Introduction

Road traffic congestion is a significant -- and growing -- problem in many parts of the world. Moreover, as congestion continues to increase, the conventional approach of "building more roads" doesn't always work for a variety of political, financial, and environmental reasons. In fact, building new roads can actually compound congestion, in some cases, by inducing greater demands for vehicle travel -- demands that quickly eat away the additional capacity.

In 1992, the Organisation for Economic Co-operation and Development (OECD) established an expert panel to study and report on the worldwide state of the practice for traffic congestion control and demand management. This article summarizes the study group's work and findings.

The Impacts of Congestion

All around the world, developed countries are discovering that road traffic congestion is no longer simply confined to commuter trips in urban areas. Congestion is everywhere. It affects the work trip and the personal trip. It affects the movement of people and the flow of goods to market. In both rural areas and intercity corridors, traffic is disrupted by incidents, maintenance operations, detours, and congestion on tourist routes, among other causes. To the traveler, congestion means lost time, missed opportunities, frustration, and waste of personal resources. To the employer, congestion means lost worker productivity, delivery delays, and increased costs. Nationally -- and internationally -- speed, reliability, and the cost of urban and intercity freight movements are increasingly affected by congestion.

The causes of congestion can be categorized as either recurring or nonrecurring. Recurring congestion is the predictable delay caused by high volumes of vehicles using the roadway during the same daily time periods (e.g., peak commute periods, holiday periods, or special events) and at critical locations (intersections, interchanges, major long-term construction areas, or toll plaza areas). Nonrecurring congestion is unpredictable delay generally caused by spontaneous, unplanned occurrences such as traffic accidents and incidents, emergency maintenance, or weather conditions.

Relieving Congestion Through Management

A growing body of evidence proves that traffic congestion problems cannot be solved simply by expanding the road infrastructure. Therefore, many countries are working to manage their existing transportation systems to improve mobility, safety, and traffic flows and to reduce demand for vehicle use. Experience and technical research have demonstrated that, when properly applied, measures taken to manage the existing transportation system can have a profound impact on trip-making behavior and traffic congestion.

A wide array of technologies for motorized and nonmotorized transport as well as economic and administrative policies have been used successfully around the world to manage congestion and influence travel demand. All of these measures, which are referred to collectively as congestion management, are designed to improve the operating efficiency of the existing transportation system -- its infrastructure, modes, and services -- in three ways:

• By increasing the use of alternative transportation modes including public transport, carpooling, and bicycling/walking.
• By altering trip patterns through the application of measures such as land-use policies, alternative work-schedule arrangements, telecommuting, and pricing.
• By improving traffic flow through measures such as route guidance systems, traffic signal improvements, and incident management.

Studying Congestion Management

The OECD Congestion Control and Demand Management Study Group investigated the congestion management technologies and policies of several countries to: (1) learn more about the measures -- both conventional and innovative -- currently in use to manage congestion and (2) share information with member countries on the broad collection of measures applied to reduce the impact of road traffic congestion problems. The group's analysis and findings are described in a final report, which presents a catalog of congestion management measures. (See sidebar.)

To conduct their investigation, the group members compiled technical information from their respective countries. Additional information came from material presented at the OECD Expert Workshop on Congestion Management held in Barcelona, Spain, in March 1993.

Categories of congestion management measures

Upon analyzing in-use congestion management measures, the study group determined that all of them fell into two basic categories. (See table 1.)

• Demand-side congestion management measures. These are measures that address traveler demand on the transportation system. These measures are designed to reduce vehicle demand on the system by increasing vehicle occupancy, increasing the use of public transport, reducing the need to travel during a specified peak-time period, and/or reducing the need to travel to a specified location.
• Supply-side congestion management measures. These are measures that address the transportation -- supply -- afforded of the existing system. These measures are designed to increase the system's existing capacity to improve traffic flow for all modes.

Table 1 -- Classification of Congestion Management Measures

Within these two strategic types, the group identified almost 40 conventional and innovative congestion management measures, which they categorized into nine strategic classes:

• Land use and zoning.
• Communications substitutes.
• Traveler information services.
• Economic measures.
• Administrative measures.
• Traffic operational measures.
• Preferential treatment.
• Public transport operations.
• Freight transport operations.

Applications of congestion management measures

Once all the measures were categorized, the study group analyzed how the various measures were being used. For example, congestion management measures can be used throughout a metropolitan area to address system, corridor, individual facility, and site improvements. They can also be implemented along major intercity corridors to relieve traffic congestion problems caused by work travel, construction, holiday or vacation travel, freight movement, and weather.

