The objective of this research is to develop guidance on the operational analysis and traffic signal design and operation of DDIs and adjacent traffic signals. The guide should be useful to (1) a modeler working with designers to optimize the geometric design, (2) a signal designer implementing operational strategies, (3) a signal technician wiring the cabinet(s) and programming the controller(s), and (4) an operator fine-tuning the operation of the interchange and adjacent intersection in real-time throughout the day.
Particular issues of concern are:
- Operational analysis
- Appropriate use of modeling tools, including calibration and sensitivity analysis
- Influence area and interactions with adjacent intersections and driveways
- Evaluation of the distance between crossover intersections
- Estimation of queue lengths and impacts
- Design of bays
- Effects of transit (including stop location and priority treatments)
- Effects of large trucks
- Interactions with the operation of the freeway
- Traffic signal design
- Design of the detection system for vehicles and pedestrians
- Selection of one or two controllers for the DDI
- Appropriate traffic control for ramp traffic at the DDI
- Appropriate traffic control for pedestrian crosswalks
- Incorporation of a performance measurement system
- Interface with ramp meters
- Traffic signal operation
- Performance metrics for operation
- Setting up the dual-ring structure in the controller
- Determination of clearance intervals
- Setting up actuated parameters, including those for failed detectors
- Setting up preemption/priority strategies
- Queue management strategies to address oversaturation
- Selection of coordinated or free operation
The interim report is under review by the project panel.
Task 1. Review pertinent domestic and international literature on traffic signal design and operation at DDIs to identify best practices and lessons learned. This literature review should be kept current throughout the project since this design is new to the United States.
Task 2. Identify leaders in the design, installation, maintenance, and operation of DDI signals. Use interviews or other techniques to gather information on best practices and lessons learned on traffic signals at DDIs, by themselves and in conjunction with adjacent intersections. Identify situations and typical failure modes that warrant the development of new control strategies to address them.
Task 3. Describe new control strategy concepts that could address the problematic situations and failure modes identified in Task 2.
Task 4. Calibrate and validate the microscopic simulation model that will be used in Task 6.
Task 5. Within 9 months, submit an interim report documenting Tasks 1 through 4. The interim report shall also include a complete plan for conducting the simulation runs in Task 6 and a detailed outline of the guide.
Task 6. Conduct microscopic simulation runs to explore issues such as (1) benefits of best and innovative control strategies that have been identified, (2) sensitivity of the operation to distances and speeds between signals, (3) impacts of different volumes and directional distributions, (4) sensitivity of the operation to timing parameters (e.g., offsets, cycle lengths, overlaps, phase sequence), (5) effectiveness of different detection schemes, (6) interactions between a DDI traffic signal and ramp meters, (7) optimal control for ramp traffic, (8) effective pedestrian strategies, (9) effective preemption and priority strategies, and (10) strategies to address phase starvation and queue spillback.
Task 7. Insofar as practical, work with a transportation agency to field validate any new traffic signal control strategies developed.
Task 8. Develop the guide on the operational analysis and traffic signal design and operation of DDIs and adjacent traffic signals that encapsulates best practices, lessons learned, and new strategies.
Task 9. Submit a final report that documents the entire research effort and includes the Task 8 guide as a stand-alone document.
The diverging diamond interchange (DDI, also known as a double crossover diamond interchange) is a relatively new design to the United States that can increase throughput and safety without widening bridge structures. An introduction to DDIs is available at http://www.fhwa.dot.gov/publications/research/safety/07048/
. There are currently more than 20 DDIs in operation in the United States and more than 50 under design.
Because the DDI is a new and novel design, traffic signal designers and operators and geometric designers lack the depth of experience that they have with traditional interchange forms. Determination of the best geometric and traffic signal design depends on the appropriate use of analysis tools, particularly microscopic simulation models. Integrating operational analysis into the design phase of a project is becoming more common and will be beneficial in these early years of DDI deployment.
One area of concern seen at installed DDIs are the interactions with an adjacent traffic signal. The unusual operation of the DDI can impair progression and waste green time at the DDI or the adjacent signal.
In many ways, the expected growth of DDIs in the United States parallels the growth of modern roundabouts several years ago. Information on best practices and lessons learned from these early installations will be beneficial in elevating the state of the practice.