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vehicle pools, thus dampening the geographical<br />

variation in the drive alone mode share.<br />

The increasing number <strong>of</strong> trips, distance <strong>of</strong><br />

trips, <strong>and</strong> dominance <strong>of</strong> private modes <strong>of</strong> transportation<br />

have all led to an increase in vmt in<br />

privately operated vehicles. In urban areas, vmt<br />

increased 34 percent between 1983 <strong>and</strong> 1990.<br />

(USDOT FHWA 1994, 2-5, 2-6) There is, however,<br />

significant variation across metropolitan<br />

areas in road transportation. Newer, fast-growth<br />

areas such as Houston <strong>and</strong> Atlanta have a vehicle-mile<br />

per capita rate more than 50 percent<br />

higher than some older metropolitan areas such<br />

as Boston <strong>and</strong> Pittsburgh.<br />

Finally, traffic growth is faster than growth in<br />

road capacity, leading to a potential for increased<br />

congestion in some areas. Between 1980<br />

<strong>and</strong> 1992, total urban vmt increased by 61 percent<br />

while urban road mileage increased by only<br />

26 percent. (USDOT FHWA various years)<br />

Wasted time <strong>and</strong> fuel are well documented costs<br />

Chapter 8 Transportation <strong>and</strong> Air Quality: A Metropolitan Perspective � 181<br />

FIGURE 8-3: JOURNEY-TO-WORK MODE SHARES IN SELECTED MSAS, 1990<br />

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�New York-Northern New Jersey-Long Isl<strong>and</strong> CMSA<br />

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Drive alone Vehicle pool Transit Work at home<br />

KEY: CMSA = consolidated metropolitan statistical area; MSA = metropolitan statistical area.<br />

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SOURCE: U.S. Department <strong>of</strong> Transportation, Federal Highway Administration, Journey-to-Work Trends in the United States <strong>and</strong> Major Metropolitan<br />

Areas, 1960–1990 (Washington, DC: November 1993), <strong>table</strong> 5-7.<br />

<strong>of</strong> congestion. (Downs 1992) Cars traveling in<br />

congested traffic have higher emissions per mile<br />

than cars traveling at a moderate speed. CO, the<br />

greenhouse gas carbon dioxide (CO 2), <strong>and</strong> VOC<br />

emissions are especially high when cars accelerate<br />

or decelerate, <strong>and</strong> may be significantly higher<br />

when a car is idling than when it cruises at a<br />

steady speed. (TRB 1995, ch. 3) It is not clear,<br />

however, whether relieving congestion necessarily<br />

reduces emissions. Congestion is a major<br />

deterrence to driving, so reduced congestion<br />

may lead to increased vmt. Also, emissions <strong>of</strong><br />

most pollutants, especially NOx, increase above<br />

moderate speeds.<br />

Empirical evidence on congestion is difficult<br />

to construct. Table 8-5 presents the Texas<br />

Transportation Institute’s (TTI’s) congestion<br />

index <strong>of</strong> U.S. cities. As the data show, most large<br />

cities have significant congestion problems,<br />

although some medium-size cities (e.g., Miami<br />

<strong>and</strong> Seattle) are more congested than some <strong>of</strong>

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