Section 26-9(a) of the city ordinances reads:
No person shall block, obstruct or otherwise hinder or impair pedestrian or vehicular traffic on the public ways of the city by placing snow or ice or permitting or causing snow or ice to be placed upon a street, sidewalk or bridge, except that snow or ice removed from a sidewalk may be piled in the adjoining gutter or on the loam border between the sidewalk and the street. This section shall not apply to municipal snow removal operations.
If you come across a residence where the driveway is cleared in such a manner as to "place" snow or ice on a sidewalk, please take a picture (cellphone cameras are fine). Take a picture of the house, too. Send pictures and the offending address to firstname.lastname@example.org and I'll post them.