Improving

U.S. Department of Transportation

Federal Highway Administration

Prepared by the Institute of Transportation Engineers

under a grant from the Federal Highway Administration,

U.S. Department of Transportation, 1995.
Institute of Transportation Engineers is an international educational and

scientific association of transportation and traffic engineers, transportation planners,

and other professionals who are responsible for meeting mobility and safety needs. 

The Institute facilitates the application of technology and scientific principles to

research, planning, funational design, implementation, operation, policy development,

and management for any mode of transportation by promoting professional development of members, supporting and encouraging education, stimulating research,

developing public awareness, exchanging professional information, and maintaining

a central point of reference and action.

Founded in 1930, the Institute serves as a gateway to knowledge and advancement tbrougb meetings, seminars, and publications and through its network of more

than 13,000 members working in some 70 countries. The Institute also has over 70

local and regional chapters and more than 90 student chapters that provide additional opportunities for information exchange, participation, and networking.

The Institute is headquartered in Washington, D. C.

Acknowledgments:

Written by Juan M. Morales, J. M. Morales & Associates, P.C

Edited by Kathryn Harrington-Hughes, Harrington-Hughes & Associates.

Designed by Debra Naylor, Naylor Design Inc.

Cover Photo: Les Moore/Uniphoto

Members of the 1994 ITE Urban Traffic Engineers Council Executive Committee and others

served as reviewers for this report:

Lyle Berg Jeffrey Paniati

Frank Dolan Joseph Pecora

Richard Edmonston Mary Anne Reynolds

R. Marshall Elizer Carlton Robinson

Kay Fitzpatrick

David Gerard

James Hauck

T. Douglas Hess

Brian Kanely

Walter Kraft

Nazir Lalani

John LaPlante 

Richard Macaluso

Dale McKinney 

Michael Meyer

Raymond Norris

Morris Rothenberg

James Sparks

Wayne Tanda

Philip Tarnoff

Sally Thompson

Norman VanNess

Graham Vincent

Charles Walter

Roy Wilshire

David Woosley

H. Nathan Yagoda

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IMPROVING TRAFFIC SIGNAL OPERATIONS
Table of Contents

1 . Traffic Signals: Tools for Improving

Safety and Traffic Flow 5

2. Simple Strategies with Big Payoffs 6

3. How Traffic Signals Work 9

4. When Is a Signal Needed? 1 1

5. Traffic Signal Maintenance 13

6. Legal Aspects 14

7. Funding Traffic Signal Improvements 15

References and Bibliography 1 6

3

IMPROVING TRAFFIC SIGNAL OPERATIONS
1 .

Traffic Signals--Tools for

Improving Safety and Traffic Flow

Traffic signal

operations can be

substantially

improved by

implementing

an aggressive,

yet relatively lowcost, management

system that will

minimize traffic

delay, pollution,

and fuel

consumption.

Traffic congestion is a major problem in cities of all sizes. People are taking moretrips, and there are more vehicles on the road. The street system is often

overtaxed, causing traffic to bog down. The resulting traffic congestion is costlymotorists’ time is wasted, and the environment is harmed by pollutants emitted from

idling engines. Even worse, the congestion often provokes motorists into dangerous

behavior, such as running red lights, in an attempt to make up lost time.

Some relatively simple, low-cost adjustments to a traffic signal system can, however, significantly improve traffic flow. This report describes how adjusting your city’s

traffic signals can reduce congestion and lead to big payoffs in time savings, environmental benefits, and safety.

When the first traffic signal was installed at a Cleveland intersection in 19 14, the

objective was to prevent accidents by alternately assigning the right of way. Not much

thought was given to minimizing traffic delay and fuel consumption. Over time,

however, as traffic volumes have increased, the objective has broadened to include

maximizing the capacity of the roadway system and improving traffic flow.

Today, there are more than 300,000 traffic signals in North America. They play an

important role in the transportation network. Traffic signals at a busy intersection in

a typical urban area might direct the movement of as many as 100,000 vehicles per

day-as if the entire population of Albany, New York, drove through the intersection.

Two-thirds of all miles driven each year occur on roadways controlled by traffic

signals. In the State of California alone, motorists drive more than 60 billion miles

each year on streets controlled by traffic signals.

Despite their important role in traffic management, traffic signals, once installed,

are often given short shrift. Maintenance activities are delayed or canceled, in reaction to shrinking budgets and staffs. More than half of the signals in North America

are in need of repair, replacement, or upgrading. If signals are not properly designed,

installed, operated, and maintained, motorists will likely

l Spend more time delayed in traffic,

l Increasingly disobey signal indications (for example, run red lights),

l Reroute themselves onto adjacent neighborhood streets, and

l Experience higher accident rates, especially involving rear-end collisions.

