New, never built one, global warming inspired
Basically I've researched the **** out of it, and the best cheapish vehicle option combining ease of use and low impact on environment is a DIY motorized bicycle and/or microcar/trike etc.
I'm not a bike nut, in a perfect world we'd all have low to the ground quads which can be pedaled or driven by clean motors.... but a motorized bike is the option which presents itself due to current laws/science/manufacturing.
Many people say "why even motorize it you lazy bum?!" to which I say, I will always have an unmotored bike as well, but when I need to pull a very heavy load up a hill, or when I'm sick and I still need to be somewhere, I will be glad I have a vehicle which gets 100+mpg.
Electric costs too much (for now), plus it nukes yer balls, propane is too expensive to be worth it YET (wait till the oil crisis really hits america), cng is too dangerous, hydrogen equipment is too expensive (for now), and compressed air power equipment (my fave, requires a microcar though, pretty bulky, but worth it) is too expensive (also for now).
Oh and they still have yet to mass produce a tiny diesel light enough to power a bike. I'm sure it's possible, using composites (need more compression but want to save weight, think beyond aluminum)
So... here I am, hoping to stand on the shoulders of motorized bicycle riding giants