I too wish that drivers would clear the snowdrifts off the roof of their cars, its only going to do 2 things, 1. slide over the windscreen the first time you brake hard, or 2. blow off onto the car behind, either way not nice for the driver that suddenly cant see.
for £0.99 you can get a cheap dustpan and brush, which can help you clear the snow off your car in a couple of minutes (brush it off or shovel it off in the pan- soft enough to not damage your car)
Grendel