In some places it is illegal to drive around with snow on the car roof or other parts of the vehicle.
The reason that driving around with snow on your vehicles roof is illegal is that the snow could fly off your roof when going down the road and fly onto someone else's windshield and cause an accident.
So make sure to remove snow off your cars roof and other parts before you drive off too far especially if you're planning on driving down the highway.
Even if it's not illegal to drive with snow on your car roof it's still not safe for other drivers behind you.
So I always remove snow from my car or trucks roof before I drive off.
It's not illegal here to drive with snow on the car roof but I clean the snow off to prevent an accident anyway.