"You have to wait here. You don't have to be bored," read the poster for a Google campaign, which ran in Boston-area bus stops. The poster let a person download Google's mobile app through a free Wi-Fi router installed in the location.

"It's an old-school media that has been around for hundreds of years but we've dressed it up and made it fresh," said Zohar Levkovitz, chief executive and co-founder of Amobee, Inc., a start-up based in Redwood City, Calif., that has developed the technology and begun selling it to advertisers.