"Why believe in a god? Just be good for goodness' sake."
According to this WP column, this "slogan" is a new advertising campaign by atheists in the D. C. Metro. Wow!
Making reference to such "ethical" practices usually associated with riding the Metro such as giving up one's seat for a pregnant woman, the article points to the work of the American Humanist Association to get "their message" out.
In case the irony does not strike you, take some time to watch Tim Keller's "author talk" at Google.