Looking for information on the percent of automobiles "on the road" in
the US in a given year that are equiped with catalytic converters,
starting 10 years before catalytic converters were first introduced,
to the present year.
Ideally, we would like this data broken down a) by year and b) by
state.
The purpose is to look at variation in adoption and penetration of
catalytic converters over time and by state.
Please note that we don't want to know what autos are being BOUGHT in
that year...we want to know about autos that are on the road (i.e.
including older cars that are still being driven regularly.)