The north end of St John's Street marks one of the entrances to Cambridge's main pedestrian zone, the other being at the south end of Sidney Street.
There are two signs marking the entrance to the pedestrian zone on St John's Street. One is a fixed sign just west of the entrance to the forecourt of St John's College, while the other is a variable sign (with two faces in use) at the junction with Bridge Street.
The fixed sign (albeit without its lower panel) was authorised by the Secretary of State for Transport (ref GT 46/20/014 dated 12th July 1994). These days one would use a slightly different sign to diagram 618.3. Either way, it ought to be lit.
The variable sign is also specially authorised (ref GT46/20/14 dated 3rd December 1998), though it doesn't quite match the authorisation since it still has a cycle symbol in place of the word "cycles".
The restrictions on entry to the pedestrian zone are imposed by the County of Cambridgeshire (Northern Historic City Centre, Cambridge) (Pedestrian Zone and Peripheral Streets) Order 1993. Article 3 corresponds with the inner, fixed sign:
3. Save as provided in Article 4 no person shall, except upon the direction or with the permission of a police officer in uniform or a traffic warden, cause or permit any motor vehicle or any horse drawn vehicle to proceed during the zone restricted hours in the roads and the parts of the roads referred to in Part 1 of Schedule 1.
4. Nothing in Article 3 shall render it unlawful to cause or permit any motor vehicle to proceed in the roads and the parts of the roads referred to therein if the vehicle is:
(i) a motor vehicle being used by a zone permit holder , upon which is displayed in the relevant position a zone permit issued to exempt the zone permit holder from the prohibitions imposed by Article 3;
etc
Schedule 1 only covers the part of St John's Street between the College entrance and Trinity Street.
The outer, variable sign is clearly intended to correspond with articles 9 to 13. They start innocently enough:
9. (1) For the purposes of this Article and Articles 10 to 13, a "specified road" shall be one of the roads or parts of the roads specified in Column 1 of Parts 1 and 2 of Schedule 2
(2) Save as provided in Articles 10 and 11 no person shall cause or permit any vehicle to enter a specified road.
St John's Street (the whole of it) is in Part 2 of Schedule 2. So were it not for articles 10 and 11, no vehicles would be allowed to enter St John's Street at all. Article 10 conveniently provides some exceptions:
10. Subject to the provisions of Article 11, nothing in Article 9 (2) shall render it unlawful to cause or permit any vehicle to enter a specified road either:-
(i) upon the direction or with the permission of a police officer in uniform or a traffic warden; or
(ii) if the vehicle falls within any of the classes cf vehicle referred to in Column 3 of Parts 1 and 2 of Schedule 2 for the specified road to be entered; or
(iii) is being used for any of the uses referred to in Column 2 of Parts 1 and 2 of Schedule 2 for the specified road to be entered.
The relevant entries in Columns 2 and 3 provide for access to premises, loading, emergency services, buses, taxis, disabled persons' vehicles, pedal cycles, and a selection of more obscure cases. However, article 10 is subject to the provisions of article 11, which applies some time limits:
11. The provisions of Article 10 shall only be applicable in respect of the specified roads referred to:
(a) in Part 1 of Schedule 2 before 10am and after 4pm on Mondays to Saturdays inclusive and at all times on Sundays;
(b) in Part 2 of Schedule 2 between 8.30am to 10am and between 4pm to 6.30pm on Mondays to Saturdays inclusive.
So that means that article 10, which contains the exceptions, only applies to St John's Street between 08:30 and 10:00 and between 16:00 and 18:30 on Mondays to Saturdays. At other times, the restriction in article 9 applies with full force and no vehicles may enter St John's Street.
Of course, this isn't what the sign says. Between 10:00 and 16:00 Monday to Saturday it doesn't indicate any entry restriction at all, and just warns of one at the next sign. Outside those times, it indicates (roughly) the entry restriction with all its exemptions, even though the exemptions don't apply between 18:30 and 08:30, or at all on Sundays.
Annotations on the authorisation for the variable sign, and on that for its predecessor, suggest that it was intended that the "no motor vehicles" face would only be displayed 08:30 to 10:00 and 16:00 to 18:30 Mondays to Saturdays, and that overnight and on Sundays the signs would be blank apart from the "no waiting" panel. This isn't want actually happens, but it hints that someone expected article 11 to qualify article 9 rather than article 10.
South of the College entrance, St John's Street is a simple one-way street (art. 30 Area Z Order). North of that, it's one-way most of the time, but is two-way between 10:00 and 16:00 Monday to Saturday. The latter restriction isn't signed at all: the first one-way sign is south of the College entrance (and unlit).