Heavy-duty road vehicles are subject to a regulatory program administered primarily by the federal government, a program that evolved out of concerns about increasing smog in California in the 1960s. Among the applicable regulations today are Clean Air Act mobile source provisions, Tier 2 standards, and the proposed Tier 3 standards. The mobile source program has existed for one-half century, but regulation of heavy-duty vehicles developed much later than the program for light-duty vehicles. More recently, the federal government has begun implementing a program to control greenhouse gas emissions from heavy-duty vehicles.