Reshus Harabim

According to the Shulchan Oruch (OC 345) a Reshus HaRabim (public domain) is an area that is at least 16 amos wide without a roof. The opinion of R Avraham Chaim Na’eh is that an amah is 18.9 inches. According to the opinion of the Chazon Ish an amah is 22.7 inches   (Frank, The Practical Talmud Dictionary)

Question: Based on each of these opinions how wide does a street have to be to be a Reshus HaRabim?

Question: The average width of an alley in the City of Chicago is 18 feet. Would this be a Reshus HaRabim by either of these opinions?

Answer:

By R Na’eh

18.9″ x 16 = 302.4″

302.4″ / 12 = 25.2′

By Chazon Ish

22.7″ x 16 = 363.2″

363.2″ / 12 = 30.27′

At 18′ a Chicago alley is not a Reshus HaRabim by either opinion.