Shishim Ribo and the Requirement of Six Hundred Thousand
Shishim ribo—six hundred thousand—comes from the Jewish people in the midbar. Many Rishonim make comparable public use a condition of reshus harabim; others define the public domain through its physical form without requiring that number.
The camp in the midbar
Six hundred thousand adult Jewish men travelled in the midbar, and the public roads of the camp provide the model for reshus harabim. The number is therefore tied to a public whose roads served the whole encampment, not merely to a modern turnstile count detached from the city around it.
Shulchan Aruch HaRav 357:7, 392:1, and 404:2 discusses the camp and its public ways in contexts used by Rabbi Shmuel Zajac and Rabbi Sholom Dovber Levin to understand the count as the city population served by its streets. That reading is an applied halachic argument, not an uncontested arithmetic rule.
A single street or the public served by the roads
One approach asks whether six hundred thousand pass through the relevant street or point each day. Another looks at the city population and the roads available to that public. The difference is decisive in a place where no single block carries 600,000 daily passages but the connected metropolis contains millions.
Rabbi Levin illustrates the practical difficulty of a literal point count: even over twelve hours, 600,000 passages would average nearly fourteen people every second. He argues that the classical sources speak instead about the public of the city and the roads serving them.
Population and traffic figures are not interchangeable
A census counts residents within a stated boundary. Road counters count vehicles; subway data count entries or rides; pedestrian counts cover selected places and hours. None can be substituted silently for another.
A useful calculation first states the halachic premise, then the geographic area, time period, unit, and source of the number. A precise figure attached to the wrong question proves nothing.
New York and Brooklyn
Rav Moshe did not treat Flatbush, Boro Park, or another Brooklyn neighborhood as a separate small town. His responsa place local streets within a borough of millions and a public network extending through New York.
In Orach Chaim IV:88, he refers to contemporary estimates of approximately three million Brooklyn residents and roughly one million additional commuters and visitors. Those are historical figures from the teshuva, not present-day counts, but they show the scale on which he framed the halachic question.
The Alter Rebbe brings both definitions
The shishim-ribo condition appears in the Alter Rebbe's presentation of the widely relied-upon lenient definition. He also brings the stricter position that a road can be reshus harabim without six hundred thousand.
A city eruv must account for how the accepted psak treats both views. Quoting the number alone does not settle a large modern city, and omitting the stricter view does not present the Alter Rebbe's ruling in full.