Eruvin 7a: A Central thoroughfare (sratya) or plaza (platya)
(excerpted from The Contemporary Eruv)
In his analysis of the definition of reshus ha'rabbim, the Aruch HaShulchan, Orach Chaim 345:14-22 takes this concept to a further extreme, and states that there are essentially no reshuyos ha'rabbim in our times. He opines that an essential component of the definition of reshus ha'rabbim is that it serve as the central thoroughfare (sratya) or plaza (platya), for the entire city. Since none of our cities have one central thoroughfare that constitutes the main access route to and from the city, nor do they have one central plaza that serves to amass the population of the city - à la the Machane Levi’ah in the desert - there are no longer any true reshuyos ha'rabbim in our midst. Many Poskim will only use the Aruch HaShulchan’s opinion as an additional reason to be inclined toward leniency (a “snif”) when there are other mitigating factors. See Igros Moshe, Orach Chaim, 5:28:9 and Nesivos Shabbos 3:1, note 9.
The Rashba (Avodas HaKodesh 3:1) - and others, see the Avodas Avoda there note 2 - holds that there are halachic distinctions between the two types of reshus ha'rabbim mentioned in the Gemara: A “sratya” - thoroughfare - and a “platya” - central public square or market. While doors serve to block a thoroughfare and render it a reshus ha'yachid, they do not render a public square a reshus ha'yachid. The generally accepted understanding of the Rashba’s position is that a platya is not a reshus ha'rabbim because of a high volume of traffic - which may, indeed, be prevented by doors - but, rather, by the presence of a static multitude - upon which doors obviously have no impact. According to the Rashba, even when Yerushalayim was fully walled and doored, the public squares within the city retained their character as reshuyos ha'rabbim and carrying remained prohibited in those areas (although not in other streets of the city, as noted by the Sefer HaBattim, Sha’ar Sha’arei Issur Hotza'a 1:15 in his explanation of the Rashba’s position).
Most sources argue with the Rashba, and the final, accepted ruling is that delasos are effective in rendering a platya a reshus ha'yachid. (See the Avodas Avoda, ibid., that even the Rashba may concede the point where the platya itself - as opposed to the surrounding city - is surrounded by doors).