Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

Some Perspectives on Congestion: The Emerging Mission of Modern Transportation Departments

David J. Hensing

444 N. Capitol Street NW, Suite 249,
Washington, D.C. 20001.

After nearly a century of existence, state departments of transportation (DOTs) have accumulated a number of key missions, including system preservation, fostering transportation safety, and enhancing mobility of both persons and goods. Travel demand continues to grow steadily, vastly outstripping the ability of these departments and the underlying will of their bodies politic to match this growth with capacity supply. The inevitable result is congestion, which is spreading everywhere in both time and space. A series of national policies and programs have grown up, including congestion management systems, special funding categories, interest in congestion pricing schemes, and the application of advanced technologies as intelligent transportation systems to deal with this phenomenon. As a result, coping with congestion has emerged as a major mission of a modern state DOT, joining the ranks of the others. Key words: congestion, state transportation agencies, missions.

Most modern state departments of transportation began as highway departments in the last decade of the nineteenth century and the first years of the twentieth. They have been given a succession of missions—perhaps emphases would be a better word—since their inception. Although many have forgotten it, the first mission of some of these departments, particularly in the East, was to provide smooth riding surfaces for bicyclists, a popular activity approaching a craze in the 1880s and 1890s. With the advent of the popularly priced motor vehicle, however, the raisons d'Ítre and missions of road departments, more or less in order of succession, became "getting farmers out of the mud," i.e., getting produce to railheads and river ports and then to markets; connecting towns and cities; supporting commerce of both the intrastate and interstate variety; expanding Americans' recreational and vacation opportunities; providing for defense mobilization; and making the road travel experience safer.

Although pockets and periods of congestion arose immediately after the dawn of the motor age, they occurred mainly in the larger cities and, of course, were not new with motorized travel. Reliable records from the Roman Republic describe steps taken to reduce chariot-induced congestion in Rome and undoubtedly elsewhere. But it is only in recent years that coping with congestion has taken its rank as a major if not dominant mission of most state transportation departments.

Congestion is a peculiar phenomenon. It is the transportation characteristic most frequently covered in the press, the source of bitter complaints, a cocktail party staple. It's the source of some recent additions to the English language, like "there's a parking lot on I-270" and "brake lights reported on I-95 approaching Dale City." It has even led to gunplay in some locations, an incomprehensible modern tragedy. In the opening scene of one of the more violent films of recent times, the triggering event of the outraged protagonist's rampage is "gridlock" congestion on a Los Angeles freeway.

And yet, how many cities, towns, and counties, many with higher than average unemployment rates and economies in recession or worse, would not wish for some of that congestion to come their way. Though unpleasant, congestion is a symbol of robust economic activity and prosperity. Many politicians and members of the business community fear that some efforts to mitigate congestion, particularly those oriented toward the demand side, will damage the local economy.

Thus we are cautious in the verbs we choose in dealing with congestion. We can "fight" it, "reduce" it, "cope with" it, and "ease" it. But we cannot "eliminate," "erase," or "destroy" it.


There is a certain amount of imprecision in the definition of congestion. In its rulemaking concerning Congestion Management Systems, the FHWA defines congestion as "the level at which transportation system performance is no longer acceptable due to traffic interference." The key word, of course, is "acceptable." What is acceptable to someone in Chicago may not be so in St. Louis, Minneapolis, or Des Moines, and so on down the population scale. Complaints about congestion may be heard in yet smaller cities, and event-based or seasonal congestion in rural areas can be

severe. Thus, there is a relative sense to the word "congestion," which is strengthened by the continuation of the FHWA definition: "The level of system performance may vary by type of transportation facility, geographic location (metropolitan area or subarea, rural area), and/or time of day."

A second observation about congestion is that it comes in two distinct varieties, recurring and nonrecurring. The first term is used to refer to repetitive, peak hour congestion. Some refer to this as the "rush hour," although one wit has observed that it usually lasts longer than an hour, and during it no one is rushing. The second type is incident-based. This could also be properly referred to as recurring, since traffic incidents—vehicle breakdowns, accidents and the like—certainly recur in every peak period. The distinction is important, however, since solutions to each type are by and large different. The FHWA has estimated that 60 percent of all person-hours of delay due to congestion result from nonrecurring causes.

Figures 1 and 2 illustrate these two types of congestion, as well as the effect of potential mitigating measures aimed at both the supply and demand sides. The vertical axis of both charts is cumulative volume, while the horizontal axis is time. The solid straight line in both charts, sloped upward and to the right, indicates capacity or supply. For a freeway lane under uninterrupted flow conditions, for example, this might have a slope of 2000 vehicles per hour. The curved line represents vehicular demand. In Figure 1, starting at a more steeply sloped rate than supply at the outset,

demand exceeds supply and congestion results. This continues until the demand slope flattens to less than supply and eventually crosses the supply line, indicating the restoration of free flow conditions. The area between the flat supply line and the curved demand line represents the hours lost by travelers to congestion.

