From the EPA website:
Collectible cars removed from Phoenix, Tucson smog program
SAN FRANCISCO - The U.S. Environmental Protection Agency today announced that Tucson motorcycles and Phoenix and Tucson collectible vehicles will no longer have to undergo emissions tests beginning April 30. The change to the state's clean air plan affects approximately 3,800 collectible vehicles in Maricopa County and about 1,400 collectible vehicles and 6,240 motorcycles in Pima County. The Arizona Department of Environmental Quality studied the effect of exempting motorcycles, collectible vehicles, and vehicles 25 model years old and older from the current emission testing program and showed that exempting motorcycles and collectible vehicles from the Tucson area program and collectible vehicles from the Phoenix area program would not interfere with meeting and maintaining health-based standards for ozone and carbon monoxide. In the event that the ozone or carbon monoxide air quality standards are violated in these areas, the state's plan can reinstate the requirements.