A think-tank in Michigan has announced a controversial new finding. It concludes that efforts to combat high emissions from cars by taxing gasoline and SUVs are misplaced; rather, governments should be providing consumers with tax incentives to purchase large cars and carpooling with their neighbors to work. They estimate that such a plan would reduce national gas emissions by 23% over the next decade.
Which of the following, if true, casts the greatest doubt on the likely effectiveness of this recommendation?