Rav Moshe maintained (ibid., 1:139:3) that even if an area such as a walled city ― which is classified me’d’Oraysa as a reshus hayachid ― has pirtzos which are open to an unenclosed area containing shishim ribo, these pirtzos would nevertheless require delasos, me’d’rabbanan. This requirement is Rav Moshe’s own chiddush. [On the contrary, the Shulchan Aruch HaRav (364:4); Bais Ephraim (O.C. 26); Bais Meir (O.C. 364:2); Mishnah Berurah (364:5), and the Achiezer (4:8) maintain that only when the area meets all the classifications of a reshus harabbim is there a requirement of delasos.]
Rav Moshe mentioned this originally about Manhattan which had shishim ribo and nevertheless was classified, me’d’Oraysa, as a reshus hayachid because it was enclosed by mechitzos ― sea walls that encompassed the island. Since Rav Moshe had seen Rav Tzvi Eisenstadt zt"l's teshuvah he knew that there were some pirtzos in the Manhattan mechitzos (Kuntres Tikkun Eruvin Manhattan, pp. 68, 168-69; Divrei Menachem, O.C. vol. 2, p. 76; Rav Tzvi Eisenstadt in his Hatzaah L’Tikkun Eruvin B’Manhattan, and in his recently published sefer Minchas Tzvi, siman 4). Nevertheless, because Manhattan is an island, the pirtzos opened to the water, which obviously did not contain shishim ribo, and therefore posed no problem to Rav Moshe since a tzuras hapesach would then be adequate. However according to Rav Moshe’s chiddush, the bridges leading from Manhattan ― which were open along their sides ― possibly would have needed to be rectified with delasos. Rav Moshe questioned (Igros Moshe, O.C. 1:139:5) whether the bridges were considered part of Manhattan, in which case the city’s population would in effect cause the bridges to be classified as a reshus harabbim and they would require delasos. Alternatively, if the bridges were not considered part of the city, but an intercity road (sratya), they would require shishim ribo crossing them in order to be classified as a reshus harabbim. As this was not the case, a tzuras hapesach would then be sufficient.
According to Rav Moshe, in a Brooklyn neighborhood, a tzuras hapesach would suffice for the following reason:
Since the tzuras hapesachim we erected in our neighborhoods are established in an area encompassed by mechitzos, the area is classified as a reshus hayachid, and the tzuras hapesachim are sufficient and delasos are not required (ibid., 1:139 see the end of anaf 3 where Rav Moshe states clearly that if the tzuras hapesach is erected in a reshus hayachid, it is sufficient; see Is There a Requirement of Delasos According to Rav Moshe Feinstein zt”l).