lane-based optimization model for the integrated design of DLT intersection types, lane markings, the length of the displaced left-turn lane, and the signal timings. The optimization is formulated as a mixed-integer non-linear program. This program is further transformed to a series of mixed-integer linear programming problems that can be solved by the standard branch-and-bound technique. Results from extensive numerical analyses reveal the effectiveness of the proposed method, as well as the promising property of assisting transportation professionals in the proper selection of DLT intersection types, and the design of geometric layout and signal timings.

Evaluation of a Risk Awareness Perception Training Program on Novice Teen Driver Behavior at Left-Turn Intersections

Collisions at left turn intersections are among the most prevalent types of teen driver serious crashes, with inadequate surveillance as a key factor. Risk awareness perception training (RAPT) has shown effectiveness in improving hazard anticipation for latent hazards. The goal of this study was to determine if RAPT version 3 (RAPT-3) improved intersection turning behaviors among novice teen drivers when the hazards were not latent and frequent glancing to multiple locations at the intersection was needed. Teens aged 16-18 with ≤180 days of licensure were randomly assigned to: 1) an intervention group (n=18) that received RAPT-3 (Trained); or 2) a control group (n=19) that received no training (Untrained). Both groups completed RAPT-3 Baseline Assessment and the Trained group completed RAPT-3 Training and RAPT-3 Post Assessment. Training effects were evaluated on a driving simulator. Simulator (gap selection errors and collisions) and eye tracker (traffic check errors) metrics from six left-turn stop sign controlled intersections in the Simulated Driving Assessment (SDA) were analyzed. The Trained group scored significantly higher in RAPT-3 Post Assessment than RAPT-3 Baseline Assessment (p< 0.0001). There were no significant differences in either traffic check and gap selection errors or collisions among Trained and Untrained teens in the SDA. Though Trained teens learned about hazard anticipation related to latent hazards, learning did not translate to performance differences in left-turn stop sign controlled intersections where the hazards were not latent. Our findings point to further research to better understand the challenges teens have with left turn intersections.

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.

Signal design for displaced left-turn intersection using Monte Carlo method

Unconventional intersection designs have been receiving increased consideration at intersections that serve high traffic demands. Although operational performance of unconventional intersections has been evaluated in a number of studies, little research has been conducted regarding methodologies to obtain optimal signal timings for them. Several studies have reported that commercial signal timing packages have not been able to be utilized to develop optimal signal timings due to the unique design features of the unconventional intersections. In this paper a Monte Carlo simulation method to minimize intersection delay is described as well as a bandwidth maximization method for a Displaced Left-Turn (DLT) intersection, one of the most popular unconventional intersections. The proposed method is applied to a representative intersection design for two different traffic demand scenarios. It is seen that the two optimization approaches may result in different optimal solutions and the Monte Carlo simulation is able to provide near-optimum parameter selection ranges for the given traffic demands. Also, it is found that the Monte Carlo method is sufficiently flexible to provide intersection-tailored solutions with intersection specific design parameters.

Light Rail Transit Urban Insertion and Safety

This paper provides a rough design tool kit to improve the safety of light rail transit (LRT) urban insertion. This tool kit was prepared after the analysis of more than 130 examples of good and bad practices related to LRT urban insertions in 13 European countries, which took place during the first working phase of the European Cooperation in Science and Technology's Action TU1103, Operation and Safety of Tramways in Interaction with Public Space. The paper deals with the design of the following aspects: pedestrian pathways at stations and stops; platform design and station and stop location; pavement treatment on shared space; pavement treatment on segregated channels; LRT separators on segregated channels; intersections (e.g., left-turn intersections, roundabouts, pedestrian and cyclist crossings, innovative solutions at intersections); LRT channel differentiation and protection by means of pavement, marks, fences, and barriers; and overhead contact system pole location.

Operational Effects of Continuous Flow Intersection Geometrics

The continuous flow intersection (CFI), also known as a displaced left-turn (DLT) intersection, increases capacity and reduces delays by eliminating the left-turn traffic signal phase. The geometric layout of the intersection is changed so that left turns cross opposing traffic lanes at a location upstream of the main intersection. Although the installation of CFIs has become more prevalent in the United States, insights into the geometrics of CFIs gained through their application have not yet been dispersed to industry or academia. This paper considers the impacts that geometric design has on the operations of CFIs. The paper presents a deterministic model that considers the effects of the DLT length and other CFI geometrics on traffic operations. Although academic in nature, this model evolved through practical application. The model takes functional and operational insights gained through the analysis, design, and observation of multiple operational CFIs in Utah and articulates them in mathematical terms. The quantitative relationships presented provide transportation professionals and officials more accurate and precise guidance to facilitate better decisions in considering, evaluating, and designing a CFI.

Performance Analysis of Parallel Flow Intersection and Displaced Left-Turn Intersection Designs

The displaced left-turn (DLT) design, also known as the continuous flow intersection, has proved to be a superior alternative to the conventional intersection in terms of handling heavy volumes during peak periods. In locations where the availability of additional right-of-way and driveway access is not a major concern, the DLT design is a cost-effective and time-saving option compared with the expensive option of grade-separated interchanges. Recently, a new nontraditional intersection design called the parallel flow intersection (PFI) has been proposed. The PFI is also an at-grade design and operates with the same number of signal phases as a DLT. This research compares the operational performance of DLT and PFI designs based on the maximum through and left-turn movement throughputs for three different high-volume scenarios using traffic simulation. The results indicate that maximum throughput values of through movement in the PFI were very close to the values obtained for the DLT. The designs produced similar results, mainly because both operate as two-phase signals (at the main intersection) with equal green times for through movements. The left-turn movement throughputs in the PFI were found to be lower than those in the DLT. For two study cases, the DLT was able to process 180 and 80 more vehicles per hour per lane than the PFI. This is attributable to left-turning vehicles experiencing a greater number of stops, on average, in a PFI than they would in a DLT.

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.

Design and Operational Performance of Crossover Displaced Left-Turn Intersections

Although the concepts of the crossover displaced left-turn (XDL) intersection (also called the continuous flow intersection) were developed approximately four decades ago, there is no simplified procedure to evaluate its traffic performance and to compare this intersection with conventional intersections. Several studies have shown the qualitative and quantitative benefits of the XDL intersection without providing accessible tools for traffic engineers and planners to estimate average control delays and queues. Modeling was conducted on typical geometries over a wide distribution of traffic flow conditions for three different design configurations or cases using VISSIM simulations with pretimed signal settings. Some comparisons with similar conventional designs showed considerable savings in average control delay, as well as average queue length and increase in intersection capacity. The statistical models provided an accessible tool for the practitioner to assess average delay and average queue length for three types of XDL intersections. Finally, a preferred signal setting was developed for each of the five intersections of the XDL network.

A Cognitive Engineering Approach to Improving Signalized Left Turn Intersections

This research evaluated the effect of providing advanced left turn information to individuals faced with deciding whether or not it is safe to turn at a signalized intersection. Younger (mean age 37 years) and older (mean age 71 years) drivers were tested in simulations of approaching an intersection with and without advanced cueing. Experiment 1 used a featureless background; Experiment 2 used an animated presentation of traffic control displays. In both experiments the subjects had to determine whether or not they had right-of-way to make a left turn. Exaggerated error rates and increased decision latencies for displays conveying unprotected turn status occurred in both studies. Cueing drivers with advanced notice of the decision rule through a redundant upstream posting of sign elements improved both accuracy and latency of younger and older drivers' decisions.