Declaring that the city is ready for what may be a historic blizzard, Mayor Thomas M. Menino today canceled school in Boston for Friday and urged people to stay off roads and out of downtown.

“We haven’t had a snowstorm of this size in many years,” Menino said at a press conference at City Hall. “We are hearty New Englanders used to these storms, but we want people to use common sense and stay off the streets.”