Drivers read the road and respond to the look and the feel of the place.
If changing the traffic signs from 30mph to 20mph does not reduce speeds sufficiently, the police are likely to insist on self-enforcement in the form of traffic calming.
Traffic calming need not necessarily mean conventional road humps and pavement build-outs into the road that may be seen by drivers as a challenge to overcome and by local residents as quite ugly and alien to the character of a place.
Bearing in mind that drivers read the whole road and adjust the way they drive to what they feel is safe, an alternative approach to the design of traffic calming is to use landscape to change the nature of the road so that drivers feel they want to slow and take more care.
For example at the edge of a 20mph area the road is subtly reduced in width, the centre of road white line is removed and paths are rerouted away from the road at the position of new planters, village name sign and hedges. They mark the point along a road where drivers clearly understand that they should reduce speed.