“What is a reshus harabbim? A street or marketplace that is sixteen amos wide, … and there are those who say that if it [the street or marketplace] does not have 600,000 people traversing it daily, it is not a reshus harabbim.”
Rashi is the original source of this criterion that the street requires shishim ribo traversing it in order that it be classified as a reshus harabbim, and not all of the Rishonim are in agreement with him. [The Bais Av (2:5:2) brings proof that the majority of Rishonim accepts shishim ribo as a fundament of a reshus harabbim (see also Shemiras Shabbos K’Hilchasa, perek 17, note 21 and Toldos Shmuel, 3:86:8).] Since there is no unified opinion among the Rishonim, when the Shulchan Aruch (O.C. 345:7) mentions the criterion that the street requires shishim ribo it prefaces it with a qualifier, “vyeish oimrim,” there are those who say. Nevertheless, nearly all of the Achronim consider the criterion of shishim ribo an accepted fundament of a reshus harabbim (Taz, 345:6 and Magen Avraham, 345:7). The Aruch HaShulchan (Choshen Mishpat 162:1) maintains that shishim ribo is an accepted fundament to the extent that if one does not want to join his neighbors in their eruv chatzeiros because he wants to be stringent and not rely on the criterion of shishim ribo, his neighbors can compel him to join them.
However, since some of the Rishonim do not accept shishim ribo as a requirement for a reshus harabbim, there is a difference of opinions among the Achronim whether or not a Baal Nefesh should adopt the stringent position if there is no shishim ribo present. Most Achronim mention the requirement of shishim ribo traversing the street without stating that a Baal Nefesh should adopt the stringent position. This omission signals that the requirement of shishim ribo traversing the street is an accepted fundament in the laws of reshus harabbim and even a Baal Nefesh can rely on it. Additionally, some Achronim state that once we utilize a tzuras hapesach for a street that is sixteen amos wide it is accepted that a Baal Nefesh could rely on the fact that the street does not have shishim ribo traversing it (Kanah V’Kanamon, 5:56; Livush Mordechai, 4:4, and Bais Av, 2:9:3). Other Achronim maintain that a Baal Nefesh should adopt the stringent position and not rely on the fact that the street does not have shishim ribo traversing it. However, that is only where the sole basis for leniency is that the streets are lacking shishim ribo (Mishnah Berurah, 364:8).
It is important to recognize that when considering the basis for leniency there are two levels: a fundamental factor in the din of reshus harabbim such as mefulash or mechitzos and a tzad l’heter such as using questionable mechitzos. The Mishnah Berurah (Bi’ur Halachah, 345:23), quoting the Elya Rabah, states that even in conjunction with a tzad l’heter, a Baal Nefesh can be lenient and rely on the fact that the street does not have shishim ribo traversing it. How much more so, if we were relying on fundamental mitigating factors like the streets not being mefulash and the area having mechitzos, there is no doubt that the Mishnah Berurah ― and all the other poskim as well ― would agree that a Baal Nefesh could be lenient and utilize an eruv of tzuras hapesachim, since we have not met all the requirements of a reshus harabbim.
It should be noted that had the Mishnah Berurah (345:23) seen the Bais Ephraim (O.C. 26) in conjunction with the Mishkenos Yaakov (O.C. 120-122), there is a possibility he would have paskened like the Bais Ephraim that shishim ribo is an accepted fundament of a reshus harabbim, and he would have agreed that even a Baal Nefesh could be lenient and rely on the fact that the streets are lacking shishim ribo. See Bi’ur Halachah (208:9, s.v. Eino M’Vorech), where he states that he does not have the sefer Bais Ephraim (see also Toldos Shmuel, 3:81:7, 3:86:8; Bais Av, 2:5:2; Divrei Yatziv 2:173:1, and Even Yisroel, 8:36).