How Much do they Really Drive in Houston?

Our friend Tory Gattis pointed out yesterday at Houston Strategies that conventional wisdom (and the US DoT Federal Highway Administration) are wrong. Quoting a recent report by New Geography contributor Wendell Cox:

In fact, this data is incorrect. The FHWA 2006 data indicates that the Houston urban area has a population of 2,801,000. According to the United States Bureau of the Census, the population of the Houston urban area was 4,353,000 in 2006.... Actually Houston’s driving is about average: If the urban area population is corrected to agree with the Bureau of the Census data, per capita driving in the Houston area is slightly below the national average for large urban areas. Houston would rank 19th out of 38 urban areas, with daily per capita driving of 23.2 miles, compared to the national average of 23.9 miles.

Even if you're not interested in Houston or that potential gaffe, check out Wendell's report for a table of per capita vehicle miles driven for 38 urbanized areas over 1,000,000 population.

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I'll wager all the data is bad.

A quick glance shows that most of the "evil" metros are rapidly growing cities of the South. Most of the "virtuous" ones are stagnant cities elsewhere. I seriously doubt any of the population numbers are correct, and they understate the populations of the growing metropolitan areas.

"Micro" studies of human behavior have shown that people tend to devote a fairly constant part of their day to transportation. Whether they are Africans walking to the nearest village, or Americans driving on the freeway, people tend to spend 15-20% of their time moving from place to place. If there is so little variation from Africa to America, there must be even less variation between Atlanta and Sacramento. Thus, I posit that any real differences in the car-miles traveled has mostly to do with how choked the freeways are.

A question of definitions

I guarantee you that the difference lies in FHWA's definition of "urban area" and the Census definition that Cox is using. If memory serves, FHWA planning is based on urbanized areas that may or may not line up with Census tract boundaries. I think the metropolitan planning organization boundaries (urban areas) used by FHWA are typically a compendium of transportation analysis zones that meet certain "urban" criteria. To resolve this issue, you would have to look at Houston's MPO transportation plan.


There's no question that FHWA's and Census's definitions of any given urban area will differ, but that doesn't explain how FHWA can claim more than 1.5 million fewer people in an urban area that it defines as 180 square miles LARGER than the US Census definition.


It might, if the Census definition includes Houston AND Galveston but the FHWA definition only includes Houston and its urbanizing hinterlands (assuming that those hinterlands cover a large area). I have no idea if that's the case or not; I'm just stating a hypothesis.

Gaffe #2?

There seems to be another gaffe in the data: New Orleans presumably only does so incredibly well because it sadly doesn't actually have that population any more.