According to Fremont Motors, driving with loads of snow atop your vehicle is not only dangerous, it may be breaking the law.
In states including Tennessee, drivers can be charged with reckless driving for not clearing ice or snow off a vehicle before driving. This is in a state that doesn't even get much snow, compared to the Midwest and virtually all northern regions of the United States.
In Wyoming, the law doesn't specifically mention penalties for not clearing off snow or ice, but it does state that no one shall drive a motor vehicle with obstructions or material that impairs vision through any of the windows.
Driving with poor visibility due to snow- or ice-covered windows is hazardous to everyone in the vehicle and potentially other vehicles and pedestrians.
It's not just about clearing the windows, though. Piles of snow won't stay atop the car once the vehicle is in motion. That snow flies off and can be a hazard to vehicles behind yours. Snow-covered headlights, turning lights and brake lights are also not as visible as they should be, so other drivers may not be aware of your intent. Covered headlights mean less road visibility while driving.
A foam-topped snow broom can remove loads of snow in a hurry, thanks to a long handle. The foam-topped models also help prevent vehicle scratches. If using a regular broom or other device, leave a thin layer of snow to protect the vehicle's finish.