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Why do some cyclists think they have a right to ignore traffic laws as it suits them and why do the police and local authorites do nothing about it ?"

The same reason as motorists do it to the extent that when challenged by another road user they pull a baseball bat from the boot and go to town.

Don't keep parroting these stupid questions. Cyclists are doing nothing motorists don't do in greater numbers, at higher speeds and with more death and maiming resulting. Furthermore, motorists evade paying tax and insurance (which are not required of cyclists - whether it should be is another matter - I do have cycle 3rd party insurance), they steal petrol from filling stations, park all over the pavements, use their vehicle for robberies, assaults and disposal of bodies - but we're getting silly here aren't we, we don't want to attribute the faults of a few to the many - oh, but that's just what you are doing!