ITS Benefits: The Case of Traffic Signal Control Systems

ITS Benefits: The Case of Traffic Signal Control Systems
Authors: Skabardonis, Alexander
Summary Information
This paper summarizes an evaluation of the benefits of optimizing traffic signal timing plans,
coordinating traffic signal control, and implementing adaptive signal control at locations
throughout the State of California. The signal timing optimization and coordination projects were
carried out during the Fuel Efficient Traffic Signal Management (FETSIM) Program, between
1983 and 1993. This program involved 163 local agencies and 334 projects, improving 12,245
signals at a cost of $16.1 million, or $1,091 per signal. Timing plans were developed using the
TRANSYT-7F software package. TRANSYT-7F also produced estimates of delay, number of
stops and fuel consumption under the recommended signal timing plans, indicating the benefits of
optimization. This created a database of the impacts of coordinated signal timing optimization
under a variety of local conditions across the many projects implemented during the FETSIM
program. Data from the Los Angeles Advanced Traffic Control and Surveillance System
(ATSAC), allows an assessment of the performance of traffic responsive signal control at several
intersections. The results presented in this paper are based on analysis of the accumulated
statistics from the local agencies carrying out the signal improvement projects.
The TRANSYT-7F model estimates from 163 of the 334 FETSIM projects (49%), representing
55% of the total number of signals retimed, demonstrated positive results from signal retiming of
coordinated signal systems. The study found an average 7.7% percent reduction in travel time,
13.8% reduction in delays, 12.5% reduction in stops and 7.8% decline in fuel use. This study
excluded TRANSYT-7F results for oversaturated approaches, as the simulation overestimates
the benefits of retiming under these conditions. Results of “before” and “after” floating car studies
reported by some of the jurisdictions support the TRANSYT-7F figures. The average reductions
reported by the floating car studies were: 7.4% in travel time, 16.5% in delay, and 17% in stops.
The differences between these figures and the TRANSYT-7F values are due to the survey routes
selected by the projects, which did not cover each link of the retimed signal network.
There was considerable variation in the improvement achieved by the retiming projects, for a
variety of reasons:
Quality of existing timing plans. When existing timing plans were already performing well,
there was often little benefit in signal optimization.
Network configuration. The largest savings were obtained on arterials, rather than
signalized grid networks.
Traffic Patterns. High volume systems with predominant through movements achieved
the greatest improvements. Savings were marginal on systems containing several
congested intersections in need of capacity improvements.
Signal Equipment. Systems with actuated signals and easily modifiable control
parameters achieved greater benefits.
“Before” and “after” field studies using floating cars assessed the performance of 76 of the
projects that implemented coordinated signal control during the FETSIM program. These field
data found an 11.4% travel time reduction, 24.9% delay reduction, and a 27% reduction in stops,
when compared to the performance of the systems prior to coordinated control. Major benefits
were reported for through traffic for signal spacing up to 0.5 mile and moderate to heavy traffic
volumes (v/c ratio > 0.6). Signal coordination generally worsened performance on system entry
The paper reports that the estimated benefit/cost ratio for the cumulative impact of all the FETSIM
projects, as determined by the recommended AASHTO method, was 17:1.
The adaptive signal control projects investigated by this study involved the application of Critical
Intersection Control (CIC) at seven Los Angeles intersections. CIC uses detector volumes to
generate green demand for each phase during a cycle based on volume data collected during the
previous cycle. The level of service (LOS) at 4 of the 7 intersections improved for a significant
percentage of the cycles assessed during the study, ranging from 27-55%. The only decreases in
LOS for any of the cycles evaluated were 5% of the cycles at 2 of the intersections studies. The
LOS for every other cycle at each of the intersections remained the same as it was prior to the
implementation of adaptive control. The study found that the benefits of CIC were greatest for
two-phase intersections with exclusive turning lanes, and/or unbalanced critical volumes. Benefits
were considerably lower for multiphase signals due to the inflexibility of phase split lengths.
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