Large outdoor advertising companies, such as Clear Channel, have for years been providing outdoor street furniture like bus shelters and stuff in exchange for billboard space that they can sell.

多年来，Clear Channel等大型户外广告公司一直提供公共汽车站的候车棚等户外街道设施，并用之换取用于销售的户外广告牌空间。

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It's smaller than Clear Channel Outdoor, now a separate company, which puts ads on 5, 000 taxicabs in New York City.

FORBES: Magazine Article

Clear Channel Outdoor Holdings Inc.

WSJ: Auction System Helps Place Digital Outdoor Ads

Thanks to a partnership with advertising giant Clear Channel Outdoor, the G-men will deploy some 150 digital billboards -- capable of displaying fugitives, missing children, or public safety info in real-time -- to 20 major cities around the country, following a successful trial launched in Philadelphia in September.

ENGADGET: FBI rolling out digital billboards in 20 cities