Milhouse is basically correct, there are many poskim who clearly state that the very concept that a ???? ???? is not sufficient to permit in a ???”? is only a d’Rabonon. (See Shulchan Aruch haRav 364, Avnei Nezer hil Eruvin, Aruch Hashulchan. )
Many here are confusing by saying “Eruv” with eruv Chatzeros or a tzuros haPesach of 4 lechis only where we say ??? ???? ?????? ?????? at least ??????.
As per Chazon Ish, most cities nowadays are by default a reshus hayachid, as most streets that are basically 2 mechitzos ???”? end up at one point in the city within a third wall, thus rendering it as a true reshus hayachid, and therefore permitted with a tzuras haPesach.
in any case, brooklyn is surrounded by at least 3 mechitzos ??”? by sea walls and gates ???”? and are not ??????? ???????? due to the layout of the streets.