Balancing Safety and Capacity in an Adaptive Signal Control System — Phase 1
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Virtually all previous research addressing intersection safety and capacity has dealt with the two issues independently. Over the past 20 years, advancements in real-time adaptive signal timing strategies for intersections and arterials have improved signal operations by improving traffic flow efficiency, but most optimization algorithms do not include performance measures for safety. At this time, little is known about the relationships between signal timing parameters (e.g., cycle time, offsets, phase sequence, etc.) and safety that can be of benefit to traffic engineering practitioners.
This research, comprising two phases, focuses on the development of real-time signal timing methodologies and algorithms that balance safety and efficiency. This report summarizes phase 1, which examines relationships between signal timing parameters and surrogate measures of safety such as rear-end, angle, and lane-change conflicts. These single variable relationship studies determine the parameters that are most likely to offer benefits in an adaptive, real-time strategy. Phase 1 also identifies an experimental design methodology to compute the effect of a change to signal timing parameters and develops both procedures for calculating performance and algorithms for improving the traffic system based on safety and existing principles of adaptive control used in the Federal Highway Administration (FHWA) Adaptive Control Systems (ACS) Lite system.
The ultimate objective of this research is to develop algorithms that can balance the performance of the traffic control system for both efficiency and safety and that can work with state-of-the-practice signal controllers.
Monique R. Evans
Director, Office of Safety
Research and Development
This document is disseminated under the sponsorship of the
U.S. Department of Transportation in the interest of information exchange. The
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U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this report only because they are considered essential to the objective of the document.
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Technical Report Documentation Page
|1. Report No.
|2. Government Accession No.
||3 Recipient's Catalog No.
|4. Title and Subtitle
Balancing Safety and Capacity in an Adaptive Signal Control System—Phase 1
5. Report Date
6. Performing Organization Code
Ziad A. Sabra, Douglas Gettman, R. David Henry, and Venkata Nallamothu
8. Performing Organization Report No.
9. Performing Organization Name and Address
Sabra, Wang & Associates, Inc.
1504 Joh Avenue, Suite 160
Baltimore, MD 21227
10. Work Unit No. (TRAIS)
11. Contract or Grant No.
|12. Sponsoring Agency Name and Address
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296
|13. Type of Report and Period Covered
September 2008–December 2009
14. Sponsoring Agency Code
15. Supplementary Notes
The FHWA Contracting Officer's Technical Representative (COTR) was Joe Bared. The project panel that reviewed progress of this report are Eddie Curtis, Raj Ghaman, David Gibson, John Halkias, and Wei Zhang.
This research focuses on the development of real-time signal timing methodologies and algorithms that balance safety and efficiency. The research consists of two phases, and this report summarizes the findings of phase 1. First, it examines the relationships between signal timing and surrogate measures of safety: frequency of rear-end, angle, and lane-change conflicts. The Federal Highway Administration (FHWA) Surrogate Safety Assessment Methodology (SSAM) was used to evaluate simulated scenarios to test the relationships between signal timing parameters and the occurrence of traffic conflicts. A single intersection and a three-intersection arterial were examined, and each parameter was tested independently. The analysis effort indicated the following results:
- The ratio of demand to capacity (i.e., the length of the split) is a factor that influences the total number of conflicts. There is an inverse linear relationship between splits and total conflicts.
- Cycle length has the most significant impact on the total number of conflicts. Increasing the cycle length beyond its optimum value on an arterial system has a significant effect in reducing all types of conflicts.
- Detector extension times have only a minor impact on changes to conflict rates.
- The phase-change interval has a marginal effect on the total number of conflicts.
- Left-turn phasing (protected/permitted) has a significant effect on the total number of conflicts.
- An offset has an insignificant effect on conflicts until the change is more than ±10 percent of the cycle length.
- Phase sequence has a significant effect on the total number of conflicts on an arterial.
These results were obtained by modifying each variable independently for specific geometric and volume conditions. As such, these results provide evidence that certain parameters have a positive correlation to changes in surrogate measures of safety, but they do not provide metrics that can be used for real-time signal timing optimization. This report also discusses a methodology based on design of experiments to calculate a safety performance function that can be used for estimating the effect of changes to signal timing parameters in tandem. The report concludes with the development of a multiobjective optimization methodology and the five principle algorithms that constitute the proposed adaptive system for tuning the cycle length, splits, offsets, left-turn phase protection treatment, and left-turn phase sequence of a set of intersections.
|17. Key Words
Surrogate measures of safety, Adaptive traffic control, Traffic signal timing, Traffic conflicts, Microsimulation traffic models, ACS Lite, Multiobjective optimization, Design of experiments
|18. Distribution Statement
No restrictions. This document is available through the National Technical Information Service, Springfield, VA 22161.
19. Security Classification
(of this report)
20. Security Classification
(of this page)
21. No. of Pages
|Form DOT F 1700.7
||Reproduction of completed page authorized
SI* (Modern Metric) Conversion Factors
TABLE OF CONTENTS
2.0 SIGNAL TIMING AND SAFETY
3.0 ANALYSIS AND SIMULATION METHODOLOGIES
4.0 STUDY SCENARIOS AND SURROGATE MEASURES OF SAFETY
5.0 FINDINGS FROM THE SIMULATION ANALYSIS
6.0 PHASE 2 RESEARCH AND DEVELOPMENT
LIST OF FIGURES
LIST OF TABLES
LIST OF ACRONYMS AND ABBREVIATIONS
|AADT||Average annual daily traffic|
|ACS||Adaptive control system|
|ADT||Average daily traffic|
|Aimsun||Advanced Interactive Microscopic Simulator for Urban and Non-urban Networks|
|ASC MIB||Actuated signal controller management information base|
|ATMS||Advanced Traffic Management Systems|
|CCD||Central composite design|
|CEP||Conflict ending point|
|CICAS||Cooperative Intersection Collision Avoidance System|
|CMF||Crash modification factor|
|CSP||Conflict starting point|
|DCS||Detection Control System|
|DeltaS||Maximum speed differential|
|DeltaV||Change between conflict velocity|
|FHWA||Federal Highway Administration|
|FRESIM||Integrated Traffic Simulator|
|HUTSIM||Helsinki Urban Traffic Simulation|
|ITE||Institute of Technical Engineers|
|MaxD||Maximum deceleration rate|
|MaxS||Maximum speed of vehicle|
|MDSHA||Maryland State Highway Administration|
|OPAC||Optimization Policies for Adaptive Control|
|RHODES||Real Time Hierarchical Optimized Distributed Effective System|
|SCATS®||Sydney Coordinated Adaptive Traffic System|
|SCOOT||Split Cycle Offset Optimization Technique|
|SPUI||Single-point urban interchanges|
|SSAM||Surrogate Safety Analysis Model|
|TEXAS||Traffic Experimental Analytical Simulation|
|TOD||Time of day|
|TRANSIMS||Transportation Analysis and Simulation System|
|TTC||Time to collision|
|V/C||Volume to capacity|
|Y+AR||Yellow plus all red|