Of course bicycle is best to ride on a solid flat surface, but it is not so easy to find. Dry asphalt or compacted snow is best option in winter. If you have bike tires with metal studs, you can feel confident even on ice. But it is more for extreme lovers. You can ride bike on snow very comfortably only if there is no too much snow. In this case the wheels reach the ground under the weight of the cyclist, and tires get good traction with the surface. Although it's a bit more difficult to ride. While biking on deeper snow wheels don't reach the surface, and bike start slipping on the uphills.
The roads with traffic is bad choice in winter. Even when cleaned the roadway is more narrow in winter. And the bike line is often covered with snow, what makes bicycles stall. Of course, you can go to the very edge of pure asphalt, but you can disturb the car traffic and it is not safe.
When driving on the roads be especially careful near bus stops, and traffic lights. Frequent braking can form very slippery ice areas not always visible for cyclists because of snow. Imagine falling from bike five meters in front of a moving bus. In this situation helmet won't save. Trails and paths in parks are the best places for winter riding. These narrow, curving paths is a great place to sharpen driving skills.
In any case try to avoid heavy braking and sharp turns at high speed, unless of course you are executing some tricks. Falling on soft snow is much more comfortable than falling on the hard ground, that is why winter is loved by extreme bike riders. In general in winter your morale and riding desire much more important then weather conditions and your equipment.