The reference to "traffic signs" in regulation 18 of LATOR 1996 has a precise legal meaning. Section 64 of the Road Traffic Regulation Act 1984 defines "traffic signs" as
signs specified by regulations (i.e. The Traffic Signs Regulations and General Directions, currently TSRGD 2016 as amended); or
authorised by the relevant authority (i.e. specially authorised by DfT in England or the equivalent Departments in the other parts of the UK).
This precludes the use of traffic signs which local authorities invent, such as Blue "flying motorcycle" signs and Motor caravan parking signs, unless they obtain special authorisation for them.
It also requires local authorities to use signs which are described in TSRGD only as prescribed. This is like a doctor's prescription: just as you're told that for this condition you are to take this drug once a day for two weeks, so the prescription in TSRGD sets out the appearance of the sign and the circumstances in which it can be used.
For most signs, TSRGD specifies a range of sizes. DfT provides guidance in the Traffic Signs Manual about which size should be used: larger sizes are advised where traffic on a road is faster. As this is guidance, local authorities are not obliged to follow the advice although, for local authorities outside London, the Statutory Guidance about civil enforcement does say:
The Secretary of State will not [agree to the introduction of civil enforcement] until a local authority’s Chief Executive has confirmed ... that all [traffic orders], traffic signs and road markings ... comply with [TSRGD 2016] and guidance on relevant chapters of the Traffic Signs Manual...
One instance where the guidance is not followed is with the blue roundels for bus restrictions: on 30mph roads, most are 600mm diameter whereas the Traffic Signs Manual advises 750mm.
The Statutory Guidance about civil enforcement also advises local authorities to:
aim to increase compliance with bus lane and moving traffic regulations through clear, well designed, legal, and enforced controls including clear traffic signing which can readily be understood from a moving vehicle.
The orists who The size of signs and their legibility to motorists are key factors which adjudicators take into account when considering appeals against PCNs on the grounds that the signage was inadequate.
. When these signs were introduced in 1994, the size recommended for 30mph roads was 600mm diameter. In the 2002 update to TSRGD this was changed to 750mm diameter, presumably because the sign was found to be harder to recognise from a distance than other signs. Nonetheless, most blue roundels placed on 30mph roads are 600mm diameter.