Cyrano,
Even taking into account the source of that diagram, it's interesting. With four pax in the car (you can bet the aircraft are assumed full to bursting) it would still look bad CO-wise, while the train would look more similar to the ATR.
But it does seem to show a stark difference between the ATR and the 70-seat jet.
I still wonder how a modern piston engined airliner would compare, and how/whether a next gen piston could improve.
It certainly looks as though there would be an environmental case for encouraging greater use of turboprops on short haul routes - I wonder what the range figure would be at which turboprops would not be acceptable to pax?
Perhaps the answer would be to tax emissions rather than fuel?