Amends the Illinois Vehicle Code. Changes the definition of "all-terrain vehicle by: (1) changing the size limitation to 55 (rather than 50) inches measured from the outside of the tire rim to the outside of the tire rim; and (2) removing specific design requirements. Changes the definition of "recreational off-highway vehicle" by changing the size limitation to 80 (rather than 64) inches or less in width measured from the outside of the tire rim to the outside of the tire rim. Effective immediately.