goldersgreener: “Nonetheless, there is a suggestion – primarily by הגאון ר’ משה פיינשטיין about brooklyn and the מנחת יצחק about London, to say that the מחבר does not refer too a literal 600k every day, rather, basically, the מחבר means a street which is מיועד for 600 k. Exactly what each pisek held, and exactly how one is קןבע the מציאות can be debated, and it remains הררים התלוים בשערה.”
Only the Mishkenos Yaakov maintains that מיועד is sufficient. Rav Moshe maintains that the criterion of shishim ribo is dependent on 12 mil by 12 mil , were the shishim ribo actually traverse the streets. The Minchas Yitzchak (8:32:1) maintains that the shishim ribo needs to traverse the street, just not on a daily basis (this is how he understands the Bais Ephraim).
The only reason why some large roads in EY are not included in the eruvin, was because originally these roads did not have mechitzos habbatim so the Chazon Ish excluded them from the eruv (http://eruvonline.blogspot.com/2008/05/would-chazon-ish-have-utilized-his.html).