In what cases are polygraphs used?

One legit angle might be cases involving memory loss from automobile or other accidents which may have taken human life- so-called Vehicular homicides. they usually call in Pyschiatrists and there is always the possibility the person being tested may be faking amnesia ( memory blot outs or loss). Another possibility is the test subject may be in fear for their own life ( from accident) and will tell the police anything they want to hear- to get them off their back- nerves, or whatever. there are balancing acts between Police fact-finding and medical compassion. if the test subject is drunk- well you know Sarge, I was coming down the Boulevard and I saw this huge Pink Dinosaur and I put on the brakes, but... oh boy.