How the Position Is Applied in a Modern City
Modern urban population, road networks, open public access, and citywide movement can recreate facts that were uncommon in earlier towns.
Begin with the city as it exists
Street widths, straight routes, public access, working gates, population, traffic, and connections to the wider network must be established before choosing a halachic label.
Modern mapping and transportation data can help, but they must be checked against the physical street and used for the particular facts halacha asks about.
Old descriptions can outlive the facts
A city described generations ago as small, walled, or lightly traveled may later absorb suburbs, remove its gates, widen its roads, and grow past six hundred thousand people.
The Rebbe made this point sharply in the sicha of Yud-Beis Tammuz 5745: people were repeating an old assumption while overlooking the present population of the city before them.
The responsible Rav applies the earlier views
After the facts are known, the posek decides how the positions of the Rishonim and later authorities apply. That decision governs the kind of enclosure that can be built.
In Crown Heights, the local Rabbonim have already given the practical psak. The advertised eruv may not be relied upon, irrespective of a general claim that reshus harabim no longer exists.