Under what circumstances a 20 mph speed limit will be introduced on a trunk road.

There are no towns or villages through which the Scottish trunk road network passes with a permanent 20mph zone in place.
In September 2003, a ministerial announcement confirmed measures to assist roads authorities in implementing 20mph speed limits at schools. The initiative is intended to enable roads authorities to raise awareness of appropriate speeds within communities and contribute to the efforts in pedestrian casualty reduction – particularly with regard to children.
In conjunction with the police forces, local authority representatives and its operating companies, the Strategic Road Safety Unit carried out a survey of all schools adjacent to trunk roads in order to determine those locations that would benefit from mandatory reduced 20mph limits. Further working groups were then set up to investigate the particular signing needs of the higher-speed roads where drivers might need improved advance warning of the part-time speed limit. The results of these investigations were subsequently agreed by all roads authorities and have been installed at many locations across the trunk road network, with many sites already benefiting from the speed management measures in operation at times when the pupils are going to and from school.
Other temporary speed limits are put in place when works are being carried out on or near the trunk roads.
Due to their strategic importance, trunk roads should be kept free from obstruction to permit the efficient movement of goods and vehicles whilst ensuring road safety is maintained.