''Either we live normal lives with normal streets and normal sidewalks, or we go back to living in fortresses as they did in the Middle Ages,'' he said in an interview.
Many prominent buildings are quite close to streets. These include Grand Central Terminal and the Empire State Building and structures under construction like the Bank of America Tower at 1 Bryant Park and the future glass-clad headquarters of The New York Times across Eighth Avenue from the Port Authority Bus Terminal. This transportation hub has for months been undergoing a security upgrade. Bare steel support columns in upstairs parking areas have been encased in reinforced concrete to enhance their fire and blast resistance.
Several experts question the location of the Freedom Tower near a state highway, three through streets and the headquarters of at least two high-profile companies, American Express and Verizon.
''The best thing from a security standpoint would be to move the Freedom Tower to Church Street,'' said Guy J. Nordenson, a structural engineer and a professor of architecture at Princeton University who is currently involved in several projects at the trade center site. ''You could control truck traffic access better on Church Street than on West Street, and you would be able to achieve setbacks more easily on that site.''
Trucks are not the only threat. ''Large passenger vehicles like S.U.V.'s are able to carry more explosives unobtrusively and are a major hazard,'' Dr. Hinman said. She said the current estimate of how much a big S.U.V. can carry is the equivalent of the charge used in the Oklahoma City bombing.
To make underground parking safe, all vehicles entering the subterranean roadways would be subject to inspection at the entrances off Liberty and Vesey Streets, said Stefan Pryor, president of the Lower Manhattan Development Corporation.
Security experts underscore that terrorists' intentions cannot be predicted, and they recommend the installation of specially designed air intakes and ventilation systems in buildings at the site to counter radiological, chemical and biological threats.
Also undergoing scrutiny is the $2 billion PATH station and transportation hub by the architect Santiago Calatrava, which poses many engineering challenges, including the routing of the PATH tracks directly under the Freedom Tower site, meaning that the tower's supporting columns must be threaded among the rails.
Mr. Calatrava said in an interview that he could deliver ''a beautiful building that accomplishes all the needs of security,'' adding that ''95 percent of people in the station will be in lower levels,'' and therefore more shielded from the possibility of explosion.