Bureau of Transportation Statistics

Special Report

U.S. Department of Transportation

Research and Innovative Technology Administration SR-006

October 2007

Trends in Personal Income and Passenger Vehicle Miles

by Jeffery Memmott, Ph.D.

For years, increasing income has been 

a principal factor contributing to the rapid growth of highway passenger travel. 

As incomes increased, the demand for 

additional transportation services manifested itself in a variety of ways:

• more individuals and households 

acquired personal vehicles;

• the proportion of multiple vehicle 

households grew;

• families moved to larger and more 

comfortable housing in the suburbs, thereby increasing commuting trip distances; and

• the number and length of discretionary trips increased. 

As a result, passenger highway travel, 

as refl ected in vehicle miles of travel 

(VMT), increased rapidly.

But there have been discussions since 

at least the early 1990s about whether 

the effects of rising incomes on travel 

demand would start to diminish.1

 That 

speculation now appears to be a reality as recent data suggest the relationship has weakened—vehicle travel 

growth has started to slow in comparison to growth in real personal income.

1

 C. Lave, 1991. Things Won’t Get a Lot Worse: 

The Future of U.S. Traffi c Congestion. Working 

Paper No. 33, The University of California 

Transportation Center, University of California, 

Berkeley, CA.

Historically, passenger vehicle miles 

traveled2 and real personal income 

have grown at about the same rate, 

and at a much higher rate than population. Both also exhibit very similar 

patterns during periods of economic 

recession and expansion. However, since 1997, a widening gap has 

emerged between the trends in real 

personal income and passenger VMT 

(fi gure 1). The growth in passenger 

VMT is slowing down even though 

personal income continues to increase 

2

 Vehicle categories represented by Passenger 

Vehicle Miles Traveled include passenger cars, 

other 2-axle 4-tire vehicles (some of which may 

be used primarily for commercial purposes 

not involving passenger transportation), 

motorcycles, and buses.

Figure 1: Growth in U.S. Highway Passenger Vehicle Miles Traveled, Real Personal Income, and 

 Population (percent change since 1970)

1970 1975 1980 1985 1990 1995 2000 2005

0

20

40

60

80

100

120

140

160

180

200

Real personal income

Passenger VMT

Population

Percent

Source: Calculated from U.S. Department of Transportation, Federal Highway Administration, Highway Statistics, Table VM-1, Passenger Cars and Other 2-axle 4-tire 

Vehicles, various years. U.S. Department of Commerce, Bureau of Economic Analysis, Table 2.1 Personal Income and Its Disposition. U.S. Census Bureau, Annual 

Population Estimates for the United States, as of July 1 of each year.
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at about its historical trend. From 1970 to 1997, passenger 

VMT grew at 3.0 percent per year, and real personal disposable income grew at 3.2 percent per year. From 1997 

to 2005, passenger VMT grew at a much lower 2.0 percent 

per year, compared to 3.2 percent annual growth for real 

personal disposable income.

Household Income, Number of Vehicles, and 

Vehicle Use

In the past, one of the most important factors infl uencing 

the rapid increase in passenger VMT has been vehicle 

availability. Not surprisingly, income is a major determinant of the number of vehicles in a household. The 2001 

National Household Travel Survey found that, overall, 93 

percent of households have at least one vehicle.3 In the 

lowest household income bracket (less than $25,000), 80 

percent have at least one vehicle, and over 10 percent 

have three or more.4 Among households with no vehicles, 

the vast majority (78 percent) are in the lowest income 

category.

3

 The data were collected from March 2001 to May 2002.

4

 Household income is used in analyzing data from the 2001 National 

Household Travel Survey because it is the income category collected in 

the survey and available in the dataset.

An important observation from table 1 is the relatively 

similar distributions of vehicles for higher income categories. The lowest income category is different from the other 

income categories. The income categories from $25,000 to 

less than $75,000 transition from the predominantly zero 

or one vehicle households of the lower income category to 

multiple vehicle households. The two highest income categories ($75,000 to $99,999 and $100,000 or more) have 

very similar distributions. For household with incomes of 

$50,000 and above, the number of household vehicles 

increase by only modest increments. This would imply that 

increased incomes would potentially have the greatest impact on lower income households. Increased incomes for 

higher income categories would likely have a much smaller 

impact.

