The most popular method of transportation at CMU is definitely public transportation. The Port Authority has a lot of bus routes that pass campus every five or 10 minutes, so it's not hard to find a bus, although students often complain that the right bus always seems to elude them when they're looking for it. The buses span all of the major neighborhoods in Pittsburgh, and the Port Authority also operates the T, which is basically like a metro system. Most students only use this if they need to get out into the neighborhoods like South Hills, Mount Lebanon, and Bethel Park—they all surround the central areas of Pittsburgh.
Students who live off campus frequently use bikes to get to their classes, as there are plenty of places to stow a bike near the academic and recreational buildings. If you choose to bring a bike, you also have to invest in a really secure bike lock because thefts are not uncommon. A fair amount of students have cars, but they have to pay exorbitant fees for permits (although these students usually can afford it). At the end of the day, most CMU students simply rely on their own two feet to get them places, because most areas of interest are within a reasonable walking distance.