How Six Hundred Thousand Is Calculated
A calculation of shishim ribo is only useful when its halachic definition, geographic area, population, route, and period are stated clearly.
Begin with the halachic rule
The calculation must first say whether it is counting one street, all streets serving the city, daily passage, resident population, or another defined public. Different poskim and different readings of the Rishonim can require different facts.
Without that premise, a total of 600,000 has no clear halachic meaning. It may answer a question that the psak never asked.
Define the geographic area
A count can be made for a single road segment, the territory inside a proposed eruv, the borough, or the connected city. The boundary used for the data must correspond to the boundary used in the halachic argument.
Crown Heights is not separated from Brooklyn by gates or municipal borders. Any decision to count only the proposed route's residents needs a halachic reason for excluding the larger public network.
Use evidence that measures the chosen fact
Census data measure population; transportation agencies measure vehicles or riders; pedestrian counts sample particular locations; GIS establishes dimensions and connections.
A precise-looking total built from unlike measures can still be meaningless. Each number should retain its unit, place, date, and method.
Use figures from the correct period
A responsum's historical estimate belongs to the city of that time. Later population growth, new highways, expanded transit, or changed boundaries can alter the facts materially.
Present-day analysis should use current evidence and explain any comparison with the period in which the earlier psak was written.