And here is another question answered where I explain the difference between a 20mph limit and a 20mph zone
There used to be a very clear distinction between areas or roads set as 20mph zones or 20mph limits. 20mph zones require any point to be within 50m of a "traffic calming device" which used to be defined quite strictly as a physical calming device. Whilst 20mph limits only require each point to be within 50m of a repeater sign that informed the driver that the speed limit was 20mph.
This reliance on largely physical calming in zones increased the cost some 50-fold over limits due to the expense of such physical calming. Hence for the same cost of treating a street of 250 people with a physically calmed zone then an entire community of 12,500 could be treated with a wide-area 20mph limit.
However this requirement was relaxed in 2011 when the DfT announced that for 20mph zones then repeater signs, carriageway roundels and mini-roundabouts could also be classed as "traffic calming devices". However any zone should include at least one physical calming device. In limits a carriageway roundel could also be used in place of a repeater sign. Note that the actual placement of roundels and repeater signs is at the discretion of the local authority. See the Special Directions Authorisation Note.
This has made a huge difference to the way that 20mph limits may be set and in practice there may be little difference between zones and limits. Where traffic authorities are implementing 20mph limits the choice of zone or limit will probably depend on the type of street, current vehicle speeds and whether isolated 20mph zones already exist. If they do then it is often more practical to widen expand these in a much larger community-wide zone. Note that 20mph zones have rectangular boundary signs with a 20mph limit sign within it, whereas 20mph limits have round boundary signs.
In order to set a 20mph limit, either as a zone or limit, then the following conditions must apply :-
1) The correct Traffic Regulation Order must be made for the road concerned.
2) Any signage or traffic calming devices (in the case of zones) must meet the Traffic Signs Regulations and General Directions. This specifies the design of signs or traffic calming devices and where they need to be placed.
Once this has been done then such 20mph limit or zone is valid and enforceable by the police. The Association of Chief Police Officers have guidance on this which is availablehere.
The Dept for Transport guidance on setting local speed limits was revised in 2013 and this very much encourages the use of 20mph limits or zones for residential streets and those with high numbers (or potential numbers) of cyclists and pedestrians. This is available here.
Note that the TSRGD is a technical document that must be adhered to, whereas the DfT and ACPO documents are guidance.
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