article in The Korea Times entitled "Smartphones kill subway ads." According to the article, "Technological advances have changed many things from how people spend their time to where their attention wanders. This trend has pushed advertisers to move away from traditional forms of media toward digital forms. As a result, subway operators are increasingly struggling to sell ad space, which is one of their big revenue sources. According to Seoul Metro, the operator of lines 1 to 4, its ad revenue has dropped to 35 billion won ($31 million) last year from 42 billion won in 2012. It said only 36 percent of available ad space has been filled this year, down from 41.8 percent in 2014."