I'm not sure that private and unadopted are always the same thing, although some of my information dates to the 1960s. The family house was on an unmade road which seemed to have public right of use but not adopted by the council (far from the only one in the area, certainly no association and delelopers long since gone - stuff built in the 1920s & 30s - and upkeep consisted of locals throwing anything available into the ruts). When the council adopted and made up the road each householder had to pay for the section of road in front of their house.
The Grange estate in Penkridge had made roads, again seemingly with public right of access, but at least the pavements were not adopted. The council agreed to take over the upkeep if the householders agreed. Some did, some didn't, so now you have a patchwork of adopted and unadopted roads along the same street.