For a Rambam Eruv, you also have the issue of what constitutes a Torah level private domain vs public domain. You cannot build an eruv around a Torah level public domain.
What is a public domain? Most city Eruvim operate with the concept that a public domain is a major street 16 amot wide and 600,000 people pass through it daily (see Tosafot Shabbat 6b). If less than 600,000 people pass through, it’s a Carmalit – middle domain that can be made into a private domain by enclosing it with an eruv. A field is a classic example of a Carmalit. Highways, by definition, are public domains.
According to the Rambam (and many/most Sephardic Rishonim) a public domain is any street 16 amot wide (no need for 600,000 people). Therefore, most – if not all – city eruvs in America, even if built to the best standards, are useless according to the Rambam (and Rif, Ramban, Ritva, etc).
RebDoniel,
You should realize that not using eruvin can cause some issues on Shabbat with babies. My rebbe pointed out the SH”A holds that carrying babies (who can’t walk at all) is carrying D’oryta.