One reason I wanted to reprint your letter is because it underlines the dangers we all face on our highways, and our need to be careful drivers and protect ourselves (and others) in every way we can - including wearing seat belts, even for short trips. The Bible says, "Wise planning will watch over you. Understanding will keep you safe" (Proverbs 2:11, NLT).
In all honesty, your question isn't easy to answer; we don't know why God allows things like this to happen. But the Bible makes it clear that God doesn't cause the evils we see around us; we must not blame Him when tragedies strike or terrible accidents happen. When God created the world, the Bible says, He pronounced it "very good" (Genesis 1:31). But later evil entered the world, and like a deadly virus it has infected everything.
The most important thing I can tell you, however, is that God loves us, and even when life brings tragedy into our lives, we can turn to Him for comfort and hope. Remember: He knows what tragedy is, for His Son died on the cross for us.
Pray that God will use this tragedy to turn people in your community to Himself - especially those who knew these teenagers. And if you have never faced your own need of Christ, open your heart to Him and thank Him that you are in His hands forever.