The reason states like CA require every dinky bit of smog stuff
to be present 20 years later is because they admit that the tail-pipe test
is far from thorough. Which, having observed a bunch of 'em (I paid, I watch)
I can't argue with. Since most mechanics AND vehicle owners are at best
semi-competent, legislation took the easy way out and required whatever
nightmare the OEM built to be maintained as-is and the tail-pipe test
is simply the most obvious way to validate the equipment is working. 
	Fiddly bits like EGR valves are still verified separately.
	With due respect to previous posters, its neither true that people
need to be forced to save the ice caps, nor is it true that all emissions
regulation, equipment and inspection are a scam by big business. The automotive
sources of smog in densely populated California cities was well documented
in the 1950s. Emissions controls have made a huge difference. The voters
and taxpayers have largely supported this. Industrial sources have been 
curtailed, though money plays a role here and more could be done.  
We old car fans simply do not show up in the polls, but we don't speak with
one voice either.
	When I say 'old' I mean Vanagons at this point- the newest Vanagon
is about the age of the 'average' car in California - '91, yes?
	Don't forget that when people see a visibly poluting car, it is
almost always an old car which has been poorly maintained. This certainly
reinforces a belief that old cars pollute. You seledom see a brand new
8 liter extended cab pickup truck, the current 'family car' in my neighborhood,
leaving a trail of grey haze behind it. You see a '70s sedan of indeterminate
color, sagging at one rear corner. 
	We're pretty lucky in the USA- In Japan and the Netherlands, auto taxes
RISE every year, so old cars are more expensive to own than new ones!
	Bill