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Balancing Safety and Capacity in an Adaptive Signal Control System, Phase I

Project Information

Project ID: 
Project Status: 
Start Date: 
Wednesday, September 3, 2008
End Date: 
Friday, October 1, 2010
FHWA Program: 
FHWA Subprogram: 
Safety Data and Analysis
FHWA Activity: 
Surrogate Safety Assessment Model (SSAM)
FHWA Topics: 
Road Operations and Congestion--Congestion Mitigation
TRT Terms: 
Operations; Safety; Research; Traffic Signals; Traffic Congestion; Highway Traffic Control; Traffic Conflicts; Traffic Accidents; Collisions
FHWA Discipline: 
TRB Subject Area: 
Operations and Traffic Management

Contact Information

First Name: 
Last Name: 
(202) 493-3317
Email Address: 

Project Details

Project Description: 

Draft Phase 1 final report, submitted by contractor.


The safety of signal variations or adjustments within Adaptive Control Software Lite.


Deliverable Name: 
Balancing Safety and Capacity in an Adaptive Signal Control System — Phase I
Deliverable Type: 
Research report or guidelines
Deliverable Description: 
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.