However the percentage of CO2 coming from the transportation sector has been steadily climbing it was 17% in 1971, and is 23% in 1997. (Of which road traffic was responsible for 58% in 1990 by 73% in 1997) [2001 State of the World pg 111, World Watch Institute]
Merely switching to higher efficiency vehicles will not protect against all of the cars down-sides. For example although roads cover 1% of the US, it's estimated that this network has effects on wildlife on up to 20% of the surface area. [ibid]
This little rant was motivated by me wondering if electric bicycles could be sufficiently easy enough to use to actually get American's to use their cars less. They're certainly more affordable than any of the other electric alternatives.