Is there streetlighting present?
There are a few situations where one sign on the nearside is correct (usually at junctions) but generally you are correct, terminal signs are required on both sides of the carriageway, according to the TSRGD 2002.
Quote:
The placing of certain signs to indicate the beginning of a restriction, requirement, prohibition or speed limit
8. - (1) This direction applies to the signs shown in diagrams 616, 617, 618, 618.1, 618.2, 618.3, 618.3A, 619, 619.1, 619.2, 620, 620.1, 622.1A, 622.4, 622.5, 622.6, 622.7, 622.8, 622.9, 625.1, 626.2A, 627.1, 629, 629A, 629.1, 629.2, 629.2A, 632, 642, 646, 663, 663.1, 665, 667, 667.1, 668, 668.1, 670, 672, 674, 951, 952, 953, 953.1, 955, 956 and 957.
(2) In accordance with the following provisions of this direction and the provisions of direction 9, appropriate signs to which this direction applies shall be placed to indicate the point at which a restriction, requirement, prohibition or speed limit applying to traffic on a road (in this direction and in direction 9 called "the relevant road") begins.
(3) Subject to paragraphs (4), (5) and (6) and to direction 9, a sign to which this direction applies shall be placed on the relevant road at or as near as practicable to the point referred to in paragraph (2) -
(a) where the relevant road has only one carriageway, on each side of that carriageway; or
(b) where the relevant road has more than one carriageway, on each side of each carriageway in relation to which the restriction, requirement, prohibition or speed limit begins.
(4) Where the relevant road has one carriageway, then signs to which this direction applies need only be placed on one side of the relevant road to indicate the point at which a restriction, requirement, prohibition (but not a speed limit) begins in the following cases -
(a) where the restriction, requirement or prohibition applies only to traffic on one side of the relevant road; or
(b) at a junction where -
(i) traffic proceeding on another road on which it is permitted to proceed only in one direction turns into the relevant road; or
(ii) the carriageway of the relevant road is less than 5 metres wide and the sign is so placed that its centre is within 2 metres of the edge of the carriageway.
(5) Where a length of road which passes under or through a bridge, tunnel or other structure is subject to a prohibition on vehicles exceeding a particular height, the sign shown in diagram 629.2 or 629.2A may be placed on the bridge, tunnel or other structure to indicate the prohibition in addition to or instead of the signs required to be placed by paragraph (3).
(6) Paragraphs (3) and (4) shall not apply to the signs shown in diagrams 667, 667.1, 668 and 668.1 which shall be placed on the side of the carriageway on which parking on a verge or footway is permitted and as near as is practicable to the point at which the length of the verge or footway concerned begins.
Beginning of a speed limit - further provisions
9. - (1) Direction 8(3) does not apply where a speed limit in force on the relevant road begins -
(a) at a point where the relevant road begins, being a point where it has no junction with another road; or
(b) at a point where the relevant road has a junction with another road and the same speed limit is in force on both roads.
(2) This paragraph applies where the relevant road has a junction ("the junction") with the side of another road ("the other road") and a maximum speed limit is in force on the other road which is different from the speed limit in force on the relevant road.
(3) Where paragraph (2) applies, it is sufficient compliance with direction 8(2), for the purpose of indicating the beginning of the speed limit on the relevant road to traffic entering it from the other road, if the sign shown in diagram 670, 674 or 675 is placed not further than 20 metres from the junction, on the left hand or near side of the carriageway of the relevant road as viewed in the direction of travel of such traffic or, where the relevant road is a dual carriageway road, on the left hand or near side of the carriageway by which traffic may pass into the relevant road from the other road.
(4) Where paragraph (2) applies, for the purpose of indicating the speed limit in force on the other road to traffic entering that road from the relevant road, the sign shown in diagram 670, 674 or 675 shall (subject to paragraph (5)) be placed not further than 20 metres from the junction and so as to be visible to such traffic, on each side of the carriageway by which traffic may pass from the relevant road into the other road.
(5) Paragraph (4) does not apply if -
(a) the maximum speed limit in force on the other road is greater than that in force on the relevant road; and
(b) signs indicating the maximum speed limit have been placed on the other road in accordance with direction 11 on each side of, and not more than 100 metres from, the junction.
hope that helps
fatboytim