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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.
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.