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Overview
“When I was in Mrs. Lavender’s kindergarten class, in a suburban public school in LosAngeles,
we were visited one day by a local police officer. The officer brought a traffic signal, mounted
on a short pole. The signal had displays that could be seen by two lines of students, as if we
were pedestrians waiting to cross an intersection. We were instructed to form two lines and to
follow directions: walk when it was green and stop when it was red.” – the author
We learn the lessons of traffic control early in life, and with good reason. There are nearly 300,000 traffic
signals today in the United States. Each traffic signal performs the task of regulating whose turn it is to go
and who must wait. Some signal systems, known as fixed time systems, provide the same amount of time to
serve each group of users and in the same sequence. Other systems respond to the volume of vehicles and
pedestrians present at the intersection and provide varying amounts of time to serve these users. Some signals
operate independently and respond to the traffic demands at that intersection alone, while others operate
together in a system so that traffic can be moved with as few stops or as little delay as possible. A traffic signal
system at its core has two major tasks: move as many users through the intersection as possible and do it with
as little conflict between these users as possible. The first task relates to efficiency and capacity while the
second relates to safety. Both tasks are performed by first clearly defining which group of users has the right
of way at a given time and second by determining how long the group has the right of way.
The traffic signal system is probably the most important kind of transportation facility in operation today,
considering the perspectives of both safety and efficiency. Two-thirds of all miles driven each year in the U.S.
are on roadways controlled by traffic signals. In some urban areas, signals at busy intersections control the
movement of more than 100,000 users each day (Koonce, et al., 2008). The signal system also has a great
impact on energy usage and the environment. The more times a vehicle stops, the higher the level of pollutants
that it emits. And, twenty percent of the fuel used by automobiles traveling along urban arterials is consumed
while waiting at a red light at a signalized intersection.
According to a 2007 report from the National Highway Traffic Safety Administration, 20 percent of all motor
vehicle fatalities in the United States each year occur at an intersection. Between 1997 and 2004, this figure
represented 76,162 lost lives. In addition, tens of thousands of drivers, cyclists, and pedestrians are injured
each year in traffic accidents at intersections (Subramanian & Lombardo, 2007).
Any one of these traffic signals, or a system of several signals, can cause a motorist to wait unnecessarily.
The Federal Highway Administration estimates that half of these traffic signals need some sort of timing or
operational improvement
(Koonce, et al., 2008). A recent national report card gave the nation’s traffic signal
systems poor grades (National Transportation Operations Coalition, 2007). While there are a number of reasons
for this poor assessment, we believe that there are three major contributing factors. First, there is a lack of
high quality and comprehensive references defining good practice. While many states and local jurisdictions
do have standards that guide their signal timing design practices, often these standards are not based on good
science or sound theory that allow the standards to be transferable to new situations or conditions. Second,
university textbooks do not cover traffic signal systems in a comprehensive and realistic manner. Too often,
the systems are assumed to be fixed time (rarely the case in the field) while the traffic controller itself is not
covered at all. Most university laboratories do not have traffic signal controllers and are thus not able to give
their students experience in their use. Third, traffic engineers often have little direct experience with traffic
controllers since their university experience is often limited to using models that often poorly emulate the
operation of a traffic controller. This results in a problematic dichotomy. A signal engineer designs the signal
system and timing plan, but the implementation of the timing plan (and many of the important timing details)