Signalized Diamond Interchanges Research Articles

Operational Evaluation of U-Turns at At-Grade Signalized Diamond Interchanges

U-turn lanes are commonly provided at at-grade diamond interchanges with frontage roads to reduce delays for U-turning traffic and for the interchange as a whole; however, there are currently many unknowns related to their design, operation, and use. Researchers of this study assessed the existing practice of U-turn implementation by evaluating operations of U-turn lanes at 25 diamond interchanges in Texas through field data collection and simulation analysis. Researchers determined the performance and limitations of U-turn lanes under various geometric and operational conditions, identified and investigated several factors affecting U-turn lane use, and determined the anticipated effectiveness of proposed solutions to U-turn operational issues. This paper provides implementable guidelines for designing and operating U-turn lanes at diamond interchanges. Among the ten countermeasures studied in this paper, closing nearby driveways, adding acceleration lane for U-turn departures, and separating U-turn departures from conflicting traffic using pylons were found to be effective solutions to improve U-turn operations.

Operational Evaluation of Two-Lane Roundabouts at Freeway Ramp Terminals: Comparison Between Roundabout and Signalized Interchanges

In the United States, roundabouts have recently been constructed to replace signalized intersections at freeway ramp terminals as both a safety and an operational treatment. In practice, this treatment is in need of guidelines specifying conditions when the roundabout or signalized intersection is more appropriate to assist practitioners in deciding which alternative to choose. In particular, research providing a comprehensive operational comparison between roundabouts and signalized interchanges is lacking. The current research—though a strictly calibrated microscopic simulation platform—analyzes and models the control delay at double-lane roundabouts and signalized interchanges. Both roundabouts and signalized interchanges were modeled in a Vissim simulation platform. Capacity at each roundabout entrance was calibrated and validated separately for passenger cars and heavy vehicles, since both vehicle types have different critical and follow-up headways. The design of the simulation experiments covered 2,880 different scenarios for roundabouts and signalized interchanges with varying ramp and arterial volumes, ramp spacing, and heavy-vehicle percentages. From the simulation results, control delay and level of service of the off-ramp and arterial approaches of roundabouts and signalized diamond interchanges were modeled and compared. Ultimately, guidelines for the selection between double-lane roundabouts and signalized interchanges were developed and presented in the form of look-up tables. These tables provide an easy-to-use tool for practitioners to determine the appropriate double-lane interchange to install under specific combinations of traffic demand, heavy-vehicle percentage, and ramp spacing conditions.

Calculation of additional lost green time for upstream approaches at diamond interchanges considering downstream queue effects

Most diamond interchanges are composed of two closely-spaced signalized intersections where the downstream queue has strong influences on the system performance. The analysis methodology of isolated intersections may not reflect properly the downstream queue effect in the internal link at diamond interchanges. This paper proposed a new analytical model to estimate the additional lost green time for the upstream intersection at signalized diamond interchanges considering the queue effects in the internal link. The model calculates the additional lost green time at TTI three-phase and four-phase diamond interchanges using field traffic and geometric data. As in the HCM 2010 procedure, the model estimates the initial internal queue length, based on which the additional lost green time is calculated. The initial internal queue length is estimated taking a diamond interchange as a simple input-output system that processes the OD demand. The additional lost green time is calculated applying a method of kinematic waves. The performance of the model is evaluated using simulated data under various traffic and geometric conditions. The model evaluation results show that the proposed model provides close values to those by the simulation. The model can be applied to the design or operation analysis of diamond interchanges.

Sequence Optimization at Signalized Diamond Interchanges Using High-Resolution Event-Based Data

Signalized diamond interchanges are pairs of ramp intersections characterized by interlocked left turns and relatively close spacing. This paper describes a series of performance measures derived from high-resolution signal controller event data that can be used to optimize the internal phase sequence and offset to improve traffic flows within diamond interchanges and to assess the progression of the interior movements qualitatively and quantitatively. The new heuristic developed in this paper improves on traditional green band optimization techniques by incorporating actual demand profiles measured in the field. A field analysis was performed on a diamond interchange at I-69 and 96th Street in northwest Indianapolis, Indiana, where the existing sequence data were collected and used to model the alternative sequences to identify the optimal sequence. Interior operations were improved under the optimized settings: the percentage of vehicle arrivals on green increased by 19% during the 09:00-to-15:00 midday plan. Video observations were used to corroborate the data and are included in a video synthesis of the time–space trajectories.

Operation of signalized diamond interchanges with frontage roads using dynamic reversible lane control

Signalized diamond interchanges (SDI), connecting major highways and surface streets in urban and suburban areas, are probably the most widely used interchange patterns. The limited storage space between the two closely joined intersections coupled with heavy traffic volumes may easily oversaturate the facility and cause spillback problems, especially with the presence of frontage roads. This paper presents an innovative design and operational model for SDI by dynamically reversing certain lanes in the internal link on a regular basis with the deployment of overhead reversible lane control signs. A Binary-Mixed-Integer-Linear-Program (BMILP) is formulated to simultaneously optimize lane markings, dynamic usage of the reversible lane, and signal timings for the new SDI system. Results from extensive numerical analyses reveal the promising property of the proposed design and operational model in expanding capacity and reducing congestion at the SDI with frontage roads.

