What Is a Reshus HaRabbim: Part 1
The Shulchan Aruch (O.C. 345:7) gives four defining conditions of what constitutes a reshus harabbim: a street or marketplace that is at least sixteen amos wide, that is not roofed [mikorim], that is open and runs straight  from city gate to city gate [mefulash m’shaar l’shaar], ...
 The Aishel Avraham (siman 345) maintains that streets are not considered mefulash if they are intersected by roofed [mikorim] roadways. As a snif l’heter there are poskim who maintain that since elevated train tracks and overpasses ― the equivalent of roofed roadways ― circumscribe our communities we can utilize the heter of the Aishel Avraham and our streets would not be considered mefulash. It is interesting to note that Rav Moshe Feinstein zt”l (Igros Moshe, O.C. 1:140) also maintained that a street would not be considered mefulash if it is intersected by a roofed area. So it would seem that if Rav Moshe had known our neighborhoods, he would agree that our streets are not mefulash and therefore our communities would not be classified as a reshus harabbim. [See also Igros Moshe (O.C. 5:28:20) where Rav Moshe posits that while elevated train tracks do not bisect an area of shishim ribo into separate parts of less than shishim ribo, he admits that the intersected street is not considered mefulash.]
 Most poskim understand mefulash m’shaar l’shaar as meaning mefulash u’mechuvanim m’shaar l’shaar, open and running straight from city gate to city gate (Bais Yosef, 345:8; Magen Avraham, 345:6; Pri Megadim, Aishel Avraham, 364:2; Bais Ephraim, O.C. 26; Tiferes Yisroel, introduction to Shabbos; U’Bacharta B’Chaim, siman 123; Shoel U’Maishiv, 2:87; Yehudah Yaleh, O.C. siman 54; Mahari Slutsk, O.C. siman 11; Minchas Elazar, 3:4, and Mishnah Berurah, 345:20). Rav Aharon Kotler zt”l states (Mishnas Rav Aharon, 6:2), that for a street to be classified as a reshus harabbim it is accepted that it would need to be mefulash u’mechuvanim m’shaar l’shaar ― although he personally maintains that it is dependent on a walled city.
 The text of Shulchan Aruch reads:
“What is a reshus harabbim? A street or marketplace that … is not walled and even if they are walled but they [the street or marketplace] are open from city gate to city gate, they [the street or marketplace] would be considered a reshus harabbim ….”
Even though the Shulchan Aruch cites the criterion of mefulash m’shaar l’shaar in conjunction with a walled city, the Tur does not differentiate between a walled city and an open city and the Bais Yosef (siman 345) does not disagree with the Tur. Therefore, mefulash would be considered a criterion of a reshus harabbim even in a city that is not walled (see Bais Av, 2:9:1 for a detailed explanation). See Part 1: Why Brooklyn Is Not a Reshus HaRabbim that most poskim maintain that concerning the criterion of mefulash there is no difference halachically whether or not the city is walled. The Gra offers a different perspective (Bi’ur HaGra, O.C. 345:7) and explains that the words of the Shulchan Aruch, “is not walled,” refers to a street [sratya]. The Dmesek Eliezer clarifies the Gra that a sratya is a road which does not have a wall around it. The Gra explains further that, “even if they are walled,” refers to a marketplace [platya]. The Dmesek Eliezer explains that a platya is a city street that is walled on two sides and the remaining two sides are open straight from city gate to city gate. What we see from this Dmesek Eliezer is that what the Shulchan Aruch refers to as, “walled,” pertains to the streets of the city and not the city walls. Therefore, it is understandable why the Gra (Sh’nos Eliyahu, introduction to Meseches Shabbos and Chidushi HaGra, Shabbos 6a) when discussing the criterion of mefulash m’shaar l’shaar does not attribute it to a walled city. It follows that our streets which are lined with houses would be classified as walled streets and would have to be mefulash u’mechuvanim m’shaar l’shaar to be categorized as a reshus harabbim. Note that since the term city gate [shaar] does not always pertain to the gate of a walled city (Rashi, Yoma 11a), mefulash m’shaar l’shaar can refer to a city that is not walled as well.