I meant to come back to this earlier. That actually is incorrect. If it's not defined in local code as a vehicle it is "Unclassified" and is in a sort of legal limbo.

The case in Ohio with the Rhoades Car I mentioned focused around that very problem. The owner said it was a bike, local law enforcement said it didn't meet the definition of a bike (which had wording concerning the number of wheels as either 2 or 3 and was later changed to read 2 or more) and was there for a toy, therefore not permitted on local streets. It went before a judge and he said that it fit no current definition and that it should be dealt with by lawmakers. The mayor and city council created a city ordnance permitting it on local roads. It was picked up by a bicycle advocacy group and eventually they got Ohio to change the state definition to remove the limiting language. Quadracycles are now legal in Ohio because of it.

Initial story:
Bike Battle: Quadricycle Could Lead to Law Change | fox8.com

edit: The story in chronological order

http://www.cleveland.com/roadrant/in...r_of_four.html
http://www.cleveland.com/roadrant/in...r_banning.html
http://www.cleveland.com/lakewood/in...quadricyc.html
http://www.cleveland.com/lakewood/in...corporate.html
http://www.cleveland.com/roadrant/in...involving.html
http://www.cleveland.com/lakewood/in..._definiti.html