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While on our way to Bryce Canyon, coming from Las Vegas, I was following a slow car and going into a curve. I was deciding to speed up and make a pass, fortunately, this unit then screamed and alerted me for a patrol. I do not know how it did knows, since there wasn't a line of sight, but so glad I didn't have to pay for a speeding ticket and I didn't have to waste time being pulled over. Just had to share this story.