GAON:
“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). ”
Rav Moshe’s shitos are definitely mechudosh. The Mishkenos Yaakov clearly does not say that the criterion of shishim ribo is dependent on a city (he argues that according to some Rishonim the possibility of shshim ribo traversing a street would classify the road as a reshus harabbim). Rav Moshe’s main chiddushim is that the tenai is 600,000 people traversing an area of twelve mil by twelve mil, and this would require a population of 3 million. No posek ever made similar arguments.