The Alter Rebbe and the Requirement of Shishim Ribo
Shulchan Aruch HaRav records the view that shishim ribo is required for reshus harabim and also brings the stricter opinion that does not depend on that condition.
The view requiring shishim ribo
Shulchan Aruch HaRav records the widely relied-upon view that a public domain requires the passage of shishim ribo. Under that view, a street with the necessary width and openness may still fall short of a full Torah reshus harabim without the required public use.
The exact application of the number must still be decided. The text does not supply a modern census boundary or traffic-counting formula.
The stricter definition remains part of the halacha
The Alter Rebbe also brings the stricter view that does not make six hundred thousand a necessary condition. A properly formed public route can be reshus harabim because of its width, openness, and use by the public.
That opinion matters when deciding whether ordinary tzuras hapesach can be relied upon. It cannot be removed from the discussion by quoting only the more lenient definition.
Applying both views to a modern city
A very large city with wide public roads, continuous access, and heavy movement requires an actual psak. The Rav must determine how the two views are treated and which city facts satisfy each one.
Crown Heights is judged within present-day Brooklyn. Neither the phrase shishim ribo nor the statement that some authorities require it decides the neighborhood without that application.