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Because it is a 3 lane carriage with excellent visibility and I seen a lot more dodgy roads that allow you to go at national speed limit.
And the people who set the speed limit set it for the best possible conditions on that road. They had access to a shitload more information than you, so why do you think anyone should accept your judgement over theirs. On what basis do you claim the right to ignore their decision? Where is it written that everyone has to obey the law, for the good of society, except if they disagree? If you think it's arbitrary, campaign to get it changed.
Not wanting to get involved in the wider debate here, but what makes you think that a particular speed limit is arbitrary? Were you party to the decision making process? Do you have access to the report created to decide the speed limit on this particular road? Or did you just happen to read something somewhere (probably on the internet) from another equally uninformed individual who said that councils just pick numbers out of a hat?
Multiple lane carriageways heading into town and city centres often have lower speed limits than motorways; there could be any number of reasons why...increased likelihood of congestion, noise levels, road feeds into smaller roads with lower speed limits....All reasons far more likely than simply to piss off drivers.