Observation.

People are more likely to acknowledge you with a nod, a smile, or even a friendly "Hello" as they pass if you are both walking on the street, rather than the sidewalk.

Curious.

What is it that prompts this? Is it that they feel more of a connection? That you are bonded by your rebellious need to avoid the bone-crushingly slippery ice on the sidewalks , and play chicken with on-coming traffic?