Why Traffic Congestion Is Here To Stay, And Will Get Worse
Brookings Institution Press Briefing on Still Stuck in Traffic

This speech was delivered on June 14, 2004, at a briefing introducing Anthony Downs's new book, Still Stuck in Traffic. The speech essentially summarizes the main points in the new book, which is a greatly expanded revision of his earlier book (1992) Stuck in Traffic

It is both a pleasure and a challenge to speak to you today about my new book, STILL STUCK IN TRAFFIC. I will try to summarize its main points briefly and then answer your questions. This is the tenth book I have either authored or co-authored or edited for Brookings, so I am well-qualified to present a Brookings book.

Everyone hates traffic congestion, but it keeps getting worse, in spite of attempted remedies. This violates American axiom that all problems have solutions. "Why don't they do something about it?" is an often heard outcry.

The answer: because rising traffic congestion is an inescapable condition in large and growing metropolitan areas across the world. Peak-hour congestion is an inherent result of the way modern societies operate, and the strong desires of their residents to pursue goals that inevitably overload existing roads and transit systems every day. This talk examines peak-hour traffic congestion and the public policy challenges it poses.

Traffic congestion is not primarily a problem -- it is the solution to our basic mobility problem. That problem is that too many people want to move at the same times each day. Why? Because efficient operation of both the economy and our school systems requires that people work, go to school, and run errands during the same hours so they can interact with each other. We cannot alter that requirement without crippling our economies and societies. This problem exists in every major urban region in the world

In the U.S., the vast majority of people seeking to move during rush hours use private automotive vehicles, for two reasons. One is that most Americans reside in low-density settlements that public transit cannot serve efficiently. Also, privately-owned vehicles are more comfortable, faster, more private, more convenient in trip timing, and more flexible for doing multiple tasks on one trip than almost any form of public transit. Therefore, around the world, as incomes rise, more and more people shift from less costly modes of travel to privately-owned vehicles.

With 87.9 percent of America's daily commuters using private vehicles, and millions wanting to move at the same times of day, our basic problem is that our road system does not have enough capacity to handle the resulting peak-hour loads without forcing many people to wait in line for that limited road space. Such "waiting in line" is the definition of congestion. The same condition is found -- often even worse -- in growing major metropolitan regions everywhere.

There are four possible ways a region can try to cope with this challenge. But three of these ways are politically impractical or physically and financially impossible in the U.S. These ways are:

• Charging peak-hour tolls. The first way is to charge people money to enter all the lanes on major commuting roads during peak hours. If such tolls were high enough and collected electronically with "smart cards," the number of vehicles on each major road during peak hours could be slashed so that all those vehicles could move at high speed. That would allow more people to travel per lane per hour than do now under heavily congested conditions. That is why economists have long recommended this tactic.
  
  But most Americans would reject this solution politically for two reasons. Using such tolls would favor wealthier drivers and harm poor ones. The former could travel whenever they wanted to, but many of the latter would be forced off main roads during peak hours. Therefore, most Americans would resent such tolls, partly because they believe they would be disadvantaged by them.
  
  The second drawback is that people think such tolls would be "just another tax," forcing them to pay for something they have already paid for through gasoline taxes. Hence no democratic politicians anywhere advocate this tactic. The limited road-pricing schemes adopted in Singapore, Norway, and London only affect congestion in downtowns -- not the kind most prevalent in American regions.
• Greatly expanding road capacity. The second approach would be building enough additional road capacity to handle all drivers who want to travel in peak hours at the same time without delays. But this "cure" is totally impractical and prohibitively expensive. We would have to widen all expressways and other major commuting roads, demolishing millions of buildings, cutting down trees, and turning much of every region into a giant concrete slab. Then those huge roads would be grossly underutilized in non-commuting hours. There are occasions when adding more roads is a good idea, but no large region can afford enough more roads to eliminate peak-hour congestion.
• Greatly expanding public transit capacity. The third approach would be to expand public transit capacity enough to shift so many people from cars to transit that there would be no more excess demand for roads during peak hours. But in the U.S. in 2000, only 4.7 percent of all commuters traveled by public transit. (Outside of New York City, only 3.5 percent use transit and 89.3 percent use private vehicles.)
  
