Listening to your surroundings while riding a bike is a helpful safety tool (which is why you shouldn't wear headphones). However, winter weather muffles sound as snow banks and freshly fallen snow absorbs noise and can reduce the hum of approaching cars. Even more, anything you're wearing to keep your ears warm are also silencing your auditory cues that keep you safe.
For those reasons, it's necessary to be actively looking around you before making any decisions about turning, merging, etc. While you might be wearing more clothing that inhibits your ability to physically turn your head, it's still incredible important to do so in order to check for hazards.
This tip come from Mr. Frugalwood's Ultimate Bike Commuter's Guide to Winter Cycling.