Are you referring to Sivak's report[1]? The problem there is that it compares average trips and not trips where car vs plane is actually an option, according to the ThinkProgress article [2].
I have not read the report itself and I don't know how credible ThinkProgress is. The Yale Climate Connection article [3] points out similar issues. Bottom line seems to be that flying is better than driving alone, but as soon as you have more than one passenger, the car looks more attractive (still depending on the efficiency of the car).
Interesting though that the train is not looking so good in the statistics in these articles. My hunch here is that the numbers would look very different in Europe.