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Sprawl as a Primary Cause of Congestion

The Texas Transportation Institute’s data indicates that the almost 70% increase in driving in the last 16 years is a primary cause of congestion. The factors that contribute the most to that increase are at least partially related to sprawling development patterns. According to the figure (right) published in a U.S. Department of Transportation study, as much as 69% of the growth in driving between 1983 and 1990 was caused by factors influenced by sprawl. These factors include the same people driving farther, as well as a decrease in carpooling and a switch from biking, walking, or transit to driving. These changes are in part necessitated by the spread of subdivisions and office parks isolated from stores and schools. Residents are often left with no real alternative to driving. One of the unintended consequences of this growth pattern has been a steadily growing number of vehicle trips that has served to clog local streets and freeways with traffic and increasingly frustrate residents and workers. At the same time, the chart shows that population growth accounted for only 13% of the growth in driving.

STPP conducted a rigorous analysis¹ of more recent data (1992-1997) to examine the relationships between the growth in driving and other factors measured by TTI. STPP analyzed the growth of vehicle miles traveled (VMT) versus the growth of population, growth in the size of the urbanized area, increase in the number of highway lane miles, and initial density of the urban area². This analysis demonstrates how the spreading out of the metropolitan area has contributed to an increase in driving. TTI’s data reveals that every 10% growth in the size of an urbanized area generally has resulted in a 2.5% increase in miles driven, over and above the increase in driving that comes from population growth or other factors. The influence of additional road capacity, another outgrowth of sprawl, is discussed later in this paper.

This analysis indicates that our current traffic congestion problems are not an inevitable result of the normal, healthy growth of our metro areas. These problems are more closely linked to the sprawling development patterns that require so much driving.

1. This analysis estimated a system of equations simultaneously; the results reported were generated with full-information maximum likelihood estimation. Contact STPP for detailed methodology.

2. With one exception, all data came straight out of the TTI database. The one exception was initial (1992) urban area density, which was developed by Professor Rolf Pendall of Cornell University and represents the average density of all urban development in the metropolitan area.

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