The recent rise of low-cost airlines in Country Y has led to great increases in the number of air travelers and flights. However, a "hidden cost" of flying and certain other forms of travel is the significant amount of air pollution, which contributes to global warming. To make people pay these hidden costs—and in the process discourage the explosive growth in flying—the government of Country Y plans to impose fees on all air travel. There is a problem, however, in that many residents of Country Y would then use automobile transportation instead, without paying the hidden costs of the resulting air pollution.

Which of the following additions to the government's plan would be likely to help most in overcoming the problem referred to in the final sentence above?