Agreed, no issues with 30's becoming 20 where required but as Ekona says its important they don't just make well sited roads 50's or even 40's.
Lots of national limit roads near me have recently become 40's for what appears to be no reason.
I think it's about time the government looked at motorway speed limits being variable and made to match the driving conditions. Slower limits if its raining, busy etc and higher limits when the motorway is quiet and fine weather.