It doesn’t have to be that way. Traffic signal operations can be substantially improved

by implementing an aggressive, yet relatively low-cost, management system that will

minimize traffic delay, pollution, and fuel consumption. Some relatively simple tasks,

such as adjusting the timing of traffic signals, can bring dramatic benefits to a city and

its citizens.

5

IMPROVING TRAFFIC SIGNAL OPERATIONS
2 .

Traffic signal

improvements

rank as one of the

most cost-effective

energy conservation strategies in

urban areas.

Simple Strategies

with Big Payoffs

Much of the delay experienced by motorists during the day occurs at signalized

intersections, as they wait for the light to turn green. Delays can be reduced,

however, by optimizing the timing of the signals. Although more than 60 percent of

the signalized intersections in the United States would benefit from equipment

upgrades, nearly 1.5 percent of the intersections would benefit from signal timing

adjustments alone-without any hardware changes.’

Traffic signal improvements rank as one of the most cost-effective energy conservation strategies in urban areas. An idling engine not only wastes fuel, but also emits

pollutants into the air. The 1990 Clean Air Act Amendments require metropolitan

areas to improve their air quality; one cost-effective, practical step toward that goal is

to cut down the amount of time vehicles spend idling at traffic lights.

The annual costs for various signal system improvements are summarized in

Table 1. Optimizing the timing of already interconnected traffic signals is the most

cost-effective project, with an annual cost of $300 to $400 per signal. At today’s gas

prices, that is very cost effective- for less than a nickel, we can save a gallon of gas.

Each dollar spent optimizing signal timing could yield a 15- to 20-gallon fuel savings.

Alternately, interconnecting and optimizing noninterconnected signals costs anyTable 1.

Summary of Annual Costs of Various Traffic Signal System Improvements*

IMPROVING TRAFFIC SIGNAL OPERATIONS

Reducing the total vehicle hours of travel-that is, the number of hours motorists

spend behind the wheel-by a mere 10 percent results in a 3.5 percent savings in

areawide vehicle fuel consumption. That amounts to almost 12 million gallons saved

annually in an urban area with a population of 1 million. 2

Figure 1.

Comparative Impacts of Traffic Signal System lmproveinents (Project Level)2

8

IMPROVING TRAFFIC SIGNAL OPERATIONS
3 .

How Traffic

Signals Work

Traffic signals alternately assign the right of way to different traffic movements at

a n intersection. They provide a degree of control that is second only to physical

barriers.

A controller is used to switch the signal displays. Two basic kinds of controllers are

used: Pretimed (also known as fixed time) and traffic actuated.

Pretimed Controllers

Pretimed controllers represent traffic control in its most basic form. They operate on a

predetermined, regularly repeated sequence of signal indications. For example, in one

complete phase of the cycle, one street-the primary street-may be assigned 40

seconds of green time, and the other street may be assigned 15 seconds of green time.

Several seconds per minute are assigned to the yellow, or clearance, interval. The

signal rotates through this defined cycle in a constant fashion, as determined by the

controller’s settings. Pretimed controllers are best suited for intersections where traffic

volumes are predictable, stable, and fairly constant. They may also be preferable where

pedestrian volumes are large and fairly constant.

Depending on the equipment, several timing sequences may be preset to accommodate variations in traffic volume during the day. The timing of pretimed signals is

typically determined from visual observations and traffic counts. Once the timing

programs are set, they remain fixed until they are changed manually, in the field.

Generally, pretimed controllers are cheaper to purchase, install, and maintain than

traffic-actuated controllers. Their repetitive nature facilitates coordination with

adjacent signals, and they are useful where progression is desired. Progression refers to

the nonstop movement of vehicles along a signalized street system. Properly timed

signal systems facilitate progression.

Traffic-Actuated Controllers

Traffic-actuated controllers differ from pretimed controllers in that their signal

indications are not of fixed length, but rather change in response to variations in the

level and speed of traffic Traffic-actuated controllers are typically used where traffic

volumes fluctuate irregularly or where it is desirable to minimize interruptions to

traffic flow on the street carrying the greater volume of traffic.

A simple traffic-actuated signal installation consists of four basic components:

detectors, the controller unit, signal heads (the traffic lights), and connecting cables.

