Snow on the stair way

You know that is a stair way and you can notice that the middle path of the ladder is free of snow.
How do you think of it?
Here may be some variants of reasons - not having snow in the middle path.
1. Because of warming up system to melt snow and avoid slipping.
2. Because of that fact that so many people are steping up and down so many times in that day.

I also don't know why the snow are located on the ridges of the ladder.

Anyway I like the style of stair way .. going in twisted form.

Photo was taken in somewhere in Hokkaido.