Ideas & Trends; Whose Sidewalk Is It, Anyway?

ASK an urban planner to define the cradle of civilization, and thoughts drift to that innocuous but vital ribbon of concrete, the sidewalk. ''Lowly, unpurposeful and random as they appear, sidewalk contacts are the small change from which a city's wealth of public life may grow,'' Jane Jacobs wrote in her classic ''The Death and Life of Great American Cities.''

So perhaps it was not surprising that earlier this winter, the city of San Francisco became the first municipality in the country to ban the Segway -- the $4,950 self-balancing scooter that resembles a balletic wheeled pogo stick -- from the city's sidewalks.

California was one of 33 states, along with the District of Columbia, to pass legislation allowing Segways on sidewalks, classifying them as ''electric personal assistive mobility devices'' rather than motor vehicles. But the law allowed municipalities to adopt their own restrictions, and senior citizen and pedestrian advocacy groups in San Francisco insisted that a 12.5 mile-per-hour jaunt on a Segway was a threat to public safety. (Segways would be allowed on streets, however.)

The Segway ban highlights the increasingly ferocious competition overtaking the American sidewalk. Should the device ever become a fixture on the American scene, it would join vendors, rollerbladers, strollers, non-motorized scooters, newspaper boxes, sidewalk cafes, fruit stands, bus shelters and benches -- all jockeying for space on sidewalks.

Although the sidewalk has seen a resurgence in recent years in New Urbanist communities like Seaside, Fla., it remains a somewhat embattled entity. Only 10 percent of American children walk to school, for instance -- down from 69 percent in 1969, according to Peds, a pedestrian advocacy organization in Atlanta.

Ever mindful of traffic flow, engineers have reduced the time allotted to cross the street, with the ''Don't Walk'' light blinking ''10 steps out from the curb,'' said Fred Kent, the president of Project for Public Spaces, a nonprofit organization in Manhattan.

''The public realm is being dramatically infringed upon,'' he added, speaking of the Segway, skateboards and other rolling intruders.

Hallowed incubators of social interaction, sidewalks are currently the focus of various legal disputes across the country. In Sacramento last year, disability rights advocates successfully argued in court that sidewalks are essential to being part of mainstream society, and that Sacramento had a legal responsibility to move obstacles like utility poles and bus benches from its 2,200 miles of sidewalks. (The city has petitioned the United States Supreme Court to review the decision.)

Some sidewalks in Las Vegas and Salt Lake City have been privatized, although in both cases, courts have ruled that sidewalks are places for public assembly and expression, even if privately owned.

The sidewalk has always been a hotbed of competing interests, said Michael Southworth, a professor of city and regional planning and landscape architecture at the University of California at Berkeley. In the medieval city, he notes, the street was not only a place for transport but was also the sewage system. ''Waste was literally thrown out windows, pedestrians beware,'' he said. ''They had oxen and chariots to contend with, not Segways.''

In our own era, there are signs that the sidewalk is finally emerging from its suburban cul-de-sac. Most cities are now requiring sidewalks in new subdivisions of more than 100 units, at least on one side of the street, said Marya Morris, senior research associate with the American Planning Association in Chicago. And growing public support for biking and walking has resulted in an increase in federal spending for bike paths, sidewalk repair and street trees, among other things.

In areas with no sidewalks, beaten-down paths in the grass, known as ''desire lines'' in planning-speak, indicate yearning, said John La Plante, the chief traffic engineer for T. Y. Lin International, an engineering firm. ''When sidewalks are provided, people do walk,'' he said.

Sidewalks are being widened in many places, especially at intersections, where curb extensions allow more people to congregate on corners.

The most notable widening may be in Times Square, where New Year's Eve throngs reveled amid bollards marking the boundaries of new plus-sized sidewalks, the result of foot traffic that on one block -- the east side of Seventh Avenue from 45th to 46th Street -- increased from 40,000 people on a typical weekend day in 1995 to 86,000 in 2002.

The most novel new sidewalk treatment may be in Seattle, where traditional curb and gutter sidewalks have recently been joined by experimental ''sea streets,'' in which swales or ponds with water-absorbing soil are placed between street and sidewalk to reduce run-off and aid salmon recovery.

PEDESTRIAN advocates often say that driving, unlike walking, does not result in ''positive casual encounters.'' But positive is not always the operative word. When Elizabeth Byrne, head of the environmental design library at the University of California at Berkeley, walks home from work, a distance of two miles, she dodges bus benches, tabloid vending machines, wastebaskets, fire hydrants, real estate sandwich boards and double-wide baby strollers with dogs attached. ''Half the time I end up walking in the street,'' she said, sounding exhausted.

''It's a miracle anybody gets home.''

Photo: In Atlanta, a sidewalk Segway. (Associated Press)