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For the last 16 years, the Texas Transportation Institute (TTI) has reported on traffic congestion in the country’s major metropolitan areas. This year’s TTI study reports that in 1997, congestion levels continued to increase in almost all of the 68 cities for which they reported data. In some areas, traffic congestion has become a daily topic as officials and citizens seek ways out of the jams. But in order to effectively fight congestion, we need to first know why it is occurring.

The Surface Transportation Policy Project (STPP) has produced the following companion analysis of TTI’s data to begin to answer the question: "Why are the roads so congested?" Using TTI’s new data, we performed several analyses to illuminate the true causes of congestion while dispelling some myths.

It is commonly felt that congestion is a natural and unavoidable consequence of ‘growth.’ But what kind of growth are we talking about? Our analysis centers on several growth factors measured by the Texas Transportation Institute.

The Role Of Population Growth

TTI’s data show that population growth is only a minor factor in the recent rise in congestion. Population in the metro areas studied grew by 22% during the study years (1982-1997). By contrast, the delay experienced by drivers grew by 235% in the same period. This was in large part due to the increase in driving in these areas. Actual population growth in these areas totaled almost 22 million people over this period, but STPP calculates that the increase in driving by each resident makes it feel as if about 70 million more drivers have been added to the highways. This ‘perceived population growth’ experienced by motorists helps explain the widespread feeling that our metro areas are "growing too fast" or "bursting at the seams."

Only 13% of the growth in driving between 1983 and 1990 is attributed to population growth. In other words, most of the growth in driving comes not from new drivers, but from more driving by the people already on the road. Why are Americans driving so much more each year? U.S. Department of Transportation data show that 69% of the growth in driving in this period was due to 3 factors: longer average trips, less carpooling, and a switch from biking, walking, or transit to driving. Each of these factors is at least partially related to changing development patterns. Americans are each driving more every year in large part because of the increasingly spread out nature of our metro areas. As growth sprawls outward, jobs, housing and services grow farther apart. Development patterns that require an automobile trip for every errand force us to drive more every year to accomplish the same things. This is confirmed in STPP’s analysis of TTI’s data, which found that the spread of our metro areas is directly contributing to the increase in driving.

The Role Of Roadbuilding

One commonly cited cause of congestion is a failure to provide more road space. Our analysis shows that on average, the highway networks in the cities studied by TTI have expanded faster than population. The amount of highway per person in these metro areas grew by 10% over the last 16 years. We are adding highways faster than we are adding people to drive on them.

In addition, our analysis found that road building seemed to have little impact on congestion. Between 1982 and 1997, metro areas that were aggressive in expanding the amount of road space per person fared no better in terms of rush-hour congestion than those that did the least to add new road space; in fact, they did slightly worse. This is due in part to what is known among transportation planners as ‘induced travel,’ a phenomenon in which newly available road space encourages additional car travel. Our analysis of TTI’s data confirms previous research on induced travel; in the metro areas studied, a 10% increase in the size of the highway network is associated with a 5.3% increase in the amount of driving.

This analysis indicates that our current traffic congestion problems are not an inevitable consequence of the healthy growth of our metro areas. These problems appear to be more closely linked to the sprawling development patterns that require so much driving. In addition, our analysis shows that congestion is not easily alleviated through adding road space. These results indicate that the traditional, road-based approach to fighting congestion is not working very well, and transportation officials might have greater success if they focus their efforts on other, more innovative congestion-fighting techniques.

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