Las,
“his view is unique, so you didn’t answer the question. What is the source where do you find an accumulation of streets? Where do you find the 12 mil by 12 mil? ”
See the link and post by ‘youdontsay’ regarding Rash in Eruvin that states a “City” of 600k.
Rav Moshe’s concept is not totally unique. The Mishkenos Yakov’s believed that is really Rash’s shitah according to the Ritvah (also in Chidushei haRa”n), that any “road” that serves to 600k is rendered as a RH”R of 600K.
However, he specifically states that it is not conditional or depending on any city population – any road that is “open” or leads to 600k will suffice, (that means even a small town with any road leading to any major highway is a Rh”R). Moreover, he is very clear that the aforementioned shitah is NOT according to the TUR and Shu”A – based on the Tur/SHu”A, all 600K need to be on one road/place, as was in Machnei Levi at the time of the Mishkan, according to the above.
(I assume Rav Moshe did not see the Mishkenos Yaakov, as he tries to say diff strategies on how his shitah will coincide with the Diglei Midbar, hence the shiur of 12 MIL, and based on the above MY, it is really a machlokas Rishonim, and the SHU”A rules that all 600k are to be on one place)
“The Aruch Hashulchan also disagrees.”
Agreed, however, I recall Rav Moshe mentioning it in his last Teshuvah in Vol 5.