From Roader's Digest: The SABRE Wiki
In England and Wales a Highway Authority is a statutory organisation empowered by the Highways Act 1980, as amended by various other acts (including the GLA Act), to carry out the function of owning and operating the highway network in those two countries. Highway Authorities outside the national government are the top-tier local authorities in their relevant areas, such as County Councils, Metropolitan Boroughs, or Unitary Authorities.
In Scotland any organisation empowered by the Scottish Government in accordance with the Roads (Scotland) Act 1984 as amended is known as the Roads Authority; whilst there is no real equivalent term in the Republic of Ireland where all local authorities have road management responsibilities.
On the SABRE Wiki, the term Highway Authority is used as a catch-all for the various bodies that manage the road network within the whole of Great Britain and Ireland. It should be noted that outside England and Wales, this terminology is technically incorrect.
Transport for London (TfL) is the Highway Authority for main routes in London, except for sections of the M1, M4 and M11 motorways which come under the jurisdiction of Highways England. Other roads within London are the responsibility of the relevant London Borough.
All other roads are the responsibilities of the top-level Local Authorities (which may be a County Council, a Unitary Authority or a Metropolitan District) in which they are situated. A list of these can be found at Category:English Highway Authorities.
See also: Welsh principal areas
Responsibility for the development, operation, and maintenance of the trunk road network in Wales is vested in the Welsh Government, acting through either the North and Mid Wales Trunk Road Agent (NMWTRA) or the South Wales Trunk Road Agent (SWTRA). All other public highways are managed and maintained by the principal area in which they are situated. A list of these can be found at Category:Welsh Highway Authorities.
In Scotland the term Highway has no legal meaning, and the Scottish equivalent to a Highways Authority is the Roads Authority.
All trunk roads are the responsibility of Transport Scotland, a Scottish Government agency. All other roads are the responsibility of the Unitary Authorities in which they are situated. A list of these can be found at Category:Scottish Highway Authorities.
The situation in Northern Ireland is simple in that all public roads are the responsibility of the Northern Ireland Assembly, acting through Transport NI. Local authorities play no part in road maintenance.
Republic of Ireland
All roads forming part of the National Roads network – i.e., all roads having numbers prefixed by the letters M (motorway) or N (other national road) – are planned and maintained by Transport Infrastructure Ireland, a state body founded in 1994 (and known earler as the National Roads Authority) under the terms of the Roads Act 1993.
All other public roads are the responsibility of the County, City, or City and County Councils (the Local Road Authorities – LRAs) in whose areas they are situated. A list of the 31 LRAs can be found at Category:Irish Highway Authorities.
Design, Build, Finance, Operate Concessions (DBFO)
A number of roads in the UK are managed by a DBFO or similar arrangement, for example the M6 Toll, Second Severn Crossing, the A74(M) and the M40 motorway. Whilst the Concessionaire is responsible for maintaining the road, Highway Authority responsibility remains with the statutory organisation.