  A major reason is that most transit commuting is concentrated in a few large, densely-settled regions with extensive fixed-rail transit systems. The nine U.S. metropolitan areas with the most daily transit commuters, when taken together, account for 60 percent of all U.S. transit commuting, though they contain only 17 percent of our total population. Within those regions, transit commuters are 17 percent of all commuters. But elsewhere, transit carries only 2.2 percent of all commuters -- less than one percent in many regions.
  
  Even if the nation's existing transit capacity were increased fourfold and fully utilized, morning peak-hour transit travel would rise to 11 percent of all morning trips. But that would reduce all morning private vehicle trips by only 8.8 percent -- hardly enough to end congestion. And that tactic would be extremely costly.

Living with congestion: the only viable alternative. There is only one way to accommodate excess demand for roads during peak periods -- by having people wait in line. That means traffic congestion. So traffic congestion is an absolutely essential mechanism for American regions -- and most other regions throughout the world -- to cope with excess demands for road space during peak hours each day. Peak-hour congestion is the "balancing mechanism" that makes it possible for Americans to pursue other goals they value, such as working while others do too,

living in low-density settlements, and having a wide choice of places to live and work.

The least understood aspect of traffic congestion is the Principle of Triple Convergence. It occurs because traffic flows in a region's overall transportation networks form almost automatically-self-adjusting relationships among different routes, times, and modes.

Visualize a major commuting expressway so heavily congested each morning that traffic just crawls for at least 30 minutes. If that expressway were magically doubled in capacity overnight, the next day traffic there would flow rapidly because the same number of drivers would have twice as much road space.

But soon the word would get around that this road was now uncongested. Many drivers who had formerly traveled on that road before and after the peak hour to avoid congestion would shift back into that peak period. Other drivers who had been using alternative routes would shift onto this more convenient expressway. Even some commuters who used public transit would start driving on this road.

Within a short time, this triple convergence upon the expanded road during peak hours would make the road as congested as before its expansion. Experience shows that peak-hour congestion cannot be eliminated for long on an initially-congested road by expanding that road's capacity, if the road is part of a larger transportation network within the region. Almost all major roads are like that.

This principle does not mean that expanding a congested road's capacity has no benefits. After expansion, the road can carry more vehicles per hour than before, no matter how congested it is. So more people can travel on it during those more desirable periods. Also, the periods of maximum congestion may be shorter, and congestion on other routes may be lower. Those are all benefits. But that road will still experience some period of maximum congestion daily.

This principle greatly affects how many other suggested remedies to traffic congestion will work in practice. An example is staggered work hours. In theory, if a certain number of workers are able to commute during less crowded parts of the day, that will free up space on formerly congested roads. But once traffic moves faster on those roads, that will attract other drivers from other routes, other times, and other modes where conditions have not changed to shift onto the improved roads during peak hours. Soon the removal of the staggered-working-hour drivers will be fully offset. The same thing will happen if more workers become telecommuters and work at home.

Similar convergence will also occur if public transit capacity is expanded on off-road routes paralleling a congested expressway. This is why building light rail systems or even new subways rarely reduces peak-hour traffic congestion. Such congestion did not decline for long in Portland, where the light rail system was doubled in size in the 1990s, or in Dallas, where a new such system opened, or anywhere else that light rail systems or even new subways have been promoted as antidotes to peak-hour road congestion. Only complete expressway road pricing or higher gasoline taxes are exempt from the principle of triple convergence.

A ground transportation system's equilibria can also be affected by big changes in the region's population or economic activity. If a region's population is growing rapidly, as in Southern California or Florida, any expansions of major expressway capacity may soon be swamped by more vehicles generated by the added population.

