2009 National Household Travel Survey

Every few years, the Federal Highway Administration conducts a major survey called the NHTS to find out how Americans travel. The 2009 survey collected questionaires from more than 150,000 different households. Some of the results from that survey are now available in several formats.

The complete dataset is about 500 megabytes in ASCII format. Much briefer are a number of frequently-asked for table, including tables showing daily trips and miles by household income, mode and purpose, and other variables. You can also design your own table, though many useful variables in the survey, such as person miles of travel, are not (yet?) included in the design-your-own tables.

The first thing that must be noted is that the 150,000 households surveyed were not an accurate cross-section of the nation. For example, they surveyed one household for every 1,000 people in the Dallas-Ft. Worth metropolitan area, but only one for every 11,000 people in the Chicago metropolitan area, and (apparently) no one at all in the Atlanta metro area. Further, though more than two-thirds of Americans live in urban areas of 50,000 people or more, only about 61 percent of the surveys came from such areas.

These differences may explain some of the differences between the NHTS results and results from the American Community Survey, which the Census Bureau conducts of about 3 million households each year. For example, according to the 2008 ACS,2.9 percent of employees walk to work and 5.2 percent take transit to work. By comparison, the NHTS numbers indicate that 5.6 percent walk and only 3.7 percent take transit to work. This may reflect the NHTS bias towards less urban communities.

There are some other problems with the data. According to National Transportation Statistics, Americans flew about 600 billion passenger miles in 2008. The NHTS reports less than half of those passenger miles. National Transportation Statistics reports 4.5 trillion passenger miles in cars and light trucks; the NHTS reports only 3.4 trillion. Finally, National Transportation Statistics reports 54 billion transit passenger miles; the NHTS says 66 billion. In all of these cases, the data from National Transportation Statistics are considered more reputable, so care must be taken in interpreting NHTS numbers.

Recognizing these and other possible biases, it is still possible to glean some useful data out of the NHTS. A table showing average vehicle occupancies by trip purpose indicates that occupancies haven’t changed much since the 2001 survey (see page 31, physical page 45). If anything, occupancies have increased slightly, which may be due to higher fuel prices or it may reflect a greater small-town bias in the 2009 survey. But the changes are slight so it is still safe to say that the average car on the street has about 1.6 people in it — though the average for intercity trips, which are not well accounted for by the NHTS, is at least 2.2.
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Unlike the American Community Survey, which asks people only about commuting trips, the NHTS asks about all kinds of trips. The data indicate that commuting represents less than 16 percent of all travel, so all the emphasis placed on funding transit for commuting may be misplaced.

Another interesting comparison is mobility by household income. Dividing the trips by number of households reveals that the wealthiest households take about 2.5 times as many trips as the poorest. But it also turns out (from the make-your-own tables) that the wealthiest households also have more people per household than the poorest, so the number of trips per person is only about 70 percent more. Similar differences exist for miles of travel, vehicles per household, and other indicators of mobility.

We don’t have as good data for 100 years ago, but I suspect that mobility differences between the rich and poor were much greater then than they are today. Blue-collar relied mainly on foot travel, and farm families mainly on foot or horsepower, while wealthy and white-collar families had much greater access to mechanized travel such as intercity trains and streetcars. The point is that mobility today is much more evenly divided among income groups than it was before low-cost automobiles and airlines.

You can also make-your-own table showing trips by type of housing, including single-family, duplex, row house, and apartments/condos. Single-family households average 8.3 trips per day; duplexes 7.4 trips; rowhouses 5.4; and multifamily 6.0. So far so New Urbanist, but what happens when you account for household size? On a per-person basis, single-family households take 3.7 trips per day; duplexes 3.8; rowhouses 4.3; and multifamily 3.2. So rowhouses aren’t all that great.

Finally, I made a table showing trips per day by household for each of the 18 metropolitan areas identified in the data. The results offer little comfort to those who argue that increased densities will reduce travel. The highest of any metro area was Portland, at 9.2 trips per day. The lowest was Cincinnati (which urban area is fairly low density), at 7.4 trips per day. High-density Boston, Chicago, Los Angeles, and San Francisco-Oakland all scored higher than low-density Houston. Of course, in some of these cases the sample sizes were pretty small, but Houston, Los Angeles, and San Francisco-Oakland were all around 4,000 households or more.

So far I haven’t tried to download and analyze the full 500-megabyte data file. Excel could not handle this much data and I don’t have database software, though it might be worth getting some to review files like this. Any other insights into the data will be welcome in the comments.