It was the mesorah through the ages that shishim ribo is dependent on a single street. The Divrei Malkiel (4:3) stated when writing to the people erecting an eruv in the city of Odessa, which had approximately shishim ribo, that, “the minhag is to erect eruvin even in the biggest of cities and it does not concern us that they have shishim ribo since the shishim ribo is dispersed over all the streets.” New York’s population in 1905 was much more than shishim ribo and the rabbanim who were involved with the eruv then relied on the fact that there was no street that had shishim ribo traversing it (Oznei Yehoshua, 1:18 and Tirosh VaYitzhar, siman 73). This is one of the reasons why in Eretz Yisroel eruvin are maintained in Yerushalayim and in the Gush Dan [Bnei Brak with all the interconnected neighborhoods] even though these regions have shishim ribo as well (Rav Yisroel Yaakov Fisher zt”l in Even Yisroel, 8:36 and Kinyan Torah, 4:40).
These are some of the additional poskim who maintain that shishim ribo is dependent on the street:
Pnei Yehoshua, Shabbos 5b; Bais Meir, Shabbos 5b; Bais Yaakov, Eruvin 6a; Yad Dovid, Eruvin 55a; Bais Ephraim, p. 46; Mishkenos Yaakov, p. 126; Chiddushi Harim, siman 4; Yeshuos Malko, siman 27; Mishnah Berurah, Shaar HaTzion, 345:25 [the Mishnah Berurah indicates this by the usage of the phrase, “derech hamavoi hamefulash,” ― it is important to note, the Mishnah Berurah’s (345:24) primary issue is whether the shishim ribo are required to traverse the street every day of the year or whether occasional use of the street by 600,000 people would be sufficient, see also Toldos Shmuel, 3:86:10]; Minchas Elazar, 3:4; Bais Av, 2:5:2; Maharshag, 2:25; Chazon Ish, 107:6; Mahari Stief, siman 68; V’yaan Yoseph, 131:1, 155:1, 195:2; Divrei Yatziv, 173:4; Rav Shmuel Wosner shlita in Shevet HaLevi, 6:41; Rav Elyashuv shlita, as cited in the sefer Yashiv Moshe p. 58; Rav Yechezkel Roth shlita, in Emek HaTeshuvah 5:19; (See also the sheilah to the Chacham Tzvi in siman 37).
Even Rav Moshe zt”l (Igros Moshe, O.C. 1:109) accepted as fact that shishim ribo is dependent on the street and only later (ibid., 1:139:5, 4:87-88, 5:28:5, 5:29) stated his chiddush that in a city, shishim ribo applied to an area of twelve mil by twelve mil. [In this chiddush, admittedly his own, Rav Moshe defines shishim ribo as meaning 3,000,000 people (see Part 1: Shishim Ribo According to Hagaon Rav Moshe Feinstein zt”l).] Even after he developed his chiddush, Rav Moshe declined to give the rabbanim of Flatbush a p’sak against eruvin, as he wrote that his chiddush was not stated in the Achronim and that the Aruch HaShulchan would not agree with him (ibid., 4:87). That is why when it came to issues concerning a reshus harabbim ― for example, men wearing gloves or a watch on Shabbos or bringing back home on yom tov a lulav or a shofar for a woman or a child in an area that is not enclosed by an eruv ― Rav Moshe maintained that we rely on the heter of shishim ribo in all cities (ibid., 3:94, 5:19, 5:24:10; see also Rav Moshe zt"l and the Walking Stick). Rav Dovid Feinstein shlita (Kuntres L’Torah V’Horaah, vol. 6, 1976) posited that the heter for a Succos block eruv is because we rely on the heter of shishim ribo. Although he was referring to lower Manhattan, which according to his father’s chiddush met the requirement of having shishim ribo (Igros Moshe, O.C. 1:139:5), Rav Dovid still allowed an eruv. In summation it is a given that shishim ribo is conditional of a street.