Here’s the unofficial explanation from Slug-Lines.com :
Slugging is a term used to describe a unique form of commuting found in the Washington, DC area sometimes referred to as “Instant Carpooling” or “Casual Carpooling”. It’s unique because people commuting into the city stop to pickup other passengers even though they are total strangers! However, slugging is a very organized system with its own set of rules, proper etiquette, and specific pickup and drop-off
locations. It has thousands of vehicles at its disposal, moves thousands of commuters daily, and the best part, it’s FREE! Not only is it free, but it gets people to and from work faster than the typical bus, metro, or train.
And here’s a teaser from an article titled “Stand in line. Ride with strangers. Zip past traffic to the city.“:
Commuters started slugging in Northern Virginia more than 30 years ago, according to the Virginia Department of Transportation. Commuters formed slug lines on their own, and no government agency oversees them.
With gasoline prices now averaging nearly $4 a gallon, the commuters at the Woodbridge lot said the practice is growing even more popular.
An estimated 6,500 commuters a day slug or pick up slugs, compared with 3,000 in 1999, according to Virginia Department of Transportation estimates.