Reply #5: There are NO laws prohibiting the use of such a Vehicle, if it was built.

[happyslug]

Honda, Vespa and others sells 50cc Mopeds, these are "Low Speed" Vehicles that are legal on the streets. Bicycles are also legal "Low Speed" Vehicles. AS to Golf-carts and other devices like them, the problem is the same SAFETY regulations apply to ANY size motorized Vehicle for use on Public Highways. If some manufacturer would take the technology of a Golf Cart, give it the basic safety requirements for a four wheel car, it would be legal. The Vehicle would have to have a Windshield that does NOT brake on impact (Laminated Glass NOT Tempered Glass), it has to have adequate lights for operation at night, it has to have seat-belts (And Airbags or other active protection in a crash). The body MUST stay intake in in rollover (Through in Pickups and SUVs there is no such roll over requirement, this rule applies ONLY to cars). Let us NOT forget the side impact rules.

If it was decided that the Vehicle could be designed for three wheel use, Motorcycle not Automotive regulations kick in (i.e. no seat-belts, airbags, or windshields). If a pickup bed is installed instead of a back seat, pickup regulations kick.

There are makers of such vehicles in the US today, rare but they exist. The problem is that the demand for such vehicles is low given that people want their cars to be able to go on the interstate highway system (Even if they never travel on such roads, they want the perceived ability to do so).

Here is the Museum of such micro-cars, it has pictures of the Isetta and Messerschmidt of the 1950s in addition to a huge number of other micro-cars:
http://www.microcarmuseum.com/

Popular Mechanic piece on Micro-cars being the wave of the Future:
http://www.popularmechanics.com/automotive/new_cars/4255883.html

Some other articles on Micro-cars:
http://www.petersen.org/default.cfm?docid=1057
http://minutia-microcarsminicars.blogspot.com/2009/02/thats-smallest-french-microcar-ive-ever.html

Micro-cars for sale TODAY from around the world:
http://www.environmentalgraffiti.com/featured/incredible-electric-microcars/4215

Here are a list of "Micro-cars" available in the US Today, these are NOT true Micro-cars, but sub-compacts (as those terms are used in the US):
http://editorial.autos.msn.com/listarticle.aspx?cp-documentid=861622

Notice all of these cars have modern bumpers, lights, seat-belts, windshields in addition to side glass parts (Not actually required by Regulation but just try selling a car without any glass in the doors and rear window, all the regulation says is you MUST be able to see to the side and rear. Such sight vision is provided by glass in most cars, but is still satisfied by nothing, i.e. open space.

BMW was reported in 2008 at bringing back the Isetta, but this time in an electric set up:
http://www.autobloggreen.com/2008/03/08/electric-isetta-microcars-currently-in-the-works/

Talk of Toyota bring in a micro-Car later this year:
http://www.treehugger.com/files/2009/03/scion-toyota-iq-urban-micro-car-fuel-efficient-high-mpg.php?daylife=1&dcitc=daylife-article

Honda Zest, a Japanese "Kei" car NOT sold in the US:
http://world.honda.com/news/2006/4060223ZEST/

I has a POWERFUL 660 cc three cylinder engine (yes Motorcycles have bigger engines):
http://carscoop.blogspot.com/2008/12/2009-honda-zest-minor-facelift-with-new.html