9

IMPROVING TRAFFIC SIGNAL OPERATIONS
The detectors are usually placed in the pavement, but they are sometimes positioned on signal poles. Commonly used types include the inductive loop detector,

magnetic detector, magnetometer, and microwave detector.

The inductive loop detector is by far the most common. A loop of metal wire is

embedded in a saw-cut slot in the pavement and then covered with a protective epoxy

sealant. As a vehicle travels over the detector, its metallic mass changes the inductance

of the loop. The detector processes this change and notifies the controller unit of the

presence of a vehicle.

There are three basic types of traffic-actuated controllers:

l Semiactuated controllers,

l Fully actuated controllers, and

l Volume-density controllers.

Semiactuated controllers assign a continuous green indication to the major street

except when a detector signals that a vehicle on the minor street is waiting to enter the

intersection. Traffic detectors are thus only needed on the minor street approaches.

Fully actuated controllers require detectors on all lanes approaching an intersection.

They are most useful when vehicle volumes vary over the course of the day, making

frequent timing changes necessary. Fully actuated controllers are often preferred

because of their responsiveness to actual traffic conditions.

Volume-density controllers are a more advanced type of fully actuated controllers.

They record and retain actual traffic information, such as volumes. Using the recorded

information, they can calculate-and recalculate as necessary-the duration of the

minimum green time based on actual traffic demand.

Both pretimed control and actuated control have application today. In Howard

County, Maryland, for example, pretimed controllers are used to coordinate the flow

of traffic on main streets during the day, with semiactuated control on minor streets.

At night, when traffic volumes drop, fully actuated control is used on all streets.

The efficiency of a traffic-actuated signal installation depends on the programming

of the unit and the location of the detectors.

Timing adjustments should be made by trained technicians and should be based on

the traffic periods. When adjusting a controller, the technician should observe

the effect on traffic and then fine-tune the settings as necessary. Intersections should

be periodically monitored to ensure the signals are operating efficiently. As traffic

volumes and other conditions change, the controller settings will need to be changed

accordingly.

Another type of actuated control uses a computer to control, operate, and supervise

a traffic control signal system. Computer-controlled systems basically consist of a

central computer, communication media (cable, telephone, radio, etc.), and field

equipment (local controllers, detectors, etc.).

1 0

IMPROVING TRAFFIC SIGNAL OPERATIONS
4.

When Is a

Signal Needed?

Unwarranted

signals often

generate an

increase in

vehicle stops,

traffic delay,

fuel consumption,

traffic accidents,

and motorist

disrespect for

traffic signals.

When installed under conditions that justify its use, a traffic signal is an effective

traffic control device. Conversely, an unwarranted or poorly designed signal is

ineffective, inefficient, and a potential danger to motorists and pedestrians. For those

reasons, it is essential that qualified personnel conduct traffic engineering studies to

determine if a traffic signal is indeed warranted.

Many motorists and community leaders believe that traffic signals are the solution to

all traffic problems at intersections. In that mistaken belief, communities have installed

signals where no legitimate need exists, making the decision on the basis of public or

political pressure, rather than professional traffic engineering judgment. Unwarranted

signals, however,. often generate an increase in vehicle stops, traffic delays, fuel consumption, traffic accidents, and motorist disrespect for traffic signals.

To help communities in the United States determine when and where signals might

be necessary, the Federal Highway Administration has published the 

form Traffic Control Devices (MUTCD), which contains equipment and location specifications for signals, as well as guidelines (“warrants”) for determining when a signal is

necessary. The MUTCD is the federal standard governing traffic control devices in the

United States. Other countries have their own specifications and warrants.

To preclude the indiscriminate use of traffic signals, the MUTCD recommends that

traffic signals be installed only when at least one of the signal warrants in the manual is

met. But satisfying a warrant or warrants is not in itself justification for a signal, as

stated in the MUTCD:

Information should be obtained by means of engineering studies and compared with

the requirements set forth in the warrants. The engineering study should indicate

the installation of a traffic signal will improve the overall safety and/or operation of

the intersection. If these requirements are not met, a traffic signal should neither be

put into operation nor continued in operation (if already installed).

The MUTCD states that an investigation of the need for traffic signal control should

include, where applicable, at least an analysis of the factors contained in the 11 warrants

(Table 2). 6 (For more information on signal warrants, refer to the MUTCD.)

When Should a Traffic Signal Be Removed?

If a signal is no longer needed, because of changed traffic volumes or patterns, it should

be removed. There are, however, three areas of concern when a signal is marked for

removal-safety impacts; traffic flow impacts, such as delay and fuel consumption; and

cost impacts.

