TRAFFIC SIGNALS
• by Susan Langdon, P.E., PTOE
Signaling a
change of way
Traffic signal engineers can now directly measure what they
could previously only estimate and model
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here are approximately 400,000 signalized
intersections in the U.S. Delays at traffic
signals contribute 5–10% of all traffic
delays on major roadways alone. What
if we had a systematic procedure for identifying traffic
operation problems—and fixing them using controller
and probe data? Well, we do, and it is something Purdue
University, the Indiana Department of Transportation
(INDOT) and the Utah Department of Transportation
(UDOT), along with several professional organizations,
are making a concerted effort to implement. But first,
traffic engineers and planners must educate the general
transportation community about new automated traffic
signal performance measures that can change the way
DOTs operate.
For decades, the practice of signal-timing management has largely gone unchanged. Traffic-signal timing
plans are developed from volume data collected at
intersections for a few hours on a typical day; timing
plans can remain in place for three to five years or more
before they are reevaluated. What’s the problem? The
plans are not always effective because they become
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outdated when traffic conditions change due to growth
or other factors. Thankfully, several bright minds set out
to solve the problem and developed a dynamic solution.
Today, we have a better way, a new approach that is
one of the most significant improvements in signaltiming management in decades. Dr. Darcy Bullock of
Purdue University, in collaboration with INDOT and UDOT,
has combined research efforts to develop a new traffic
and signal management approach, known as Automated
Traffic Signal Performance Measures (ATSPMs). These
measures allow technical professionals to analyze
intersection operations like never before by using highresolution data. It has been a long process that required
many organizations coming together to institute change,
starting with the “Pooled Fund” study.
SOME BACKGROUND
In 2011, 12 organizations collaborated on the Federal
Highway Administration (FHWA) Pooled Fund study. The
study TPF-5(258) was led by INDOT and was supported
by nine other state transportation agencies, the FWHA
Office of Operations and Resource Center, and the
Everyone has a vested interest
in reducing congestion, saving fuel
costs and improving safety.
Chicago Department of Transportation (CDOT).
The study focused on developing automated traffic
signal performance measures (ATSPMs) based on highresolution controller event data (from here referenced
as event data). Event data encompass vehicle detection information and traffic signal phase information,
recorded at 0.1-second time intervals. The ATSPMs use
these data to allow agencies to visualize traffic being
served at intersections and along corridors at all times
of the day. These tools allow agencies to detect issues
more quickly, measure travel times along corridors, and
optimize signal operations like never before.
Why is this event data different than the data
collected from previous methods? This new highresolution data contains a real-time log of just about
every “event” the traffic-signal controller records. These
events include changes in detector states (on or off),
signal phases (begin green, begin yellow, begin red), and
all other actions the equipment controls and monitors.
The idea to record this high-resolution data came from
Purdue University, which originally partnered with
INDOT and signal controller manufacturers to develop
a data-logging system that could be used across the
board by all manufacturers. Once established, Purdue
took data-logging information to another level when
they developed a series of performance measures. INDOT
tested the data and discovered the value of the information provided. The data offered INDOT staff a modern
new way to adjust and monitor their traffic-signal
system. They built a website that graphed the ATSPMs to
improve the ease of use for their team.
UDOT was introduced to SPMs in 2012 by Purdue and
INDOT. Mark Taylor, traffic signal operations engineer at
UDOT said, “In 2012, UDOT’s executive leaders wondered
what it would take for UDOT’s traffic signal operations
to be world-class. We knew we needed to implement
real-time monitoring of the system.” So they used the
performance measures created by Purdue and INDOT.
Their staff followed INDOT’s lead and developed
software to digest the data and customize the reports for
their needs.
WHAT DO THE AUTOMATED
PERFORMANCE MEASURES DO?
The new performance measures use high-resolution
data recorded at 0.1 per second intervals, which are
stored in the signal controllers. The information from
each intersection is downloaded to a database server.
Software on the server converts the recorded data into
meaningful information. The performance measures
are then displayed in easy-to-understand graphics. This
real-time, continuous information provides agencies a
much more detailed and accurate understanding of their
intersection and corridor operations. Some of the metrics
currently available include:
• Purdue coordination diagrams (PCDs);
• Approach volumes;
• Approach speeds;
• Purdue phase termination charts;
• Split monitor;
• Approach delay;
• Arrivals on red;
• Pedestrian delay;
• Preemption details;
• Turning movement counts; and
• Purdue split failure.
Some of these metrics, such volumes, speed and
delays, are familiar to most traffic engineers. But
there are some exciting, new metrics that provide a
succinct visual summary of how coordinated signals
are operating. The Purdue Coordination Diagram
(PCD) (Figure 1) shows a 24-hour timeframe for
an individual signal phase at the intersection,
identifying when the phase was green, yellow
or red. It also shows each actuation
(vehicle arrival) throughout the
day represented by
a dot on the graphic
identifying whether
the car arrived during
the green, yellow
or red phase. Most
traffic engineers,
Susan Langdon is a
principal with Stantec
and was on the
development panel for
FHSA’s Signal-Timing
Manual, Version II.
