Roadside billboards, posters on buses and subway escalators, ads in airport terminals—a type of publicity known as out-of-home advertising—used to be the dull end of the industry. No more. The falling price and improving quality of flat-screen displays mean that static posters printed on paper are being replaced by stylish digital commercials with moving pictures, sound and sometimes interactive features.
    William Eccleshare, who runs the international operations of Clear Channel, an American firm which is one of the largest out-of-home ad companies, thinks that in some countries more than 90% of its business will be digital by the decade's end. His arch-rival, Jean-Charles Decaux, the boss of France's JCDecaux, agrees that there will be a significant switch to digital, but mainly inside airports, railway stations, shopping malls and other controlled environments. Ads in bus shelters and other outdoor spots at risk of vandalism will take a lot longer to move away from paper, Mr. Decaux thinks. Digital displays already account for about one-quarter of his company's sales in transport hubs, but for less than 5% in street furniture and billboards.
    Clear Channel is so optimistic about digital posters because it believes they offer enormous potential for making advertisements more effective. McDonald's can advertise its sausage and egg McMuffin at breakfast time, change to its regular Big Mac fare at lunch and follow that with ads for apple pie and ice cream during teatime. When Spain won the football World Cup last year, digital billboards in Madrid, sponsored by Nike, showed the result within seconds.
    Advertisers constantly talk about wanting to "engage" with consumers, so they are taking great interest in the potential for interactivity that digital technology will bring. JCDecaux, for example, is offering a free iPhone application called U snap: when a consumer sees a poster (paper or digital) for something that attracts his interest and takes a photo of it on his phone, the app recognises it, gives him product information and discount vouchers and directs him to the nearest retailer.
    Then there is "gladvertising" and "sadvertising", an idea in which billboards with embedded cameras, linked to face-tracking software, detect the mood of each consumer who passes by, and change the advertising on display to suit it.
    Such Big Brotherish software would no doubt detect a satisfied grin on the faces of out-of-home advertising bosses as they contemplate the next 18 months, in which a string of big events will boost their business: the Rugby World Cup, the American presidential election, the Euro 2012 football championship and the London Olympics. Wherever you go, there will be no escape from ads linked to these events, and the out-of-home advertising firms will be raking it in.  What did people think of out-of-home advertising in the past?