Auto availability, in turn, has a dramatic effect on the mode 

used to make trips. Figure 2 shows the distribution of trips 

by mode and number of household vehicles. Not surprisingly, zero-vehicle households make a much larger proportion of their trips using transit or walking than do households with one or more vehicles. Households with a vehicle 

use that vehicle for most travel. Households with one or 

more vehicles have similar modal use patterns, with some 

variation. For example, households having three or more 

Table 1: Distribution of Households by Number of Household Vehicles, for

 Each Income Category

Numer of household vehicles

Household income

0 vehicles 1 vehicle 2 vehicles 3 or more vehicles Total

Percent SE Percent SE Percent SE Percent SE Percent

0-24,999 19.5 0.61 47.9 0.73 22.2 0.53 10.4 0.41 100.0

25,000-49,999 3.3 0.29 33.7 0.61 40.6 0.54 22.5 0.51 100.0

50,000-74,999 1.4 0.23 20.6 0.69 45.7 0.85 32.4 0.65 100.0

75,000-99,999 0.8 0.24 11.8 0.89 47.9 1.22 39.5 1.02 100.0

100,000 + 1.3 0.26 9.6 0.66 48.2 1.10 40.9 0.98 100.0

Overall 7.2 0.20 30.8 0.31 37.7 0.26 24.3 0.24 100.0

Note: Sample covers about 26 thousand households, SE = standard error. Numbers may not add to 100 percent due to rounding.

Source: Calculated from the 2001 National Household Travel Survey, Household File, U.S. Department of Transportation, Bureau 

of Transportation Statistics and Federal Highway Administration.

Table 2: Number of Trips and Average Trip Length, for Each Income Category

Number of daily trips Trip length (miles)

Household income Mean SE Mean SE

0-24,999 3.5 0.04 8.2 0.16

25,000-49,999 4.1 0.03 9.5 0.19

50,0000-74,999 4.3 0.04 10.4 0.39

75,000-99,999 4.4 0.05 11.1 0.49

100,000 + 4.6 0.05 11.8 0.64

Overall 4.1 0.02 10.0 0.16

Note: Sample contains about 250 thousand daily trips, SE = standard error.

Source: Calculated from the 2001 National Household Travel Survey, Person and Day Trip Files, U.S. Department of Transportation, Bureau of Transportation Statistics and Federal Highway Administration.
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vehicles make a higher proportion of their trips driving alone 

than households with one or two vehicles. In addition, the 

proportion of walking trips goes down as the number of vehicles in the household increases. Households with one or 

more vehicles make very few transit trips, and transit trips 

fall to less than 1 percent of all trips among households with 

two or more vehicles.

Income also affects the number of trips individuals take, 

as well as the distance traveled in each trip. Table 2 

shows the average number of trips per day and the average length of those trips by income class. Higher income 

households make more trips and travel more miles than 

lower income households. The differences are substantial. 

Households in the highest income class make about 30 

percent more trips, and the average length of those trips 

is more than 40 percent longer than trips by those in the 

lowest income class.

Income Related Factors Affecting Future 

Highway Travel

Many factors, including household vehicle ownership, 

number and length of trips, and the number of new immigrants, will infl uence income’s effect on future highway 

travel. A widely cited report by Polzin, making the case 

for moderate future growth in VMT, points out that vehicle availability may be reaching a saturation point.5

 The 

number of zero-vehicle households is already low, and 

5

 S.E. Polzin, 2006. The Case for Moderate Growth in Vehicle Miles of 

Travel: A Critical Juncture in U.S. Travel Behavior Trends. Center for 

Urban Transportation Research, University of South Florida, Tampa, FL, 

April. 

while economic growth may continue to lower that number 

somewhat, some part of the population will have limited 

means and other limitations of a legal, mental, or physical nature that will preclude driving a vehicle. Also others 

may choose not to own a vehicle or not to own as many 

vehicles as their income could support. Polzin estimates 

that only about 25 percent of the households in the United 

States have more adults in the household than vehicles. 

This means that because the vast majority of households 

already have one or more vehicles per adult, the impacts 

of vehicle availability on passenger VMT growth may not 

be as strong as in the past.

Other income-related factors, such as increases in the 

number and length of trips, are affected by the cost of 

travel. Polzin found that real costs per vehicle mile have 

already started to rise slightly, and average speeds have 

started to drop, reversing the historical trend of lower costs 

and higher speeds. There may also be constraints on additional time available to devote to travel, though Polzin 

found no evidence that the trend of increasing time devoted to travel has begun to slow down.6

Litman also looks at some of the moderating demographic 

and behavioral trends related to travel demand.7 He concludes that per capita vehicle ownership and vehicle mile6

 Polzin predicts that the number of trips and trip length will increase 

only at about a third as fast in the future as they have historically. As a 

result of these moderating infl uences, he predicts overall VMT will grow 

at about 2 percent per year through the year 2025. Historically VMT has 

grown, on average, by about 3 percent annually since 1970.

