Abstract
Smog check programmes have failed. Official studies in the US show reductions in emissions of only about half those predicted when the programmes were adopted. Other studies show no effect at all. The failure arises for several reasons: (1) current programmes only encourage cars to be clean on one day every two years - the inspection day; (2) current programmes do little to deter motorists from tampering; (3) although most emissions come from a few vehicles with high emissions, current programmes require inspection of all cars; and (4) garages often do not repair cars properly; indeed, about half the cars repaired following an inspection have increased emissions. (Author/publisher).