Speed and angling.
We angle our blades to the left. Our head mechanic explained that if you were to get into a head on collision with a car a lot less damage would occur at that angle. The plow is low and close to the truck, it would push the car out of the way.
If it were angled right, the plow blade is a foot or two off the ground and would rip a car to shreds, if the plow hit it. Our trucks (F450 PSD 2x4 duallies w/ 8.5+ Diamond) weigh 9500+lbs. empty. With a pallet or two of bagged ice melt, you are talking some serious damage.
And a story about truck speed.
Last year i was on I-95 in the right lane with the plow on, headed back from the mechanics shop. I was going about 55-60, no problem. Out of the corner of my eye, I notice another landscape company's truck, chevy 3500 crew cab duallie w/ a 8-10' plow and a 1 yarder in the back, barreling down the road in the far left lane. I noticed he was blowing out heavy black smoke (prob. the tranny.). So I'm driving along and a few minutes later see the guy parked on the side of the road, with the hood up. Guess he burned out the transmission doing 70+ with a completely loaded truck.
Some people have no clue.