7

 T. Litman, 2006. The Future Isn’t What it Used to Be, Changing Trends 

and Their Implications for Transport Planning. Victoria Transport Policy 

Institute, Victoria, BC.

Figure 2: Distribution of Trips by Mode and Number of Household Vehicles

0 vehicles 1 vehicle 2 vehicles 3 or more vehicles

0

10

20

30

40

50

60

70

80

90

100

Percent

Personal vehicle

Transit or walk

Note: Zero-vehicle households have some drive-alone trips because the vehicle used in making a trip does not have to belong to the household.

Source: Calculated from the 2001 National Household Travel Survey, Day Trip File, U.S. Department of Transportation, Bureau of Transportation Statistics 

and Federal Highway Administration.
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For related BTS data and publications

This report was prepared by Jeffery L. Memmott, Transportation 

Specialist, of the Bureau of Transportation Statistics (BTS). BTS is a 

component of DOT’s Research and Innovative Technology Administration.

The estimates in this report were developed from a variety of data 

sources. The principal data sources are:

• U.S. Department of Transportation, Bureau of Transportation 

Statistics and Federal Highway Administration, 2001 National 

Household Travel Survey.

• U.S. Department of Transportation, Federal Highway Administration, Highway Statistics, various years.

• U.S. Department of Commerce, Bureau of Economic Analysis, 

Personal Income and Its Disposition.

• U.S. Census Bureau, Annual Population Estimates.

For questions about this or other BTS reports, call 1-800-853-1351, 

email answers@bts.gov, or visit www.bts.gov.

Data — 

• National Household Travel Survey – survey of daily and longdistance passenger travel in the United States, 2001.

• Commodity Flow Survey – survey reporting value, weight, 

and ton-miles by commodity, mode, origin, and destination.

Publications — 

• Highlights of the 2001 National Household Travel Survey

• Transportation Statistics Annual Report 2006

• Estimated Impacts of September 11th on U.S. Travel

About this Report

age have peaked and will not increase in the future. He 

bases this conclusion on the saturation of vehicle ownership and anticipated improved travel and location options. 

A frequently overlooked factor on highway vehicle travel is 

the impact of new immigrant households. New immigrants 

are typically just starting the process of growth in household income and vehicle acquisition. In addition, a large 

proportion of new immigrants immediately become part of 

the workforce, with corresponding needs for transportation 

to and from work. A study by Sum, Fogg, Harrington, et al. 

found that new foreign immigrants accounted for 47 percent of the increase in the civilian labor force from 1990 to 

2000.8

 The additional vehicle demand by new immigrants 

may not be fully anticipated in VMT growth projections. 

While it is diffi cult to predict how individuals will use 

increases in their real income in the future, it is clear that 

historically some of that increase has been used to acquire 

a household vehicle or increase the number of household 

vehicles. Higher incomes, along with increased vehicle 

availability, have enabled individuals to travel more and 

for longer distances. However, some indicators suggest 

8

 A. Sum, N. Fogg, P. Harrington, et al. 2002. Immigrant Workers and 

the Great American Job Machine: The Contributions of New Foreign 

Immigration to National and Regional Labor Force Growth in the 1990’s. 

Center for Labor Market Studies, Northeastern University, Boston, MA., 

August.

these effects have begun to diminish. Whether income 

effects will have smaller impacts in the future depends on 

many factors, such as vehicle availability and responses to 

changing travel costs and speeds. 

One important question is how many individuals will be 

willing to pay for higher quality and more reliable transportation services in the future as their incomes rise in lieu 

of increasing the number and length of trips? If demand 

increases for more costly, but faster and more reliable 

transportation services, this would likely have a moderating effect on future passenger VMT growth. 

Another factor that may affect future VMT growth is the 

possibility of rising costs of transportation, both in terms 

of time (lower average speeds due to increased congestion and longer trip distances) and vehicle operating costs 

(higher real prices of fuel). Increased transportation costs 

would tend to have some dampening effect on passenger 

VMT growth over time.

However, these potentially moderating factors on VMT 

growth could be partially offset by the additional work trips 

by new immigrants entering the labor force, along with 

increased vehicle availability for those new immigrant 

households and corresponding increases in trip frequency 

and distances for those households.