The Union of Concerned Scientists did a study to reveal that taking the bus is the greenest mode of transportation (other than walking or taking a bicycle) over planes, trains and individual automobiles when traveling city to city. According to the study, a couple traveling on a double-decker bus cut back their carbon emissions by as much as 75 percent versus flying or driving. The average miles per gallon on a full bus is 583, whereas a full flight has just 44 miles per gallon. And what's the number of individual cars one double-decker bus takes off the road? About 81. Now that discount bus services like Megabus come outfitted with Wi-Fi and power ports, it might be time to rethink the bus the next time you travel.