Flying vs. Driving. Which is better for the enviroment?

http://environment.about.com/od/greenlivingdesign/a/fly_vs_drive.htm

Driving a fuel efficient car (25-30 miles per gallon) is considered better than flying. This is so because driving releases fewer greenhouse gases. On a 300 mile trip (from Phili to Boston), driving would generate about 104 MG of carbon dioxide, while flying would generate around 184 MG of carbon dioxide. Flying is also more efficient because many people can fit on a plane while only a few can fit in a car. Doing away with flying would significantly decrease the size of the average carbon footprint on Earth. However, pulbic transportation remains the most economical and greenest form of transportation.