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.