It should scare you, because every year in our country tens of thousands of people lose their lives through automobile accidents, and hundreds of thousands more are seriously injured or have their lives turned upside down.
I don't know why some people become different behind the steering wheel. Perhaps it has to do with their impatience; perhaps it's related somehow to a feeling of power they get when they're driving. And some people, I've noticed, have a streak of anger in them that they've never conquered, and it comes to the fore when they're driving. The Bible says, "Better a patient man than a warrior, a man who controls his temper than one who takes a city" (Proverbs 16:32).
Whatever the reason, such behavior is wrong and also dangerous. Some of the saddest letters I have ever received came from people whose reckless driving led to someone's death or serious injury. Pray for your cousin, and ask God also to give you an opportunity to share your concern with him. If he is a Christian, he has a special responsibility to do what's right.
Then ask God to help you be the best driver you can be. You can't always control what others do, but you can be alert and obedient to the rules of the road. Jesus' "Golden Rule" applies to everything including our driving: "In everything, do to others what you would have them do to you" (Matthew 7:12).