Objective

The objective of this study is to develop adaptive signal control strategies along arterials, evaluate their effectiveness, and develop recommendations for possible Statewide deployment

Background

Most traffic control systems today are based on time-of-day schedules where the traffic signal timing and ramp metering rates are set by time-of-day based on historical data on traffic demand (e.g., am peak hour turning movement counts). Some systems operate in a traffic responsive mode where the traffic signal timing plans or ramp metering rates are selected based on observed volumes and occupancies. Traffic responsive systems find the best match between a plan that was developed based on design volume and occupancies values and observed values. There is no guarantee that a traffic responsive system will have a plan for the observed conditions. An adaptive control system will adjust the timing parameters at traffic signals (cycle length, green times, offsets) and/or ramp metering rates based on real-time data on traffic conditions.

Several adaptive control algorithms have been developed in the US and overseas. However, the practical implementation of adaptive control is limited especially in California. It is hard to evaluate the effectiveness of these algorithms for general use because most of the cited benefits are based on limited data and do not relate to the geometric, traffic and control characteristics of the specific project areas. More important, there is limited information on the costs associated with the deployment of such systems. There is a need to develop adaptive control algorithms, evaluate their performance through a field test, and develop a deployment plan for possible Statewide application.

Proposed Research Scope and Products

The proposed study will identify and select the most promising of existing adaptive control algorithms, develop improved algorithm(s) as appropriate, conduct field operational tests on real-world arterial corridors, and develop recommendations for deployment of adaptive control.

Specific products of the proposed research will be:

1. Improved adaptive control algorithm(s) for arterial streets that explicitly addresses the California Department of Transportation needs
2. Field testing and evaluation of the proposed algorithm in a real-world test site(s)
3. A deployment plan with guidelines for statewide implementation of adaptive control

Methodology

The research approach will consist of theoretical development, simulation modeling, and field implementation and evaluation of the proposed strategies. The primary emphasis on this project is to produce and field test operational adaptive control algorithms. The issues to be addressed in developing adaptive control algorithms including but not limited to control philosophy, surveillance requirements, theoretical formulation, computational requirements, controller hardware & software, communications requirements, and interfaces with other systems (e.g., freeway TMCs) for integrated corridor management or other local agencies' TMCs for multi-jurisdictional control. Key considerations and proposals for their resolution include the following:

Development of Adaptive Control Strategies: The approach for developing an adaptive control algorithm will be to select the most promising control strategy from the existing strategies identified in the literature, review and design and implement new functions and features that are required to meet the needs and requirements of selected test sites that are representative of arterials (including freeway-arterial corridors).

Control philosophy: The common assumption is that adaptive control continually changes the signal settings to match the measured (or predicted) traffic patterns in the network. However, this approach may not be beneficial on congested conditions because it may create spillbacks upstream from the critical intersection(s). Furthermore, coordination with adjacent metered ramps or congested off-ramps may necessitate overriding the blind adaptation to traffic patterns. It is critical to understand the difference between "control" and "adapt" and design an algorithm that satisfies the objectives and constraints for the specific project area.

Compatibility with Caltrans & local agencies signal control practices: We will consider the existing operation of signal systems on California State highways and local major arterials. Most of the signals are equipped with 170 or 2070 signal controllers and stopline detection. Algorithms that are designed to operate with such hardware framework would be easier to implement than algorithms which require customized controllers or detection location.

Surveillance requirements: The effectiveness of adaptive control depends on the availability and accuracy of the data on traffic conditions, i.e., size, shape and speed of traffic platoons approaching the intersection. The cost of installation and maintenance of conventional surveillance systems (loop detectors) is very high, particularly for systems that require multiple detection points upstream. We will investigate the potential of new detector technologies and how they can work with adaptive algorithms.

Test Sites: It is important that suitable test sites are selected for the evaluation of the proposed algorithms. Criteria for test site selection include: a) traffic volumes and patterns, b) proximity to freeway, c) hardware/software capabilities of existing signal system, d) data availability, e) cooperation with local agency staff.

Evaluation of the Proposed Adaptive Control Strategies: The performance of the proposed strategies will be first evaluated through simulation. The most promising strategies will be evaluated in the field through "before" and "after" studies. We will differentiate the benefits of the proposed adaptive control against both the existing system with existing signal settings, and the existing system with optimized signal settings to estimate the true benefits of adaptive control. Proposed measures of effectiveness (MOEs) include: arterial through traffic travel time, arterial link travel times, arterial link, cross-street and intersection delays, number of stops on the arterial links, and queue lengths on the arterial and cross-streets. System measures may include VMT (veh-miles traveled), VHT (veh-hrs traveled) and throughput, and cycle failures. Travel time reliability measures may include distribution of travel times, and the 90th percentile travel time under the different control scenarios

Schedule

The project will last two years. In Year 1, we will develop improved adaptive control algorithms and evaluate through simulation on a selected test site(s). In Year 2 we will perform field implementation and evaluation of the proposed algorithms.