Snow

Frozen water is more slippery than liquid water.  And ...

1. You can't see road edges
2. Ice may have formed underneath
3. Signs and markings can become invisible
4. People don't clear all their windows or lights
5. Vehicles get abandoned, blocking your route
6. Pedestrians may stumble, not hear you, or throw stuff at you
   
7. Road-crossers may have to jump over slushy puddles at the sides
   
8. Your gaze may get drawn upward, toward falling flakes, in the dark
   

Consider if you really need to go out.  Take food, extra clothes, a shovel, gloves, a source of heat, a torch, boots.