Speed Limits - Bringing The Law Into Disrepute.

There is a road close to my home where a 20 mph speed limit has been imposed.
There are green fields to one side and the houses on the other are well back from the road.
The road itself is broad and just looking at it, you would be hard pressed to guess whether the limit should be 30 or 40 mph.
The side roads are all at 20 mph - which is sensible.
Why has no serious thought been put into this?
As a former magistrate and one who respects the law immensely - I freely confess that I usually ignore this limit as it is so absurd.
Silly laws invite people to break them. I am not sure whether I have a conscience on this matter or not.