One question: If the entire nation is using hemp as a fuel for their vehilcles, would the everyone in the big cities be walking around stoned from the second-hand hemp smoke polution? :-)

Marijuana can produce several different kinds of fuel. In the 1800′s and 1900′s hempseed oil was the primary source of fuel in the United States and was commonly used for lamps and other oil energy needs. The diesel engine was originally designed to run on marijuana oil because Rudolf Diesel assumed that it would be the most common fuel. Marijuana is also the most efficient plant for the production of methanol. It is estimated that, in one form or another, marijuana grown in the United States could provide up to ninety percent of the nation’s entire energy needs.
Source: Schaffer Library of Drug Policy

Hemp is 4 times more efficient than corn as biofuel. Hemp pellets can be used to produce clean electricity.

… so powerful it could replace every type of fossil fuel energy product (oil, coal, and natural gas).

… This plant is the earth’s number one biomass resource or fastest growing annual plant for agriculture on a worldwide basis, producing up to 14 tons per acre. This is the only biomass source available that is capable of producing all the energy needs of the U.S. and the world…

Hemp will produce cleaner air and reduce greenhouse gases. When biomass fuel burns, it produces CO2 (the major cause of the greenhouse effect), the same as fossil fuel; but during the growth cycle of the plant, photosynthesis removes as much CO2 from the air as burning the biomass adds, so hemp actually cleans the atmosphere. After the first cycle there is no further loading to the atmosphere…
Source: USA Hemp Museum