Tables 2 and 3 show how the various congestion management measures are being applied to address these conditions. The tables show which congestion management measures are most effective -- as judged by repeated application -- for addressing which conditions.

Table 2 -- Application of Demand-Side Congestion Management Measures

X = significant application, Y = some application, Blank = no application

Table 3 -- Application of Supply-Side Congestion Management Measures

X = significant application, Y = some application, Blank = no application

For example, the study group found that the demand-side, economic measure of congestion pricing can be applied effectively to address congestion in a wide range of situations, including in urban areas, in interurban corridors, for managing holiday travel, and for controlling traffic at special events. The study group found that the supply-side, preferential treatment measure of bus lanes can be applied effectively in urban areas for both peak and off-peak periods.

Objectives of congestion management measures

Implemented individually or in concert with one another, congestion management measures can help achieve one or more of eight clearly measurable objectives. These objectives, which can also be considered as positive impacts, are:

1. Reducing the need to make a trip.

2. Reducing the length of a trip.

3. Promoting nonmotorized transport.

4. Promoting public transport.

5. Promoting carpooling.

6. Shifting peak-hour travel.

7. Shifting travel from congested locations.

8. Reducing traffic/traveler delays.

The study group assessed how well each of the various congestion management measures achieves these individual objectives. (See tables 4 and 5.)

Table 4 -- Potential Impacts (Objectives) of Demand-Side Congestion Management Measures

X = significant impact, Y = some impact, Blank = no impact

Table 5 -- Potential Impacts (Objectives) of Supply-Side Congestion Management Measures

X = significant application, Y = some application, Blank = no application

Continuing the previous example, the study group found that the demand-side, economic measure of congestion pricing can have a significant, positive impact on promoting public transport and carpooling. Congestion pricing can also have a significant, positive impact on shifting peak-hour travel, shifting trips from congested locations, and reducing traffic delays. The study group found that the supply-side, preferential treatment measure of bus lanes can significantly promote public transport. The bus lanes can also have some positive impact on shifting trips from the peak hour or from congested locations and on reducing traveler delays.

Study Conclusions -- What Was Learned and What Needs to Happen

Although the OECD Study Group classified, categorized, and analyzed the various measures individually, it found that comprehensive programs involving both demand-side and supply-side measures produce the most successful results. Some examples of comprehensive approaches investigated by the study group are the Congestion Management System Regulations in the United States, the London Red Routes Program in the United Kingdom, the Structure Plan in the Netherlands, and the Olympics Traffic Management Program in Barcelona.

For a variety of reasons, however, such comprehensive approaches are more the exception than the rule. Given this observation on comprehensive approaches, the following conclusions and recommendations are presented as a result of the study:

• Road traffic congestion can be better managed.
• Low-cost conventional measures can be effective.
• Pricing techniques can be effective in congestion relief.
• Public support for congestion management is essential.
• Traveler information is important to congestion relief.
• Coordination between public agencies is an essential aspect of congestion management.
• Congestion management efforts need to start small then grow.
• The private sector should be a partner with the public sector in managing congestion.
• Comprehensive policies and laws are needed for effective congestion management.
• New technologies will offer tools for congestion management.
• Accessibility to commercial, residential, retail, and resort areas must be maintained with congestion management.
• Evaluations are needed in congestion management.
• Training in the practices of congestion management is needed.

Reference

Congestion Control and Demand Management, Final Report of the Scientific Expert Group TT1/TT2, Organisation for Economic Co-operation and Development, Road Transport Research Program, Paris, France, December 1994.

Sheldon Strickland was chief of the Traffic Management Division in the Office of Safety and Systems Application until his retirement in September 1994 after 30 years of service to the Federal Highway Administration (FHWA). During his tenure with FHWA, he worked both in headquarters and in the field in the management of traffic operations and planning. He was appointed to the Senior Executive Service in 1985. He was active in both the Institute of Transportation Engineers (ITE) and the Intelligent Transportation Society of America (ITS AMERICA). He was chairman of the OECD Scientific Expert Study Group TT1/TT2 that conducted the investigation reported in this article.

Wayne Berman is a highway engineer in Office of Traffic Management and Intelligent Transportation Systems. Since joining FHWA in 1974, he has been responsible for research, analysis, and technical assistance programs designed to help relieve traffic congestion, reduce vehicle demand, and improve mobility. He is an active member of the Transportation Research Board (TRB), ITE, and the Association for Commuter Transportation (ACT). He served as the technical secretary and U.S. writer for the OECD Scientific Study Group TT1/TT2.