High-Resolution Event-Based Data at Diamond Interchanges

Signalized diamond interchanges are unique pairs of intersections characterized by interlocked left turns and relatively close spacing between ramps. A diamond interchange has four external entry points (origins) and four external exit points (destinations). Examination of the external origin–destination paths and evaluation of their impact on the interior storage and progression are critical for operating a diamond interchange effectively. This paper describes a series of performance measures derived from high-resolution signal controller data that can be used to (a) assess the quality of progression of the interior movements qualitatively and quantitatively and (b) optimize the internal offset to improve traffic flows within the interchange. Additional performance measures for identifying internal and ramp queuing are discussed. The integration of graphical performance measures into controller front panel displays and central systems is recommended to assist engineers in tuning and maintaining efficient operation of diamond interchanges.

Control Delay at Signalized Diamond Interchanges considering Internal Queue Spillback

Control delay is the primary measure for determination of the level of service of signalized intersections. The existing analytical delay models usually work well for isolated intersections, but are not as effective when applied to diamond interchanges. The limited storage space between the two closely spaced intersections of a diamond interchange may cause queue spillback from the internal link to the outside roads. This property would give rise to unrealistic delay calculations for diamond interchanges with high traffic volumes. This paper describes the development of a new analytical delay model that takes into account the effects of internal queue spillback at diamond interchanges. Simulation studies are conducted to compare the effectiveness of the proposed approach with existing methods. The study shows that for low-overlap time conditions, the proposed model tends to agree with the Synchro and VisSim simulations and is better than Elefteriadou's method, which tends to overpredict delay. For high overlaps, the Elefteriadou method, VisSim simulation, and the proposed model tend to agree, while Synchro diverges significantly by overestimating delay. The study contributes to the literature and practice by providing an open-source analytical model that can either be used as a stand-alone delay calculation model or as a supplement to the existing methods.

Application of Diamond Interchange Control Strategies at Closely Spaced Intersections

Signalized diamond interchanges are one of the most common interchange types in U.S. urban areas. Special signal control strategies have been developed for diamond interchanges to address their unique traffic flow and geometric characteristics such as tight spacing and one-way cross streets. The application of diamond interchange signal control strategies on a site consisting of six closely spaced intersections in Reno, Nevada, is documented. These intersections represent or closely resemble a standard diamond interchange. Advance control strategies are derived on the basis of diamond interchange signal control schemes. Simulation results of the timing strategies indicate significant reductions in the number of stops; however, no significant change is found for overall travel time and delay for the study site. Because the proposed control strategies emphasize maximum progression between closely spaced paired signals, the external approaches normally experience increased delay and stops. The proposed control is considered more efficient than the existing control because of significant reduction in stops and much improved driving experience and drivers' expectations. Public reaction to the new timings has been positive. Similar applications could be easily adopted at other locations with similar site characteristics. The proposed control enhances the knowledge of traffic signal control and coordination for closely spaced intersections.

Implementing Actuated Control of Diamond Interchanges

The design of signal timings for diamond intersections is well documented in the literature, and there are a variety of software packages to assist one in generating timing plans. However, the actual implementation of diamond intersection control procedures on modern dual ring actuated controllers is not documented in the literature, primarily because early implementations used vendor specific features on modern controllers. Consequently, none of the implementation procedures were transferable. In recent years, vendors have greatly increased the flexibility of their traffic controllers and now provide configurable ring structures, overlaps, detector modes, and detector mapping. This paper synthesizes current practice and defines procedures for implementing both three-phase and four-phase diamond control logic using common features on modern traffic controllers. The procedures introduced in this paper should allow this important area to mature and gain widespread implementation, leading to more efficient traffic signal operation at signalized diamond interchanges.

Fuzzy-logic-based incident detection for signalized diamond interchanges

This paper documents a fuzzy-logic-based incident detection algorithm for signalized urban diamond interchanges. The model is capable of detecting lane-blocking incidents whose effects are manifested by patterns of deterioration in traffic conditions that require adjustments in signal control strategies. As a component of a real-time traffic adaptive control system for signalized diamond interchanges, the algorithm feeds an incident report (i.e., the time, location, and severity of the incident) to the system's optimization manager, which uses that information to determine the appropriate signal control strategy. The performance of the model was studied using a simulation of an actual diamond interchange. The simulation study evaluated the model's performance in terms of detection rate, false alarm rate, and mean time to detect. The model's performance was encouraging, and the fuzzy-logic-based approach is considered promising.