Sealed off security zones in significant parts of America's most prominent downtowns post 9/11 attacks has led to blighted landscapes and create an 'architecture of fear', said a study.

The restricted areas not only affect the appearance of landmark buildings but also reflect an 'architecture of fear', as evidenced, by the bunker-like appearance of embassies and other perceived targets, according to the study.

The study by University of Colorado Denver professor Jeremy Nemeth said the security zones have led to blighted landscapes, limited public access and a need for a new approach to urban planning.

By limiting access and closing off space, we limit the potential for more 'eyes on the street' to catch possible acts in the process, said Nemeth. Ultimately, these places impart a dual message - simultaneously reassuring the public while causing a sense of unease.

The study, which looked at areas of downtown Los Angeles, New York City and San Francisco, found that while each city values and protects potential targets equally, what is deemed off-limits varies widely.

35.7 percent of New York's civic center district is within a security zone, meaning it is accessible only to for those with proper clearance. Only 3.4 percent of San Francisco's civic center area has the same designation, while 23-acres of public space in Los Angeles sit in a security zone.

Nemeth said security zones must now be considered a new type of land use similar to parks, open space and sidewalks. They must be planned and designed in ways that involve the public and are useful to downtown built environments.

Overt security measures may be no more effective than covert intelligence techniques, Nemeth concluded.