Footing the bill, so that it might fit
The boffins in the British monitoring and technical surveillance arm GCHQ are supposed to be smarter than the average Joe.

However, the idea of using ubiquitous city space to advertise certainly is a smart innovation. Not only are they usually free of charge — at least until municipal corporations wake up to their moneymaking potential — pavements are also city surfaces that do not attract the ire of anti-graffiti authorities unlike walls and pillars. Indeed, the city authorities should have even less of a problem with ‘reverse graffiti’ advertisements as the stencil designs come out when pressure washers wipe out street dirt to create the slogan — which would obviate any permanent ‘defacement’.
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