For centuries, pedestrians had undifferentiated dominion over both the sidewalks and the roadbed - sidewalks were not pedestrian cattle pens, but off-limits zones for vehicles. "The street" meant the entire open area, from building line to building line.

This changed in the 1880s with the advent of electric and cable streetcars, with their much greater weights and speeds than horse-drawn vehicles, not to mention their guillotine-like wheels. It is a comment on how we viewed our streets that, by design, passengers were meant to board streetcars in the middle of the roadway.

But the installation of trolley tracks also created a kill zone. In 1894 The New York Times reported that a speeding streetcar on Prospect Avenue in Brooklyn had killed 10-year-old Theodore Cox, "a bright little boy" who was "ground under the wheels." The police helped the motorman escape the crowd, which was crying "Lynch him!" This was not to be a bloodless war.

The pedestrian's domain was further challenged by the decade-long bicycle fad of the 1890s. In 1893, when The Times described upper Broadway as "a regular race track" for speeding cyclists, one of them, operating without bell or whistle, killed 7-year-old Katie McGlynn just as she was exiting a streetcar at Broadway and 67th. "They make no noise and go by you with a rush," said Police Capt. Elbert Smith. "You shout at them to slow down, but they are off before you know it." But the fashion for bicycles soon waned.

When cars entered the fray, the fact that their owners were usually rich added an element of class war. After Charles Gates, a broker, was caught in 1906 driving 25 miles per hour in an 8-mile-an-hour zone in the Bronx, The Times carefully noted that he appeared in court wearing "a blue serge suit and light-colored spats." He expected to get off, but Magistrate Leroy Crane told him, "You are a Wall Street man with millions, and you think that you can do what you please."

It took a while for cars to catch on. Dr. Albert Lamb, whom I interviewed years ago, said that around 1920, when a neighbor fell ill in front of his house on East 70th Street, "my father, also a doctor, just had him lie down in the street."

But there was no room for roadway medicine once automobiles increased in weight, speed and numbers.

Over time, without express agreement or even acknowledgment, the streets gradually became off-limits to the unwheeled. The pecking order was further clarified in 1950, when permanent street parking, which had long been forbidden, was allowed - you couldn't walk in the street, but car storage was fine. Again the map was redrawn.

The increase in the numbers of bikes in recent years has put new pressures on the real estate of the street/sidewalk. The cyclist's natural vulnerability in auto traffic has changed the stakes; you can say "share the road" all you want, but the mix is irrevocably divergent.

It is difficult to walk 20 blocks in Manhattan without encountering one or two cyclists who prefer dodging pedestrians to dodging trucks - often doing so, in my experience, with a serious attitude of entitlement. The sidewalk, the last redoubt of pedestrian safety, has been breached.

So what does this have to do with the New York streetscape? The retreat to what is left of the sidewalks changes the very essence of the common public realm, just as certainly as if, say, tourists had to stay within the arcades surrounding St. Mark's Square in Venice, or look out on Red Square from the porch on St. Basil's. New York's gridiron allows precious few vistas or plazas, but a citizen could at one time have viewed each block as an entirety, with walls and a floor. Now everyone must hug the baseboards.

There are only a few places where one can recapture the old relationship of the buildings to the full width of the street. One is the annoying street fair. If you can survive the kielbasa smoke, you get the old, wider idea of the street, as easy and relaxed as the Piazza Navona in Rome. Streets closed for school recess do the same thing, as does the occasional oil truck blocking traffic - at those times, in those places, New York is a different city. And there are some streets entirely closed to vehicles, like several blocks of Broadway above 42nd Street, a change that has infuriated cabdrivers.

It is difficult to weigh the competing claims of the combatants in the current conflict - cars and trucks fully dominate the character of the city; bicyclists have the roadway and an increasing number of bike lanes; and pedestrians dodge traffic and hedge red lights with astonishing sang froid.

But as the domain of the pedestrian - the everyman of the city - is gradually curtailed, so too is the sense of the city as a democracy of public space, open to all.