The briefing focused mainly on flood prevention efforts, but reporters asked a couple ice questions at the end.

Wu said property owners are responsible for clearing sidewalks in front of and on their property.

"I know it can be a real challenge to remember to do this in the midst of all the other responsibilities," Wu said. "But it is very important for the collective safety of everyone that we remember sidewalks need to be cleared within three hours after the end of a snowfall event."

In addition, Wu said, recent temperature fluctuations can "have long-lasting impacts throughout the next several days until the temperatures rise enough for patches of ice to melt."

The icy patches on city blocks are causing falls and sending an increasing number of the walking wounded to hospital emergency rooms with fractured ankles, broken arms, and damaged ribs.

At Tufts Medical Center, the emergency room since midnight Sunday has treated about three times the typical daily number of people for falls, said Dr. Matthew Mostofi, the hospital's associate chief of emergency medicine. Since the beginning of the week, the hospital had treated 36 people for falls, including 12 fractures, most of which were directly related to ice, Mostofi said Wednesday.

Normally, the hospital sees three or four patients a day who have fallen, Mostofi said. But since bitter cold descended on the region this week, the litany of injuries in the emergency room has "run the gamut," Mostofi said, including broken ankles, pelvises, and legs, lower-back compression, and a collapsed lung.

"The city and public entities are responsible for clearing the roadways, and property owners are responsible for clearing the sidewalks in front of and on their property," Wu said Thursday. "We need to do a better job of this collectively."

She when the city's the property owner, "we work really hard and hold ourselves accountable to that, whether it's the plaza in front of City Hall or pathways through the Boston Common, Public Garden, and other public parks."

However, the mayor continued, "we have seen many instances where the storm hits, and then there is a freeze. The temperatures drop for the next several days, so that whatever ends up not being cleared right away turns into a very dangerous, slippery patch of ice."