The 10 and 10 Rule
When conditions are getting worse, drive 10 less miles per hour than you normally would and drive 10 MORE yards behind a car than you normally would. It might sound like a pain, but don't try to be macho in these situations. Rear-ending somebody will be a much bigger pain than getting to your destination 10-15 minutes later. Basically, expect everybody else to suck at driving in the winter.