You're not clearing the ice off of your roof for you, your cleaning it off for the car behind you. 

It's very disconcerting to have ice fly off the roof of a car traveling in front of you and hit your car following a storm. I've had this happen before and it will wake your butt up, as well as generate a little anger at the car whose driver was way too lazy to clean off a little ice.

Keep an eye on the left lane of this video from Friday near Washington DC. That's a hefty dollop of ice, and when it decides to all come off at once, the result is a busted windshield, and I also think it takes out the dash cam.

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