So how many "alternative fuel" vehicles are there in the US?

[NNadir]

According to the DOE, in 2002 there were 471,098 such vehicles.

http://www.eia.doe.gov/cneaf/alternate/page/datatables/afvtable4_03.xls

Of these, 317,100 were fueled either by compressed natural gas, LPG, or methanol, meaning that these "alternative fuel vehicles" actually do close to zero to address global climate change, especially if they leak during fueling or operation. (Methanol is theoretically available from biologically produced syn gas, but as a practical matter almost all of it today is natural gas derived.)

Ethanol, which is from time to time is represented as a magical global climate change solution, fuels - including E85 and E95 - fuels 120,951 vehicles. Ethanol is available on an industrial scale from the hydroxylation of ethene (aka ethylene), a fossil source, but I believe that the majority of the ethanol burned in cars is biogenic in origin, primarily from corn fermentation, and therefore the greenhouse gas impact is certainly lower for ethanol fueled vehicles.

Thirty-three thousand and forty seven of the vehicles were electric, and the greenhouse impact is defined by the source of electricity. Nuclear electricity, hydroelectric electricity, geothermal electricity and solar electricity, listed in the order in which they are available generally in the United States, can make these cars have a lower greenhouse gas impact, but there is to my knowledge just one state in the union, Vermont, that produces almost all of its electricity through non-greenhouse dependent means.

Missing from the "alternative fuels" list here is biodiesel. Intuitively one feels that biodiesel is not making that large an impact, but certainly biodiesel exists and is in use and is more or less greenhouse gas neutral, not counting production costs and the fact that the esters are often methanol esters.

According to the Energy Information Agency, the total number of vehicles in the United States in 2001 was 191,000,000. Thus, ignoring the differential of one year's time, in 2002, about 0.24% of vehicles were alternatively fueled, and nearly all of these were greenhouse gas contributors.

http://www.eia.doe.gov/emeu/rtecs/rtecs1991/rtecs1991.html

It is said that, in spite of improvements to emissions control technology, that motor vehicles are responsible for 34% of the nitrogen oxide emissions and 51% of carbon monoxide emissions.

Thirty-three percent of the greenhouse gas emissions in the United States come from motor vehicles.

http://www.bikesatwork.com/carfree/automobiles-and-environment.html