Reshus Harabim #3

Rabbi Moshe Feinstein points out that the laws of public domain are derived from the encampment the Jews lived in when they left Egypt.  Based on this he suggests that the 600,000 people would need to be in an area the size of the Jewish encampment in order to constitute a public domain. (Igros Moshe 139)

The encampment was 12 mil by 12 mil (Sanhedrin 5b). A mil is based on the distance and average person can walk in 18 minutes. One mil is 2,000 amahs. There are 12 inches in a foot.  There are 5,280 feet to a mile.

Assuming an even population density through the city which of these cites would not be eligible for an eruv based on the opinion of Rav Moshe Feinstein.

Miami pop 2010 362,470 over 35.67 miles

Philadelphia pop   1,517,550 over 135.09 miles

Chicago pop 2,695,598 over 227.13

Boston 645,149 over 48.43 miles

Tel Aviv (metro area) 1,318,300 over 585 sq. miles

Greater Manchester, UK 2,553,379 over 243.4 Qu miles