I've seen 4-5 accidents this year already in NASCAR that may have seriously injured (if not worse) the driver 10-15 years ago. The latest being Brian Scott's head-first shot at Vegas this past weekend. All have walked away without a scratch.
Fences likely can be made safer. But their job is to be strong enough to protect spectators FIRST. Can't compromise that. And AOW that get airborn and hit at the wrong angle doesn't really matter what kind of fencing is used. Its going to be real bad.