Federal Study Shows Car Travel Keeps On Falling

(MoneyWatch)  The number of miles traveled by American drivers fell 5.6 percent in August, according to an Oct. 23 report. That was the biggest single-month drop ever recorded by the U.S. Department of Transportation, which started keeping track in 1970.

It also brings to a record 10 months in a row, the number of months "vehicle miles traveled" have fallen versus the year-ago month.

The state of Florida, which has been hard hit by the housing bust, fell the most in August of any state, down 9.7 percent, the DOT said.

All told, the latest data means Americans have driven 78.1 billion fewer miles in 2008 year to date, than the same eight-month period a year ago.

That's such a mind-boggling number, I decided to bounce it off the U.S. population, just to see if it's credible to believe Americans could be driving that much less. According to the U.S. Census Bureau, there are now about 305.5 million Americans, divided among about 110 million households.

So 78.1 billion miles works out to about 710 fewer miles per household; divided by eight months, that's only about 89 fewer miles per month per household. Call it roughly three fewer miles per day, per household. Considering most households have two cars, that's an easily believable number, after all.

For the auto industry, fewer miles driven means less demand for vehicles, and indirectly, more demand for fuel efficiency. Both trends are confirmed by a glance at the U.S. auto sales numbers. Total sales are down 12.8 percent for 2008 year to date. Within that lower total, trucks are down 20.6 percent, and cars are down 4.1 percent, according to AutoData.

For the Federal Highway Administration, as I pointed out here last week, fewer miles traveled means less fuel taxes collected. Fuel taxes are the agency's main source of highway funds, so the FHWA has been issuing increasingly shrill warnings that it needs a new source of funds.

The FHWA collects vehicle-miles-traveled data through more than 4,000 automatic traffic recorders operated round-the-clock by state highway agencies.