I only put these blocks in when I'm going to plow. I never drive very fast while out in the snow or while plowing. maybe i'm taking a chance, but the way I built it, and the additional height of the utility body bed walls makes it safer than a normal pickup bed. I've been in one accident when I wasn't plowing or even had weight in the back, just normal construction debris, I was coming down a hill that had a dusting on it, but ice underneath(that I didn't know about because only the steepest part of the hill had ice on it.) and it was around a sharp corner. I had 2x4s and some other loose stuff in the back, as i came around the turn, my front wheels lost traction and at about 25 mph I skidded off the road and into a very deep ditch. the ditch was perpendicular to the road, so I hit a wall of dirt. nothing came out of the truck, it slid forward, but nothing came off, and the ladders on top never moved at all.
i'm not saying that it can't happen, but I don't drive around all winter with these blocks in my truck. I have no big garage or hoist to lift one solid weight into the back, nor do I have a flat bed truck.