There are three or four conditions required to define an area as a reshus harabbim on a Torah level. The conditions are:
The source for these conditions is the Camp of Israel in the wilderness.
To permit carrying in a reshus harabbim, a tzuras hapesach is ineffective; only doors that are locked at night, as existed in Jerusalem in earlier times, are valid.
Where does this apply to us?
Today, the overwhelming majority of eruv boundaries are constructed from tzuras hapesach. There are eruvin within which run roads that may be considered reshus harabbim, such as Highway 4 (Geha), Route 40, Route 90, Ayalon, the Coastal Highway, and others. If these roads are classified as reshus harabbim, the entire eruv becomes invalid. Therefore, when planning an eruv, it is advisable—whenever possible—to exclude major highways from within the eruv boundary. When this is not feasible, expert consultation is required.
