Once the snowfall is more than a few inches, or if it's particularly wet and slippery, running becomes impossible. If there's ice, that's another warning sign you should stay off the roads.
Instead, you have two options: run inside on a treadmill, or hope that your local government plows enough of the road for you to safely get in your run outside. Be careful of narrower roads and traffic, which may pose some safety risks. Wear a reflector vest or very bright clothing if you're running in the early morning or dusk hours.
Winter running necessitates a "make the best of it" attitude. Freezing temperatures, snow and ice don't provide an ideal training environment, especially if you need to run fast 5K workouts or intervals for other short races. But by modifying your runs slightly, you can still run the majority of your workouts outside or on the treadmill.
Remember, safety is your first priority when running outside in the snow and cold. Winter conditions sometimes make injuries more common—by straining a muscle on snow or falling—and sidewalks sometimes aren't cleared for running, forcing you onto the roads. Prioritize safety by carrying ID, leaving the MP3 player at home, and staying vigilant for cars.race.