Nev,
“Gaon: There’s a shittah that the 600K only applies to cities and intra-city roads require fewer than 600K.”
I am aware of that shitah – it is the shitas haRamban in Eruvin 59 (i think that’s the daf). that a “sratya” does not require 600k, he says that “bederch efsher” to answer shitas Rashi. However the Shu”a and the TUR seem like they don’t go with that shitah. (I recall the Avnei Nezer ruling that most Rishonim clearly say not like the above Ramban)