“The collapse of a billboard along the Brooklyn-Queens Expressway on Jan. 13 has turned the city’s attention again to the presence of highway billboards: how many are legal, how many are safe and what are they doing there in the first place? “In the realm of outdoor advertising in the city, definitive answers can be elusive,” Patrick McGeehan reported in The Times on Friday. What is beyond doubt is how much of an impact billboards have on the cityscape.”