IS 20 Plenty?

It will be for local authorities to determine whether speed limits or zones should be used. They will need to decide whether the proposed type of speed limit is

• appropriate to the area, and
• beneficial in road safety and environmental terms.

Equally important is that the form of speed limit chosen does not require unreasonable levels of enforcement by the police.

20 mph speed limits by signs alone would be most appropriate where 85th percentile speeds are already low and further traffic calming measures are not needed. 20 mph zones should be used where excessive speeds occur, and where traffic calming measures would be needed to ensure speeds are at or below 20 mph. 20 mph zones would be particularly appropriate where there is an existing record of accidents to children occurring over an area, or where concentrations of pedestrians and/or cyclists exist or are anticipated. They can help to protect children walking and cycling to and from school, and may encourage other children to walk or cycle.

With new road layouts, where suitable features can be included in the design, the preference should be for 20 mph zones. Design Bulletin 32 and the companion guide "Places, Streets & Movement" provide further advice.
http://www.dft.gov.uk/pgr/roads/tpm/tal/trafficmanagement/20mphspeedlimitsandzones?page=1#a1000