I think this illustrates the problem with drivers generally. Everyone overestimates their ability.But there are plenty of 30 limits that really don't need to be 20, and I don't see the benefit of a blanket change.
Speed limits are introduced if there are accidents. There is not somebody that just decides there should be a blanket application. There are accidents.
My feeling is it should be very, very easy to lose your driving licence for operating a car dangerously. People are really far too blase. They drive while looking at Facebook. They drive while angry, and lose their tempers at people who they percieve are holding them up. If someone has so little control of themselves, how could you expect them to have control over a vehicle that will probably kill anyone it touches?