A Federal Highway Administration study found that when 191 urban intersections

were converted from signalized control to two-way stop control, neither the total

1 1

IMPROVING TRAFFIC SIGNAL OPERATIONS
accident rate nor the rate of injury accidents (those accidents resulting@ personal

injury) was significantly affected. 7

Factors known to affect the accident rate at intersections where traffic signals have

been removed include: (a) adequacy of the side-street sight distance, (b) traffic volumes,

and (c) the average annual accident frequency at the intersection prior to signal removal.

When a signalized intersection is converted to two-way stop control, studies have

found that the total delay per vehicle is reduced by 10 seconds, the idling delay per

vehicle is reduced by 5-6 seconds, stops are reduced by half, and fuel consumption is

slightly improved. In addition, by eliminating electrical costs and reducing maintenance costs, the switch to two-way stop control can save the highway agency money.

Before deciding to remove any signal, however, the highway agency must conduct a

thorough review process. Engineering factors, such as impact on accidents, must be

carefully evaluated, and other factors such as political barriers and public reaction,

must also be considered.

Table 2. Warrants for Traffic Signal6

IMPROVING TRAFFIC SIGNAL OPERATIONS
5 .

Traffic Signal

Maintenance

Traffic signal

timing must be a

routine, ongoing

activity involving

regular review of

timing plans in

light of actual

traffic volumes

and patterns.

An effective maintenance management program, consisting of both preventive and

remedial maintenance of traffic signal hardware and software, is essential to the

successful performance of any traffic control system, whether computerized or not. A

poorly maintained signal system can compromise travel efficiency and safety. And, as is

the case with most equipment, signal systems that are neglected will likely perform

inefficiently and experience premature failure, which in turn could lead to traffic delays

and even accidents. Preventive maintenance really does pay off with traffic signals.

To ensure that a signal system is working properly and thus that traffic is flowing

efficiently and safely, highway agencies should regularly monitor traffic at intersections

and then update their traffic control strategies, including signal timing plans, in response.

Unfortunately, because updating timing control strategies is labor intensive and thus

costly, many jurisdictions fail to do this, with the result that the original traffic signal plan

for an intersection often remains operational long after changing traffic volumes have

made it outdated. Those jurisdictions instead choose to reevaluate their traffic control

strategies only when forced to, such as when equipment fails, motorists complain, or

congestion becomes unbearable. Traffic signal timing must be a routine, ongoing activity

involving regular review of timing plans in light of actual traffic volumes and patterns.

Traffic signal maintenance can be divided into three categories: preventive maintenance, response maintenance, and design modification.

Preventive maintenance practices involve inspecting, cleaning, and adjusting signals

at regular intervals and replacing components as necessary. The goal is to avoid signal

failures through timely maintenance procedures. Examples include replacing signal

lamps, cleaning signal lenses, aligning signal heads, tuning vehicle detectors, and

inspecting and testing signal control equipment. Keeping detailed records of all these

tasks is an important part of an effective preventive maintenance program.

Vehicle detectors are an all-too-often overlooked component of the signal system.

They do, however, play a critical role in the efficient operation of a signal system. If a

detector stops working properly, the actuated controller becomes useless; the signal

system will no longer respond to actual traffic conditions.

The second category of maintenance- response maintenance-involves procedures

that are undertaken when traffic signal and control equipment fails, either fully or

partially. It also includes troubleshooting and record-keeping tasks. A highway agency

can minimize the need for this type of maintenance services by implementing a

well-planned preventive maintenance program.

The third category-design modification-involves changing the signal display,

timing plans, or equipment to reflect changed traffic conditions.

The overall level of maintenance will vary, depending on a jurisdiction’s commitment

to safe and efficient traffic operations and available funding and staffing resources.

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IMPROVING TRAFFIC SIGNAL OPERATIONS
6 .

Legal Aspects

I mproperly maintained and operated traffic signals can result in an increased number

and frequency of traffic accidents. A liability judgment against the highway agency

could result. Courts have held that state highway departments, for example, have a

duty to use reasonable care in creating a safe highway environment. This involves, in

essence, regularly inspecting the highway system, anticipating and correcting defects in

the system, and conforming with applicable standards and practices, such as the

Manual on Uniform Traffic Control Devices.

Although driver error contributes to many accidents, the highway environment is

also often culpable. The environment-which includes signs, signals, and markingscould cause motorists to err or to respond inappropriately at signalized intersections.

