Default Auto reject calls when using map navigation

I'm using stock unrooted, what us the best way to stop people almost killing me when I'm driving and following maps (other than buying a separate sat navigation device)? I'm thinking tasker which I haven't yet figured out how to use but surely as this must be a common annoyance there must be a simpler way ?

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My right hand took a break long enough to write you this message, and you should be grateful for that.