Agree with Lee -- there is nothing wrong with the idea in principle but in
practice it can only have a marginal effect.
In the UK the aviation industry sometimes reacts to proposals for emissions
trading by saying that it will buy the emissions permits it needs. What gives it
the idea that sufficient permits will be for sale ?
As far as road traffic is concerned, there is the additional factor that the
greenhouse gas emissions issue is only one of a number of reasons why reducing
traffic is essential to maintain our quality of life.