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The wreckage had not been cleared from ground zero when planners and neighbors began imagining how the devastation of Sept. 11 could be redeemed, in some way, by a new World Trade Center, one that would be fully joined to the rest of Lower Manhattan rather than standing apart in chilling isolation.

Building Blocks How the city looks and feels — and why it got that way.

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It was a chance, they believed, to correct a painful planning error of the 1960s, when a lively, jumbled gridiron of riverfront streets, anchored by a clamorous appliance district called Radio Row, was bulldozed into oblivion to create a barren, windswept plaza whose only evident purpose was to serve as a pedestal for the twin towers.

With the blank slate offered by a catastrophic attack, planners, soon joined by the mayor himself, saw a chance to re-establish a great crossroads: Fulton and Greenwich Streets, tying the second World Trade Center into New York — north, south, east and west.

Now, however, they see that vision slipping away, as security concerns trump urban planning.

The Police Department has proposed encircling the site with a fortified palisade of guard booths, vehicle barricades and sidewalk barriers. And neighbors and planners worry that the trade center will once again feel cut off from its surroundings, a place where security credentials prevail, traffic is unwelcome and every step is scrutinized, as at the New York Stock Exchange or 1 Police Plaza security zones.

The police plan calls for nine guard booths, each about 6 by 12 feet in area and 11 feet tall. Eight street intersections would be restricted by a double barricade system known as a sally port, from 30 to 160 feet long. The trade center site would also be bounded by bollards, the barrier posts that have cropped up around many important structures since 2001.

This appears to depart from the expansive vision for Lower Manhattan offered by Mayor Michael R. Bloomberg in December 2002. “We can examine innovative ways to manage streets and traffic downtown, reinforcing the feeling that this is one place,” he said at the time. “Getting around easily means community, and that’s what we’re trying to create.”

Brendan J. Sexton, former president of the Municipal Art Society and former director of the city’s Office of Operations, said this month that the Police Department’s security plan “appears to bend every effort to undo this connectivity, the fluidity and connectedness the neighborhood hoped it was getting back.”

And Alexander Garvin, who was the vice president of the Lower Manhattan Development Corporation for planning, design and development from 2002 to 2003, said the planned security measures made “a complete mockery of the public participation process of 2002-2003, the design competition, and the mayor’s speech of December 2002.”

But the Police Department argued that the measures allow for plenty of public access. “The campus security plan will not isolate the World Trade Center from the Lower Manhattan community,” said Richard C. Daddario, the deputy police commissioner for counterterrorism. In fact, he said, it would make the site a more desirable for pedestrians and bicyclists, who will be able to pass freely. That, in turn, might knit it more seamlessly into the fabric of downtown.

The security measures are described in a draft environmental impact statement that is open for public comment until Wednesday.

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A key goal of the early planning of the trade center redevelopment was to recreate parts of two important thoroughfares, the east-west Fulton Street and the north-south Greenwich Street, that had been eliminated by the trade center superblock.

“Greenwich Street is not only symbolically important,” Amanda M. Burden, the chairwoman of the city’s Planning Commission, said in April 2002, when the first reclaimed segment was established. “It’s about beginning to interconnect the totally disconnected elements of Lower Manhattan.” And Mr. Bloomberg said eight months later that “re-establishing Fulton Street through the World Trade Center site would make it a thoroughfare that stretches from river to river.”

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Those streets have been recreated and will once again crisscross as they did 50 years ago. But under the police plan, they would be closed to through traffic at the trade center.

Vesey and Liberty Streets would also be closed to through traffic at the north and south ends of the site, as would one lane of Church Street, the eastern boundary. Only vehicles with business at the trade center would be permitted. They would undergo credential checks and inspection. Vehicles and drivers enrolled in a trusted-access program could go through faster screening.

Installation of the security measures would occur as construction progresses on the site, which may extend until 2019. The Police Department expects to share the estimated $40 million cost with the Port Authority of New York and New Jersey, which owns the site.

Mr. Sexton recalled that what had come through most clearly in the planning process was the community’s desire to restore Fulton and Greenwich Streets. “We can, with one change in municipal behavior, defeat all that design and engineering and urban planning, and vacate all that civic good will and good work,” he said in a May 1 letter to Mr. Daddario.

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A year ago, leaders of Community Board 1 in Lower Manhattan told police officials of their worries. “We are very concerned that the implementation of this plan will adversely affect our growing residential and business community,” Michael Levine, the director of planning and land use for the board, said at the time. “We see ourselves surrounded by a ‘fortresslike’ environment with the creation of a security perimeter around the W.T.C. site.”

This week, Mr. Levine said, “We knew our statements were falling on deaf ears.”

Julie Menin, the former chairwoman of the community board, who is now running for Manhattan borough president, said the community “cannot be in complete lockdown.”

Restrictions in zones that the police have established around the Stock Exchange and Police Headquarters have strangled some small businesses, she said.

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Mr. Daddario disagreed with such assessments.

“Pedestrians and bicyclists will be able to enter the site and travel along its streets and sidewalks just as they can everywhere else in the city,” he said in an e-mail. “Vehicles and tour buses having business at the site will have access after screening to guard against the threat of a car bomb. The booths at entry and exit points are approximately the size of existing newsstands you see about the city.”

In fact, the booths may be designed to complement the newsstands, and bus shelters installed by Cemusa under a franchise from the city, thereby making them less conspicuous.

“The argument that these measures will disconnect the site from the rest of Lower Manhattan ignores the fact that people largely experience the city on foot and on bikes,” Mr. Daddario said. “The measure of connectiveness should not be the volume of vehicle traffic passing through the site.”

There is a school of thought that pedestrian-only zones can seem lifeless, but some planners believe that the traffic restrictions will have a positive effect on the public’s experience.

“It makes the entire World Trade Center more of a pedestrian precinct than it had been in the past,” said Jeff Zupan, a senior fellow of the Regional Plan Association.

Mr. Zupan did not say that guard booths, sally ports and bollard lines were ideal. But he added: “If you accept the fact that vehicles have to be checked, that’s unavoidable. I’m not quite sure how to overcome that. It might be the byproduct of the fact that the World Trade Center was a place where 3,000 people died.”