There's a concept developed by Amory Lovins called the Negawatt. In electricity terms, a megawatt is a unit of electricity and when power utility companies build electric generating capacity they do so in megawatts. But if you promote energy efficiency, for example by supporting the development and sale of efficient refridgerators, then you've changed the rising demand story for electricity.

The lower energy demand gained by efficient gadgets is energy production capacity that does not have to be built. He created the word negawatt to capture that idea.

Obviously the same concept can apply to vehicles, where using highly efficient vehicles is a kind of negabarrel effect that would be the same end effect as switching some of the vehicles to alternative energy sources.