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TRAFFIC SIGNALS
Figure 1.
The Purdue
Coordination
Diagram.
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when working on signal timing, spend hours
in the field watching the signal operate. It
is time-consuming, and it does not always
depict the full picture. The PCD shows an
entire day of operation—what is working
well and what needs adjustment—all from
the ubiquitous vantage of the office. The
software even provides daily email alerts.
UDOT’s software has moved the agency
a long way down the road towards offering
a “world-class traffic signals” program. UDOT is so
invested in the implications its software may have on
the national transportation community, it has made
its software available to the public for free. Taylor
notes, “UDOT created a website that does not require
any special software or passwords to access. In fact,
you can pull this website up on your smartphone. This
allows anyone who has a desire for the data to use it.
We have approximately 1,700 intersections statewide
in Utah bringing back near real-time SPM data. Using
these ATSPMs, UDOT has been able to improve mobility,
increase safety and use resources more effectively. We
now find most traffic-signal problems before the public
notices.”
Centralized traffic management software developers
are also taking note of the benefits of using ATSPMs.
Many “off-the-shelf” systems that are under development
or available now offer many of these features in their
new systems. The software has become so beneficial
that the FHWA made it available on its website for
open-source software. The federal agency also included
software documentation and a venue for users to share
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improvements in an open forum for online discussion.
According to Dr. Bullock, the combination of agencies and strong vendor participation was critical to the
success of this research. “Agencies reached consensus on
performance measures that help them more effectively
manage their traffic signal assets. Vendors were quick
to pitch in and deploy the requisite high-resolution
data collection capabilities needed to compute the new
performance measures,” says Bullock. “It’s truly been a
collaboration of all the various user groups that makes
up the transportation community to implement these
new practices. It’s an exciting time in the history of
traffic-signal management.”
WHY USE AUTOMATED MEASURES?
What can an agency accomplish using ATSPMs?
There are many opportunities in these measures to
improve daily operation of signal systems and the longterm health of the overall signal network, including:
• Troubleshoot complaints and reduce wasted time for
maintenance staff;
• Identify problems more quickly, without waiting for a
complaint call;
• Operate and optimize system more efficiently;
• Retime signals as needed, not on a schedule;
• Communicate system performance to public agency
leaders;
• Transition from reactive management to proactive
signal management; and
• Improve overall safety.
Everyone has a vested interest in reducing congestion,
saving fuel costs and improving
safety—from transportation
departments and metropolitan
planning organizations, to
municipalities and other
agencies seeking to improve
mobility in their community. By
maintaining safe and efficient
signal timing, DOTs can relieve
congestion and help people get
home safely.
In the past, UDOT tracked how
often staff took action based on
ATSPMs and how these actions
improved both operations and maintenance. Figure 2
reflects the number of actions taken to correct different
problems and the ATSPM tool that was used to find
those problems between Jan. 1, 2014 and June 18, 2014.
“You can see that we identified and fixed more than
250 detector problems, used the data more than 200
times in our traffic signal models, made more than 150
signal coordination split adjustments, and corrected or
optimized more than 50 time-of-day schedules for signal
coordination, Taylor said. “These ATSPM’s have enabled
us to do more with less, focus our resources on the areas
that need it most, and allowed us to more effectively
prioritize resources and workload.” That’s why Taylor is
not surprised to see that transportation departments
across the country are now starting to realize the
benefits and efficiencies the ATSPM software can deliver.
Currently, the city of Richardson, Texas, is working
alongside professional consultant Stantec to set up the
software for ATSPMs to manage its signal system. The
software is not something you buy off the shelf. Having
a traffic-signal consultant onboard can help agencies
understand the software, coordinate installation and
train staff on best practices. The ATSPMs are another tool
DOT and municipal staff can use to provide them with
better-quality information in traffic-signal monitoring to
make their jobs easier.
The FHWA’s Every Day Counts program includes
ATSPMs as one of 11 innovations featured in the
Figure 2. Tracking
actions taken and
Phase 4 2017-2019 program. The mission
ATSPM tool employed
of the EDC-4 initiative is to accelerate the
to correct.
implementation of automated traffic-signal
performance measures by providing tools,
technical assistance, and outreach to advance
objectives and performance-based approaches
to traffic-signal management and operation. The UDOT
software is available on the FHWA Open Source Application Development Portal (www.itsforge.com), including
documentation. The software’s availability is made
possible thanks to collaboration between the FHWA
and UDOT, which will hopefully lead to increased traffic
monitoring throughout the nation.
So, with all this great opportunity, where can DOTs go
from here? Agencies can reach out to their local FHWA
support team for information on funding opportunities
to enhance their signal system and incorporate ATSPMs
into their agencies. They can contact agencies that have
already begun using ATSPMs (see Figure 3) to understand lessons learned and discover how they made use
of the information from a local
perspective. Moreover, the
FHWA website provides
discussion boards on
ATSPMs, and there are
webinars and professional society meetings
within the industry for
further information, as well as
professional consultant teams
who offer assistance in making
ATSPMs a reality for any
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Figure 3. A diagram of agencies
employing ATSPMs.
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