The dashed flat line, sloped more steeply than the solid one, reflects a congestion-fighting measure on the supply side. This might be the elimination of a downstream bottleneck, the installation of a ramp metering scheme, or other capacity-enhancing measure. The dashed curved line represents a demand-reduction measure, which might be an Advanced Traveler Information System, for example, which would encourage some travelers to defer their trips or to select an alternative, less congested route. Or it might be a ridesharing program, serving to raise average vehicle occupancy. In this example, congestion is not eliminated, but significantly reduced.

Figure 2 shows the situation where demand is well less than supply, until the supply, or capacity, is sharply curtailed by an incident, represented by the break in the solid line to a much flatter slope. The loss of one lane of a three-lane freeway to a vehicular breakdown is usually much greater than one third. The effect of secondary accidents, "rubbernecking," and speed loss due to vehicles moving from the impacted lane to the others results in a capacity loss of 50 percent or more. Thus, the speed with which the incident can be cleared and full capacity restored (the length of the shallowly sloped solid line) is crucial to minimizing delay losses.

A third characteristic of congestion is that it is closely related to safety performance. Incident-based congestion in particular leads frequently to secondary accidents due to sudden stoppages, efforts to bypass or avoid delays through unsafe actions or maneuvers, and other causes. Successful efforts to mitigate congestion nearly always lead to fewer accidents and improved safety.

A final aspect of congestion is that it is inevitable. It is growing everywhere, evidenced in longer and longer peak periods and its spread to locations and times heretofore congestion free. A glance at Figure 3 shows why. From 1950 to 1990, total vehicle miles of travel more than quintupled, while centerline miles of roadway has increased by only 17.5 percent. This figure includes the entire 43,500 mile Interstate system, and obviously lane miles have increased at a much faster pace, but the rate of physical capacity growth has slowed dramatically in the last decade-and-a-half of this period. Sanguinity about the ability of even small- and medium-sized metropolitan areas to build their way out of the demand-supply problem during the fifties, sixties, and even part of the seventies has disappeared everywhere. Even if funding were not limited, which it most assuredly is, no community will tolerate the negative social, economic and environmental impacts such a massive construction program would imply.


Coping with congestion, usually without significant physical capacity additions, has begun to emerge as a major mission of modern state and local transportation agencies. The succession of surface transportation acts by the U. S. Congress dating back to at least the 1960s clearly evidences the growing national concern with the problem. The very title of the most recent of these acts, the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) seems to illustrate this. The term "intermodal" means many things, but at least one of its implications is to shift demand to a less taxed transportation mode. And "efficiency" is frequently used as a counterpoint to congestion.

More substantively, many features of this law are unambiguously aimed at supporting the mitigation of congestion. These include the Congestion Management System requirement, one of six management systems embedded in the law; a new funding category called Congestion Mitigation and Air Quality (CMAQ); a call for study and possible experimentation with congestion pricing; support for Intelligent Transportation Systems development and deployment; and broader eligibility for many operational programs intended mainly for congestion relief such as incident detection and management programs, employer-based ridesharing, improved freeway operations, and others.

Originally a mandate, the recently passed National Highway System Designation (NHSD) Act has converted most of the six management systems to voluntary programs, although a Congestion Management System in so-called nonattainment (i. e., areas failing to meet ambient air quality standards) Transportation Management Areas (TMAs) continues to be required. A congestion management system is intended to be a process, aimed at identifying, prioritizing, and solving congestion problems, and it has been successfully implemented in the metropolitan areas of many states. A key focus of many of these systems is to speed the implementation of many congestion-fighting minor projects, such as more responsive modern signal systems, improved intersection geometry, employer-based ridesharing programs, and the implementation of incident management programs.

Until the recent passage of the NHSD Act, the name given to the CMAQ funding category was something of a misnomer, since the ISTEA legislative language restricted the use of these funds to projects whose purpose was aimed primarily toward air quality improvement. Many of the projects selected, however, had a beneficial effect on congestion as well, and with the new Act, projects funded under this category may also have as their primary goal congestion relief. Congestion has a serious degrading effect on air quality, and thus many projects benefit both kinds of needs.

