Findings from the 2001 National Household Travel Survey (NHTS) have been pored over by some of us with as much interest as the JPL people have in photos from the Mars Rover.
The sponsoring Federal Highway Administration has just released a helpful summary, Our Nation's Travel: Current Issues.
Here are some tidbits worth thinking about (page nos. refer to hard-copy version):
From 1969-2001 (the years of the various surveys), U.S. population grew 41% and auto use 191%, but, "[t]here was a 43% decrease in carbon monoxide, a 59% decrease in volatile organic compounds (VOC), and a 5% increase in nitrous oxide (NOx)". p. 3.
"Since 1969, the number of households without vehicles has declined (to about 8% of all households), while the majority of households now have two or three vehicles. In 2001 the number of household-based vehicles, 202 million exceeded the number of drivers by 12 million. In fact, in 22.7 million or 21.2% of all households, there are more vehilces that drivers." p. 6.
"Travel time for a 10-mile trip does increase as area size increases, but a larger dterminant seems to be the density of population." The accompanying graphic shows that the longest trip durations are in the largest, densest metros with rail. p. 15.
"The NHTS asked respondents how much of a problem congestion was in their life. Almost half (49.3%) of the survey respondents reported that congestion was not a problem or a little problem. ... For those living in the largest metro areas (three million or more population) only 37.2 percent said congestion was not a problem or a little problem ..." (p. 16).