America's most pervasive urban designer is Fear.

Fear-fully designed cities, however, are fearful. There is no place to sit on streets or plazas, for fear that bums or hippies might become comfortable. There are no benches in the parks, for fear that drunks might sleep on them. There are no vendors, for fear they might compete with established stores. There are no street performers, for fear they might become an attractive nuisance. There are no hot dog or fruit stands, for fear of litter.

Years ago, when Bassin's on 14th Street and Pennsylvania Avenue applied for Washington's first sidewalk cafe license, the Daughters of the American Revolution said they feared that outdoor eating and drinking would encourage prostitution.

Chicago's grand old Buckingham Fountain is surrounded by an electrified fence for fear kids might splash in the water.

In Baltimore, federal judges tried to stop the erection of a sculpture in front of their new courthouse for fear assassins might hide behind it. Don't laugh. That's what the federal judges said.

In recent years, to be sure, a growing number of Americans have come to like ledges you can sit on, park benches, street vendors and artists, sidewalk cafes, fountains and public sculpture. We argue that public spaces, art and amenities attract people downtown and that the more "eyes on the street," as Jane Jacobs put it, the safer the streets become.

Or is that wishful thinking?

Enough well-meant efforts at exterior decoration and designed ambitance turned out to be such dismal failures as to plant doubts in the minds of the most ardent Jane Jacobists. (Just think of Washington's miserable "Streets for People," our insipid 10th Street Mall that leads to the vacuous L'Enfant Plaza, or that pathetic half-mall on downtown F Street.)

But now we can take heart. We have evidence. It's on film (which is every bit as evident as FBI videotape), "Fear proves itself," says William (Holly) Whyte.

In a superb little book, "The Social Life of Small Urban Spaces," Whyte has summarized a decade of meticulous and ingenious research of how people use streets and plazas, what works and what doesn't.

Whyte is one of a small band of generalists who, 22 years ago, began to arrest and reverse the horrendous damage specialists were doing to our cities. Whyte was an editor of Fortune magazine at the time who with Jane Jacobs and Grady Clay, was part of a small band who published "The Exploding Metropolis."

This book first opened our eyes to the fact that the central city was beging "renewed" by architects, planners, politicians, bankers and big businessmen who did not like cities. It pointed out that the surge to suburgia was no romantic Sturm und Drang but, for the most part, a reluctant emigration forced by the absence of decent middle-income housing, affordable mortgage loans and good schools and nurseries in the city center. It showed that roughly half the suburbanites would have preferred to commute by public transportation, had there been any, and that highrise public housing in the ghettos was a disaster.

The most important message in this book was that downtowns must be lively to live, and that live they must because their death would be far too expensive, even in the 1956 dollars of Eisenhower America.

The to work with the city. Bedraggled and abused as they are, our downtowns do work. They need help, not wholesale razing...The remarkable intricacy and liveliness of downtown can never be created by the abstract logic of a few men. Downtown has had the capability of providing something for everybody only because it has been created by everybody," wrote Jane Jacobs in her contribution to Whyte's book. At the time hardly anyone listened. The idea is just now beginning to sink in.

In his study of small urban spaces, Whyte has now returned to this theme, not with a sermon but a prescription.

He does not tell us what ought to be. He doesn't present pretty but untested designs, or the computerized hypothesizing and statistical jugglery that usually passes for urban research. He does what real, as opposed to social or behavioral, scientists do: He observes. Often, he records observations with absolute accuracy and a stamina far beyond human endurance by employing time-lapse or slow-motion cameras running for as long as 48 hours.

This method often yields surprising discoveries and will surely soon become an indispensable tool of city planning.

The merchants along Cleveland's Euclid Avenue, for instance, insisted that curb parking in front of their stores was essential to their economic survival. Time-lapse cameras, mounted above the avenue, produced a film which showed cars flitting to the curb and ejecting their drivers who would then, in the jerky fashion of old flicks, rush for blocks up and down the avenue and disappear in side streets. Not one entered a store.

As to planning places with fear and distrust, Whyte's observations show that planners get what they are looking for. The fear is mostly of undesirables."

"they are not themselves much of a problem" he says. "It is the measures taken to combat them that is the problem. Many businessmen have an almost obsessive fear that if a place is attractive to people it might attract undesirable people. So it is made unattractive. There is to be no loitering, no eating, no sitting. So it is that benches are made too short to sleep on, that spikes are put in ledges; most important, many needed spaces are not provided at all, or the plans for them are scuttled.

Who are the undesirables? For most businessmen, curiously, they are not muggers, dope dealers, or truly dangerous people. They are the winos, derelicts who drink out of half-pint bottles in paperbags -- the most harmless of the city's marginal people.

For retailers, the list of the undesirables is considerably more inclusive: there are the bag women, people who act strangely in public, "hippies," teenagers, older people, street musicians, vendors of all kinds...

"The best way to handle the problems of undesirables is to make a place attractive to everyone else. The record is overwhelmingly positive on this score. With few exceptions, plazas and smaller parks in most central business districts are probably as safe a place as you can find during the times that people use them."

The only serious problem Whyte encountered in his nine years of studying plazas and small parks in New York City, was a plaza in which marijuana dealers began operating. The management took away about half of the benches, and constructed steel-bar fences on the two open sides of the plaza. This effectively cut down the number of people who used the place, to the delight of the dealers, who now had it much more to themselves and their customers.

The reverse happened on the plaza of the New York Telephone Company's building. It was used by "undesirables." The company's president, John R. Mulhearn, wanted people to enjoy the place and decided to liven it up with chairs, tables and a buffet. It was an immediate success. Employees and passersby enjoy it. Most of the "undesirables" have gone somewhere else.

Won't outdoor chairs be stolen?

The New York Metropolitan Museum of Art provides some 200 chairs alongside its front steps, which people can move in or out of the sun, to join others or to enjoy privacy. The chairs are out 24 hours a day, seven days a week. The Met figured that it is cheaper to trust people and buy replacements periodically than to have guards gather the chairs every night. And that's the way it has worked out. There is little vandalism.

There are startling findings and lessons on virtualy every page of Whyte's manual. The most starteled readers will be the architects of some of these small urban spaces. Real people usually behave differently than the pin-headed little figure on their drawings.

"The Social Life of Small Urban Spaces" and Whyte's film on the subject appear just as downtown Washington begins a surprising building boom.

It should be mandatory for every downtown developer to buy and pass a test on this book before he gets his building permit. (Copies are available for 10.50 including postage and handling from the publications dept., Conservation Foundation, 1717 Mass. Ave., N.W., Washington, D.C. 20036.)