During the 1970s, the contruction of the eruv in Flatbush (a neighborhood of Brooklyn, New York) aroused great controversy. To this day, its permissibility remains disputed. The Va'ad Harabanim of Flatbush permits carrying inside the Flatbush eruv, while many rabbis and rashei yeshivah there, such as Torah Vodaath's Rav Yisroel Belsky (personal communication), forbid its use.

Rav Yosef Eliyahu Henkin (Kitvei Hagaon Rav Y.E. Henkin 2:25) strongly encourages the construction of eruvin in New York's five boroughs, including Brooklyn (whose population easily exceeded 600,000 already in hsi day). Although Rav Henkin does not explain why these places are not reshuyot harabim [official public domains - GS], a number of arguments have been offered to support his cotnention that Flatbush is not in this category. First, Rav Shlomo David Kahane's argument regarding the Warsaw eruv seemingly applies to Flatbush, too, because no street within the Flatbush eruv runs straight from one end of the city to the other.[6]

Second, the ruling of Rav Chaim Ozer Grodzinski and the Chazon Ish also seems to apply to Flatbush. The faces of the buildings and the fences along the Belt Parkway appear to constitute the majority of a wall on three sides.[7] (Ironically, this lenient consideration is most often applicable in densely populated urban areas rather than smaller suburbs, which frequently have much empty space between buildings.)

Third, the Aruch Hashulchan's unique (but highly questionable) approach might be taken into account (Orach Chaim 345:19-24)...

Rav Moshe Feinstein (Teshuvot Igrot Moshe, Orach Chaim 4:87) vigorously disputes the Aruch Hashulchan's argument, citing a proof to the contrary from the Gemara (Shabbat 96b). The Divrei Malkiel (vol. 3, p. 267) also writes that one may not rely on the Aruch Hashulchan's novel insight... Rav Aharon Lichtenstein conveyed sentiments similar to those of the Divrei Malkiel and Rav Moshe...

A fourth defense of the Flatbush eruv is the opinion of Rav Efraim Zalman Margoliot (Beit Efraim, Orach Chaim 26) that only pedestrians count when determining that 600,000 people travel in a street. He argues that the requirement for 600,000 people is based on a comparison to the encampments in the desert. The comparison can thus be made only to pedestrians, as the 600,000 people who were in the quintessential reshut harabim were all pedestrians... Both Rav Moshe (Teshuvot Igrot Moshe, Orach Chaim 1:139:6) and Rav Binyamin Silber (Teshuvot Az Nidberu 6:70) reject this argument, pointing out that wagons (agalot) were used in the desert encampment's thoroughfares. [I believe the Mishkenos Ya'akov, a contemporary of the Beis Efraim, also disputes this position - GS]

Despite all of the arguments in favor of being lenient, Rav Moshe did not endorse the construction of the Flatbush eruv (see Teshuvot Igrot Moshe, Orach Chaim 4:87-88). He explicitly rejects all of the arguments presented and rules that the 600,000 people who regularly travel the streets of Brooklyn render it a reshut harabim.[10]

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[6] Even Flatbush Avenue and Bedford Avenue bend at various points; Ocean Parkway does not extend from one end of Brooklyn to the other.
[7] The Chazon Ish (O.C. 107-5-7) requires that there be at least one street in the town that either bends or ends inside the town. Brooklyn meets this requirement, as we have explained in the previous footnote.
[10] Rav Moshe's concern was not for the 600,000 residents but for 600,000 people traveling the streets at any time (drivers and pedestrians) within an area that is twelve mil (approximately eight miles by eight miles). He thus requires that the population be so great that 600,000 people are regularly found in the streets. Rav Moshe estimates that this requires at least 2.4 million residents. Rav Moshe is the lone authority who requries such a large populations, and even he (O.C. 4:87) expresses reservations about his view, noting that no other authorities mention it. Nevertheless, Brooklyn is so populous that even Rav Moshe considers it a reshut harabim.