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.