The Halachic Definition of Reshus Harabim
Reshus harabim is the Torah's public domain. Its classic form is drawn from the roads of the midbar, and the poskim apply that model through the width, openness, access, and public use of the road before them.
Sixteen amos wide
The roads in the midbar were sixteen amos wide, making that width the classic starting point for a public way. A modern measurement must identify which shiur of an amah it follows and which parts of the road belong to one continuous public surface.
Travel lanes, parking lanes, medians, service roads, and sidewalks do not all receive the same treatment automatically. Painted traffic markings are not walls; a raised and substantial division may affect the road's form. The physical plan and the public's use of it are considered together.
Mefulash and open public routes
Mefulash describes a public way open through the city. In the classic walled-city case, the road runs from one gate to the gate opposite it. Walls, working gates, closed ends, and meaningful turns can affect that form.
Exact gate-to-gate alignment belongs most plainly to a walled city. Rabbi Sholom Dovber Levin, citing Rashi, the Raavyah, the Tzemach Tzedek, and Igros Moshe, argues that crooked or intersecting streets do not by themselves remove reshus harabim from an unwalled city.
Open to the public
A city street that the public may enter at will differs from a private road behind working gates. Government ownership is not the test by itself. A privately owned passage can function as a public way, while real restrictions may change the use of publicly owned land.
Residents, workers, visitors, vehicles, pedestrians, buses, and subway riders may all belong to the public served by the road, depending on the halachic view being applied. Census totals and traffic counts therefore answer different questions.
The road within the city
A neighborhood street may lead into a major avenue, bridge approach, commercial corridor, or transit center. A low count on one block does not necessarily detach it from the larger public system.
Rav Moshe's New York responsa repeatedly consider the population and roads of the larger city. Cultural neighborhood names, ZIP codes, and a line chosen for an eruv are not themselves walls that divide public movement.
Why the definition changes the enclosure
If the area is a karmelis, a properly constructed tzuras hapesach may enclose it. If the road is a reshus harabim, Shulchan Aruch HaRav requires the stronger enclosure of walls and doors that close at night.
The definition is therefore practical. It determines whether the proposed boundary can permit carrying at all.