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Speaking as an old beat up insurance broker, I can tell you that it is your responsibility to yield to oncoming traffic if you are turning left. Regardless of the oncoming vehicles speed. You have a clear view of oncoming traffic, and in 25 yrs in the business never knew anyone turning left that was not deemed responsible for the accident. Some states with contributable neglagence laws would assign a certain percentage of blame(liability) on a speeder, but never 50% or more.