Speed limits are introduced to ensure greater road safety.
Measures for influencing the speed of vehicles generally fall into two categorie; legislative and physical. Speed limits fall into the first category whereas traffic calming devices would fall into the second. Comprehensive information on the speed limits you would expect to come across on the different category of road is given in chart form in The Highway Code (External link).
Requests for speed limits to be lowered, raised or extended will be considered by the area traffic engineer, and as appopriate, by other parties such as the emergency services. Account will be taken of the characteristics of the road, such as its alignment, the level of activity alongside the road, the accident record and the degree of severance caused to a community by the speed of vehicles.
If it is considered that a change in the speed limit is warranted then a new Speed limit Order has to be made. This involves a statutory legal process that takes approximately 9 months to complete.