In 2010, the County Council imposed a 20mph speed limit on much of central Cambridge through the Cambridgeshire County Council (Cambridge Historic Centre) (20 mph Speed Limit) Order 2010. It took them a few months to get around to putting up signs to indicate the effect of the order, and even now they're not quite right.
At the Magdalene Street entrance to the central area, there's a 20mph speed limit sign (diagram 670) on the left-hand side of the road. This might be sufficient, though the Traffic Signs Manual (ch. 3 para. 14.18) strongly recommends putting in two. Also, it recommends (ch. 3 para. 14.60) that when multiple signs are mounted on a single post, the speed limit sign should be above everything other than Stop (diagram 601.1) and Give Way (diagram 602) signs.
Outbound, there's just one 30mph sign on the right, whereas the direction 8 of the Traffic Signs General Directions requires one on each side of the road. The sign looks like it's on a backing board which is white with a black border, neither of which is permitted by the regulations (reg. 42). As with the inbound sign, the arrangement on the post disregards the recommendations of the Traffic Signs Manual.
Strangely, the change of speed limit on Silver Street is signed as the entrance to a 20mph zone rather than an ordinary 20mph limit. The inbound sign (diag. 674) is only on the left-hand side of the road, which is arguably correct but contrary to the recommendation of the Traffic Signs Manual (ch. 3 para. 14.18). The outbound sign is also only on the left, in contravention of direction 8.
The signage as a 20mph zone is only permitted if the whole area of the speed limit indicated by the sign is either a short cul de sac or within 50m of a traffic-calming feature (direction 14). In this case, the sign indicates almost the entire central area speed limit and while I haven't checked, I rather suspect that some part of it is further than 50m from a traffic-calming feature.
Inbound on Trumpington Street, there is a pair of speed limit signs to diagram 670, one on the left-hand side of the carriageway and one on the central island. Direction 8 requires that speed limit signs must be placed on either side of the carriageway a where the road only has one, and regulation 1 defines a dual carriageway so as to specifically exclude a road that just has a traffic island in the middle.
The outbound 30mph signs are similarly situated, and don't even have the excuse that they might be the entrance to a side road.
Despite being covered by the Order, Regent Terrace doesn't seem to have any speed limit signs at all.
Inbound, Parkside only has a 20mph sign on the left-hand side. This is arguably permitted, though the Traffic Signs Manual strongly recommends putting signs on both sides of roads at signal-controlled junctions (ch. 3 para. 14.18). On the other hand, only having a single 30mph sign in the outbound direction is definitely contrary to direction 9.
Like Regent Terrace, despite being listed in the order, Burleigh Place has no speed limit signs at all.
Again, Severn Place is listed in the order, but has no speed limit signs.
Inbound, there's a 20mph sign on the left-hand side of the road, and another one on an island at the exit of the roundabout. There's the same arrangement in 30mph signs on the outbound side. As on Trumpington Street, a traffic island isn't enough to make a dual carriageway, so direction 9 requires a sign on the right hand sign outbound, and the Traffic Signs Manual recommends one inbound too.
There's a similar arrangement here, with inbound and outbound signs on the left-hand side of the road and on the island.