Since the passage of ISTEA, about $1 billion has been spent annually under the CMAQ program. Approximately one third has been devoted to high occupancy vehicle lane projects, signallization improvements, and new turn lanes. Half the funding has been used for both rail and bus transit projects, and the remaining one sixth has gone into ridesharing efforts and bicycle and pedestrian projects. Most of these projects have helped to reduce congestion.

Congestion pricing is a concept beloved of economists but with few real-life applications. The idea of congestion pricing is to curtail driving during peak periods through tolls on roads, bridges, or certain designated travel lanes of roads and bridges; fees to enter congested areas; or changes in transit or parking pricing. A TRB policy study (1) of the topic essentially concluded that congestion pricing is technically feasible, would cause some motorists to change their behavior, and would result in some net benefits to society in terms of reduced congestion, improved air quality, and reduced energy consumption. If structured properly, it would benefit all income groups but would create some loser motorist classes. And it is highly uncertain in terms of political feasibility. One application currently being tried is in Southern California where HOT (high occupancy-toll) lanes with tolls varying by time of day (congestion level) are assessed against vehicles with one or two occupants. Three or more occupants are not charged. The toll charged ranges from $0.25 for off-peak users to $2.50 per vehicle during peak periods. Users must be equipped with a readable toll "tag," and violators are vigorously prosecuted. Though evaluation of this scheme is not complete, it appears to have promise and is being considered in other locations.

Incident management is the term referring to techniques aimed at nonrecurring congestion problems. A recent study (2) sponsored by the Truck Research Council of the American Trucking Associations has helped establish the dimension of this problem and through a series of case studies has identified some of the more effective solutions. Although lending itself to advanced technology solutions, a key to any effective incident management program is a solution to the need for effective institutional arrangements across both jurisdictional and functional boundaries, including fire, police, emergency medical, and street and freeway operations agencies. Stimulated by the creation of this study, a coalition of several national organizations including ATA, AASHTO, FHWA, ITE, Highway Users Federation, Towing and Recovery Association of America, Urban Land Institute, and affected state and local transportation departments have conducted a series of seminars in most of the larger and medium-sized metropolitan areas across the country, informing mayor, traffic engineers, fire and police chiefs, and other local leaders of the benefits and techniques of effective incident management. This has led to continuing programs in many of these areas.

The most potent of the congestion-fighting programs and activities to arrive in recent years is Intelligent Transportation Systems (ITS). Having gone by several names in the three venues in which it has manifested itself, North America, Western Europe and Japan, the term ITS now seems to have been adopted in all three. It refers to the application of advances in computing and communications to a wide range of transportation needs and is the primary technological reason for the feasibility of some of the activities heretofore mentioned such as congestion pricing and incident management. A system for classifying ITS applications has arisen, of which at least three are aimed largely at congestion mitigation. These include Advanced Transportation Management Systems (ATMS), aimed at integrated, infrastructure based traffic control systems; Advanced Traveler Information Systems (ATIS), providing pre-trip and en route real-time information on travel conditions; and Advanced Public Transportation Systems (APTS), offering real-time information to transit users based on advanced vehicle locator systems (AVL) and other technologies. Table 1 is a brief list of some of the ITS components being deployed now.

Table 1 Some Operational ITS Systems
ITS Systems
Integrated traffic signal systems with bus priority
Freeway management systems with ramp metering and traffic monitoring
Transit fleet management with AVL and in-vehicle monitoring
Incident response systems
Electronic toll and fare collection systems
Providing real-time travel information to the public

One of the longer range ITS programs with truly remarkable potential effects on congestion is the automated highway, a demonstration of which is called for in ISTEA. A consortium including General Motors, the California Department of Transportation (Caltrans), and others is actively engaged in this effort.


When asked what is his or her most important mission as the chief administrative officer of a modern state department of transportation, as AASHTO asks its members from time to time, the invariable response is system preservation. This is without doubt the way it should be, since after nearly a century of existence, the publicly owned transportation physical plant represents an enormous capital asset, the protection of which is paramount. As a second priority, providing for safety in transportation always appears high. Various formulations for preserving and enhancing mobility for both persons and goods appear next in most lists, including connecting rural areas to their markets and communities, suburbs to other suburbs and their central cities, and towns and cities to each other. These are the historical missions of a transportation agency. To this list must now be added the problem of mitigating congestion in both a metropolitan and rural context, which, due to its inevitable spread in both time and space joins the others as a front rank mission of a modern transportation agency.

  1. Curbing Gridlock, Peak Period Fees to Relieve Traffic Congestion. Transportation Research Board, National Academy Press, 1994.
  2. Incident Management. Trucking Research Institute, 1990.

CTRE is an Iowa State University center, administered by the Institute for Transportation.

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