A person that has 80% of their head up their ass... Often used on the road to refer to people driving on I476 or I76 in the Philadelphia region because of their lack of ability to merge onto a moving highway. Also used to describe all of New Jersey's drivers... They cause traffic jams because they apply the brakes instead of gas when trying to merge onto a fast moving highway thus causing accidents, or making everyone stop causing a chain reaction traffic jam that doesn't end until sometime around 7:30pm... These roads would otherwise be clear if these people had the ability to access the other 80%.
Also used to describe everyone that drives to the beach on late friday afternoons during rush hour.
Driver in front: "Hey, look at me. I think I'm a safe driver because I go 35MPH to merge onto a highway that has an average speed of 75MPH!"
Smart Driver stuck behind: "What a fucking twenty percenter. Now none of us can merge smoothly."