I race circle-track race cars, and will never race without a HANS. I'm thankful that I made my HANS purchase before my hard wreck, rather than after. I learned exactly how valuable a HANS device is almost a year ago..
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Coming to the green flag, a guy a few cars ahead of me had ran into the back of the guy in front of him, which resulted in a hole though the guy's radiator. He was putting water down on the track from the exit of turn 2, all the way down through turn 3 & 4. On the start, I had fallen back about a half-car, to almost a full car length. My car was awesome entrance to mid-corner, so I knew that "stuffing" it into the corner wouldn't be a problem, and I'd be right back on the guy's bumper. Unfortunately, slick tires don't work well on water, and at my corner entrance & braking points - the car didn't turn. I ended up locking the wheels and skidding straight into the tire that protects the pit entrance. We're guessing I got it slowed to about 80-85mph at the point of impact. I deflected straight up into the air. The tire itself (from what I have been told from the photographer that took the picture) was about twice as high in the air at one point, and that she caught the image on both the tire and the car's return to earth. I actually ended up landing on top of the tire...
Regardless, I FELT the Hans lock up. I walked away fine... and was fine until I tried to lift some heavy items the next day. Even then, it was only minor pain - so i just kept from lifting heavy items for a few days.
While my wreck isn't anywhere near as bad as Jay's, I just want to help re-itterate the importance of a Head & Neck restraint. Your neck is built to support YOUR HEAD. When you tie up the chin-strap of your brain bucket, you add extra weight that your neck needs to support. With that added weight, you really need something to help support your head & neck in a wreck.
I've never tried the Hans in a Viper, as my dad doesn't track his, nor does he have a 5-point harness in it - but from my other racing experience, you've heard my story.