This result is strengthened because America's vehicle population has been increasing much faster than its human population. From 1980 to 2000, 1.2 more automotive vehicles were added to the vehicle population of the United States for every 1.0 persons beings added to the human population (though this ratio declined to 1 to 1 in the 1990s). The nation's human population is expected to grow by around 60 million by 2020 -- possibly adding another 60 million vehicles to our national stock. That is why prospects for reducing peak-hour traffic congestion in the future are dim indeed.

Shifts in economic activity also affect regional congestion. During the internet and telecom boom of the late 1990s, congestion in the San Francisco Bay Area intensified immensely. After the "bubble" burst in 2000, congestion fell markedly without any major change in population. Thus, severe congestion can be a sign of strong regional prosperity, just as reduced congestion can signal an economic downturn.

Why has congestion increased almost everywhere?

The most obvious reason is population growth, but total vehicle mileage traveled has grown much faster. From 1980 to 2000, the total population of the U.S. rose 24 percent, but number of vehicles per 1,000 persons rose 14 percent and the number of miles driven per vehicle rose 24 percent. So total miles traveled grew 80 percent -- more than three times faster than population growth. Even without any population gain in those two decades, total miles driven would have risen 47 percent.

A combination of declining real gas prices (corrected for inflation) and more miles per gallon caused the real cost of each mile driven to fall 54 percent from 1980 to 2000! That helped raise the fraction of U.S. households owning cars from 86 percent in 1983 to 92 percent in 1995.

Furthermore, American road building lagged far behind increases in vehicle travel. Urban lane-miles rose by 37 percent vs. an 80 percent increase in miles traveled. As a result, the amount of daily traffic that was congested in the 75 areas analyzed by the Texas Transportation Institute went from 16 percent in 1982 to 34 percent in 2001.

Yet another factor consists of accidents and incidents, which some experts believe cause half of all traffic congestion. From 1980 to 2000, the absolute number of accidents each year remained amazingly constant, and the annual number of traffic deaths in the U.S. fell 18 percent, in spite of the great rise in vehicle miles traveled.

But incidents are non-accident causes of delay, such as stalled cars, road repairs, over-turned vehicles, and bad weather. No one knows how many incidents occur, but the number is much greater than for accidents. And the number of incidents probably rises along with total driving. So that could have increased congestion.

Another crucial factor contributing to more traffic congestion is the desire of most Americans to live in low-density settlements. In 1999, the National Association of Homebuilders asked 2,000 randomly-selected households this question:

"You have two options: buying a $150,000 townhouse in an urban setting close to public transportation, work, and shopping or purchasing a larger, detached single-family home in an outlying suburban area with longer distances to work, public transportation, and shopping. Which option would you choose?"

83 percent of respondents chose the larger, farther-out suburban home. New workplaces have also been spreading out in low-density areas in most regions.

Past studies have shown that public transit works best where (1) gross residential densities are above 4,200 persons per square mile, (2) relatively dense housing is clustered close to transit stations or stops, and (3) many jobs are concentrated in relatively compact business districts. But in 2000, at least two thirds of all residents of U.S. urbanized areas and over 75 percent of all citizens resided in settlements with densities of under 4,000 persons per sq. mi. Those are too low for public transit to be effective. Hence their residents are compelled to rely on private vehicles for almost all of their travel, including trips during peak hours.

Can Anything Be Done To Slow Down Likely Future Increases In Traffic Congestion?

There are some tactics that could, if carried out well, help slow down future increases in peak-hour and other traffic congestion, though they would not eliminate all increases.

