Public domain - property rights assured to the public?
A roman system jurist would probably distinguish between property rights enjoyed by the public at large and liberties. Liberties are however possession-protected, as anyone can (in some jurisdictions) claim possession (even not exclusive...) of a throughway if a squatter prevents the liberty to use it.
Therefore, possession, as the free use of a public domain utility may be recognized as an economic right even though property does not actually falls into the hands of the public at large.
This analysis of public domain (on American legal concepts) has been very convincingly advanced by the market failure theorists.