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• • 11/1/1999 Why are the Roads so Congested? Sprawl as a Primary Cause of Congestion Back to Table of Contents Sprawl as a Primary Cause of Congestion factors Contributing to the Growth in Driving The Texas �icreasc in Ta�cE'i3 Transportation Trips Ta Institute's data Increase in indicates that the Trip Lengths almost 70% increase 35c:� in driving in the last 16 years is a primary Decrease cause of congestion. in vehicle The factors that Occupancy` contribute the most to 17% that increase are at least partially related to sprawling switch to development Drivingincrease in patterns. According to 17% Population the figure (right) 13% published in a U.S. Department of Transportation study, Source: Travel Behavior Issues in the 90's. U.S. Department of as much as 69% of Transportation, Federal Transit Administration. Washington, DC, July 1992: the growth in driving p• 14. 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 analysis' 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 areae. 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.