Here's my rule of thumb - if the lane is wide enough to share safely, keep to the right far enough to allow it to be shared and only move center when the situation requires it - if the lane is not wide enough to share safely, take the lane and yield to allow faster traffic to pass when you have a safe opportunity to do so.
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"Let us hope our weapons are never needed --but do not forget what the common people knew when they demanded the Bill of Rights: An armed citizenry is the first defense, the best defense, and the final defense against tyranny. If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government -- and a few outlaws. I intend to be among the outlaws" - Edward Abbey