There are two main types of private or unadopted road: those on new developments such as housing estates and those which, usually by historic accident, have existed for a long time, often since the nineteenth century. A Department of Transport survey in 1972 found that there were then approximately 40,000 unadopted roads in England and Wales, making up some 4,000 miles of road. No later survey has been undertaken but the figure is thought not to have changed much. The Labour Government estimated in 2009 that it would cost £3 billion to make up these roads to an adoptable standard.
The law on the maintenance and adoption of private roads in England and Wales is highly complex. It is largely contained in Part XI of the Highways Act 1980. Briefly, a private or unadopted road is by definition a highway not maintainable at public expense. The local highway authority is therefore under no obligation to pay for its maintenance. Responsibility for the cost of maintaining a private road rests with the frontagers (the owners of properties which front onto such roads).
Statutory provision does exist for unadopted roads to be adopted and thus become highways maintainable at public expense. Statutory provision also enables the street works authority to require frontagers to undertake repairs if there is a danger to traffic in a private street. Where the frontagers fail to act as required the authority may execute the repairs itself and recover the costs from the frontagers.
Andrew Barsby, Private Roads: The Legal Framework (5th ed.), 2013, contains a good description of the law.
There is separate legislation for Scotland and Northern Ireland, not covered in this paper.
Information on other roads-related matters can be found on the Roads Briefings Page of the Parliament website.