Public road – sometimes published on OS maps as an ORPA (Other Route of Public Access)
ORPA’s often have public vehicular rights (ie, motorised) but are not necessarily tarmac serviced,
legally the above route would appear to be a road maintainable at public expense (ie. recorded on the “list of streets” by the council rather than on the rights of way map) so would have full vehicular rights unless a TRO was imposed) I would imagine the sign was there to clarify that the route was not a private track.
In other words, you’ve just found a new legal trail that was not on the map 😀