Subject to the provisions of this section, a highway authority may erect and maintain fences or posts for the purpose of preventing access to—
a highway maintainable at the public expense by them,
land on which in accordance with plans made or approved by the Minister or a strategic highways company they are for the time being constructing or intending to construct a highway shown in the plans which is to be a highway so maintainable, or
land on which in pursuance of a scheme under section 16 above, or of an order under section 14 or 18 above, they are for the time being constructing or intending to construct a highway.
A highway authority may alter or remove a fence or post erected by them under this section.
The powers conferred by this section shall not be exercised so as to—
interfere with a fence or gate required for the purpose of agriculture; or
obstruct a public right of way; or
obstruct any means of access for the construction, formation or laying out of which planning permission has been granted under Part III or, in relation to England, Part 13 of the Town and Country Planning 1990 (or under any enactment replaced by the said Part III) and, in the case of a trunk road, consent has been given under section 175B (consent of highway authority required for trunk road access) ; or
obstruct any means of access which was constructed, formed or laid out before 1st July 1948, unless it was constructed, formed or laid out in contravention of restrictions in force under section 1 or 2 of the Restriction of Ribbon Development Act 1935.
As respects—
a highway that is a trunk road, and
land on which the Minister or a strategic highways company is for the time being constructing or intending to construct a highway that is, or is to be, a trunk road, either in accordance with plans made by him or it in which the road is shown or in pursuance of a scheme under section 16 above,
the powers under this section may be exercised not only by the Minister or a strategic highways company but also, where the road or land is outside Greater London, by the council of the county or metropolitan district in which it is situated, or where the road or land is in Greater London, by ... the council of the London borough in which it is situated.