1. Building more roads. Opponents of more roads claim "we cannot build our way out of congestion," because more highway capacity will simply attract more travelers. That criticism is true for already-overcrowded established roads. But the large projected growth of the nation's population means that we will need a lot more road and lane mileage just to cope in growth areas.
2. Using peak-hour road pricing. This tactic is not politically feasible if we try to put tolls on all major commuter lanes. But so-called HOT lanes (High Occupancy Toll) can increase traveler choices by adding some new toll lanes onto existing expressways while leaving present lanes without tolls. This allows anyone who needs to move fast on any given day to do so, without forcing all low-income drivers off the roads during peak periods. But HOT lanes will only work if the accompanying lanes remain congested. So HOT lanes do not eliminate congestion.
3. Ramp-metering means letting vehicles enter expressways gradually. It has improved freeway speed during peak hours in both Seattle and the Twin Cities. It could be much more widely applied across the U.S. and elsewhere.
4. Using Intelligent Transportation devices to speed traffic flows. These include electronic coordination of signal lights on local streets, large variable signs informing drivers of traffic conditions ahead, Global Positioning System equipment in cars and trucks, and radio broadcasts of current road conditions. These technologies exist now and can be effective, but they will not end congestion.
5. Responding more rapidly to traffic-blocking accidents and incidents. Removing accidents and incidents from major roads faster by using roving service vehicles run by Traffic Management Centers equipped with television and electronic surveillance of road conditions is an excellent tactic for reducing congestion delays.
6. Adopting "parking cash-out" programs. Demonstrations have shown that if firms offer to pay persons now receiving free employee parking a stipend for shifting to carpooling or transit, significant percentages will do so. That could reduce the number of cars on the road. But it will be partly offset by triple convergence.
7. Restricting the outward movement of new development. Severely constraining far-out development may reduce total driving at the edges of a region. However, it takes very large percentage increases in peripheral densities to cause significant declines in regional average driving distances. Also, even if a region's population grows 2.4 percent per year -- double the 1990s' national average -- its present population will still be 62 percent of its 2020 population. So it is difficult to cut overall population densities via marginal changes except over very long periods.
8. Requiring higher densities in both new-growth and established settlements. Proposing to raise existing settlement densities will arouse opposition from current residents. Most suburban governments are politically dominated by homeowning voters who do not want local changes likely to reduce the market values of their homes. Hence they usually oppose more multi-family housing and higher-density single-family units. The Portland, Oregon region has had the nation's most stringent urban growth boundary for over 20 years plus rapid population growth. Yet in 2000, it had a relatively low density of 3,340 persons per square mile.
9. Clustering high-density housing around transit stops. Such Transit Oriented Developments (TODs) would permit more residents to walk to transit, thereby decreasing private vehicles on the roads. But (1) the number of such "transit circles" within each region would have to be very large, (2) the residential density within each circle would have to be much greater than the average central city density in 2000 and (3) the percentage of workers living in the TODs who commuted by transit would have to be at least triple the 10.5 percent average for central cities in 2000. Moreover, the shift of TOD residents from using private vehicles to transit would soon be offset by the principle of triple convergence.

What conclusions can be drawn from this in-depth but objective analysis?

Peak-hour traffic congestion in almost all large and growing metropolitan regions around the world is almost certain to get worse during the next few decades because of rising populations and wealth, no matter what policies are adopted to combat congestion.

This outcome should not be regarded as a mark of social failure or wrong policies. In fact, traffic congestion often results from economic prosperity and other types of success. People congregate in large numbers in those places where they most want to be. The paradox of congestion was depicted best by Yogi Berra when he said, "No one goes there any more because it's so crowded."

This conclusion does not mean nothing can be done to slow down the rate at which congestion intensifies. Several policies could do that effectively. But nothing can eliminate traffic congestion from large metropolitan regions anywhere, and only serious recessions could forestall its increasing.

So my advice to traffic-plagued commuters is: relax and get used to peak-hour congestion. Get a comfortable air-conditioned vehicle with a stereo system, a tape deck and CD player, a hands-free telephone, perhaps even a microwave oven, and commute daily with someone you really like. Learn to make congestion part of your everyday leisure time, because it is going to be your commuting companion for a long time.