For example, an improperly timed signal may not provide an adequate yellow interval,

which could lead a driver to inadvertently run a red light and cause an accident.

Improperly timed and unnecessary signals can also lead to motorist disrespect for

signals in general, with the result that they take more risks when approaching and

entering intersections. The result could be an increase in accidents, with a corresponding increase in the highway agency’s liability exposure.

A highway agency’s failure to respond in a timely fashion to complaints about traffic

signals may be interpreted as negligence on the agency’s part. A series of accidents at

the same intersection could be construed as notice of an existing hazard. Although the

agency might be somewhat protected by a design immunity, a court might waive the

immunity in cases where there is an apparent notice of a hazardous condition. Once an

agency is aware of a dangerous condition, it must act reasonably to correct the hazard.

1 4

IMPROVING TRAFFIC SIGNAL OPERATIONS
7.

Funding Traffic

Signal Improvements

Funding sources for traffic signal improvements include a number of transportation improvement programs at the local, state, and federal level. In many

jurisdictions, the cost of installing and improving traffic signals near new developments is funded by fees assessed on the land developers.

Federal funds are available to assist highway agencies in making optimum use of

their signal systems so as to minimize delay and thus reduce the amount of pollutants

emitted from idling vehicles. Two pieces of legislation-the 1990 Clean Air Act

Amendments and the 199 1 Intermodal Surface Transportation Efficiency Act-set the

stage for funding transportation improvements. Both statutes emphasize improving

traffic operations, rather than building more highways.

Federal funds for traffic signal improvements are now available as part of the

National Highway Program, the Surface Transportation Program, and the Congestion

Mitigation/Air Quality Program. In addition, for the first time, federal funds can now

be used to help pay for the operation of traffic control systems.

Local highway agencies should work closely with their state department of transportation and their metropolitan planning organization to ensure that their traffic

signal improvement projects and other needs are fully considered for funding as part

of the transportation improvement program.

15

IMPROVING TRAFFIC SIGNAL OPERATIONS
For more information

about publications

of the Institute of

Transportation

Engineers, call

202-554-8050

References

1.

.2.

3.

4 .

5 .

6 .

7.

Twelve Tools for Improving Mobility and Managing Congestion. Washington,

D.C.: Urban Land Institute, 1991.

F. A. Wagner. Energy Impacts of Urban Transportation Improvements . Washington, D.C.: Institute of Transportation Engineers, 1980.

A Toolbox for Alleviating Traffic Congestion . Washington, DC: Institute of

Transportation Engineers, 1989.

Fuel-Efficient Traffic Signal Management: Results from the 1983 Program. 

California Energy Commission, 1984.

Urban and Suburban Traffic Congestion: Working Paper No. 10. Washington, DC: Federal Highway Administration, 1987.

Manual on Uniform Traffic Control Devices. Washington, D.C.: Federal

Highway Administration. 1988.

H. H. Bissell and L. G. Neudorff. “Criteria for Removing Traffic Signals.”

In 1980 Compendium of Technical Papers. Washington, D.C.: Institute of

Transportation Engineers, 1980.

Bibliography

Gluck, J. S Mastering Traffic Engineering. Military Traffic Management Command, 1981.

Kraft, W. H., and J. R. Doughty “Traffic Signal Maintenance Requirements.”

In Strategies to Alleviate Traffic Congestion. Washington, D.C.: Institute of

Transportation Engineers, 1987.

Lindley, J. A., and L. G. Neudorff. “Implementing Traffic Control Systems:

Effective Management is the Key.” In 1990 Compendium of Technical Papers,

Washington, D.C., Institute of Transportation Engineers, 1990.

Poteat, V. P. “Traffic Signal Retiming: It Works.” In Strategies to Alleviate Traffic

Congestion. Washington, D.C.: Institute of Transportation Engineers, 1987.

Rodgers, L. M., and L. G. Sands. Automobile Traffic Signal Control Systems. First

ed. Philadelphia, Pa.: Chilton Book Company, 1969.

Schoene, G. W., and G. W. Euler. “Energy Conservation Through Traffic Signal

Optimization.” In 1981 Compendium of Technical Papers. Washington, D.C.:

Institute of Transportation Engineers, 1981.

Traffic Improvements-Legal Aspects and Liabilities. Seminar Notebook. Washington, D.C.: Institute of Transportation Engineers, 1980.

Transportation: Efficiencies and Alternatives. Draft report. San Francisco, Calif.:

The Global Cities Project, 1991.

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IMPROVING TRAFFIC SIGNAL OPERATIONS