There is a certain slug etiquette. The first person in line is the "caller" that announces the driver's destination to others waiting for a ride. Once a slug has taken a ride, there is no eating or smoking, no exchange of names, not much talking, no window adjustments, no cell phones, and no messing with the radio!
It is believed that slugging began in Washington DC in the early 1970s. While slugging is not for everyone, it has become so "advanced" in Washington DC that it even has it's own website, Slug-Lines.com.