[Lamplighter]

... and I thought the advice was going to be about hitchhiking in DC.

See About Slugging*
Slugging is a term used to describe a unique form of commuting found in the Washington, DC area.
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.
It's unique because it is not a government sponsored commuter program,
but one created out of ingenuity from local citizens to solve commuter problems.

A longtime slug has created a set of signs designed to help improve both slugs and drivers.
The site has uniquely designed signs that a driver can display to show slugs the cars destination.