7th arrondissement, Paris.

This week the Parisian transit authorities started a "liberty of expression" program, putting up 47 blank billboards in 24 stations and inviting riders to write on them. (Bad autotranslation.) This is to discourage people from writing anti-advertising slogans over the regular ads, a surprisingly popular hobby here. My favorite quote from the linked article:

"You have nothing better to do, band of idiots!" grumbled one man upon seeing young people writing on the panels.