Saw this on Mythbusters, and they kept saying how dangerous it is. I don't see how doing that is any more dangerous than being behind an SUV. If anything it is less dangerous, as long as you're not too close, 40-50 feet behind.
I can see being in the big rig's blind spot off to the sides or in front of it can be dangerous, but if you're behind the big rig, the only way you'd get in trouble is if the big rig hit a wall and stop immediately. Other than that, I don't see how a 50,000 lbs truck can stop quicker than you can.