The LSE prof slash blogger writes:

I’m not convinced of the environmental benefits of the cash-for-clunkers scheme.  I would be a major beneficiary of the scheme should the UK decide to introduce one.  I drive an 18-year old hooptie that has bits falling off all the time (it’s easy to stick them back on, however, with superglue, miracle putty and duct tape).  These policies to promote accelerated obsolescence of consumer durables also leave a more generic bad taste.  I’ll keep my old banger on the road as long as the laws of physics allow it.

I think he makes a decent point in the piece in suggesting that the environmental impact of the plan depends on what are probably unknowable factors. Obviously, the direct effect — trading dirty cars for clean ones, is a positive one. But is the plan creating new automobile demand or merely shifting the time and distribution of purchases? New cars have to be built, which is not a carbon neutral process. One should also consider the opportunity cost of the program’s funding; what if it were instead used to reduce transit service cuts and fares?

Which isn’t to say it’s an awful idea, but the net effects of such plans are rarely clear?