.... is (most often) a bit of common sense.
Through towns or villages I won't pretend that I never exceed 50 kph but sometimes I am down at 30 or less, depending on what is going on. Entry and exits to the built up area... take care and watch the oncoming vehicles, who will often give you the 'warning flash'. If they are going unnaturally slowly, it's often for a reason.
Other than that, if you see the brake lights of a vehicle in front come on
for no immediately apparent reason it's worthwhile thinking very carefully about slowing down yourself, using in effect the bod in front's eyes to work for you. Who knows what is going on where you cannot see? Any clue of something possibly untoward up ahead is a good one, it may be a jackknifed lorry

or 'just' plod with their hair-dryer.