To the question, “How much security apparatus is needed on city streets?” the agencies charged with guarding public safety seem to answer, “As much as we say.”

Against the memory of two successful attacks on the World Trade Center and numerous foiled plots, no one wants to sign off on security measures that could later be breached or compromised, with a loss of life, limb and property. So barriers, gates, fences and checkpoints proliferate. Whether they are reasonable and prudent — or excessive results of worst-case thinking — is a matter often left unspoken.

And that leaves the public with no meaningful way to assess the transformation of some city streets into obstacle courses.

Hoping to cast some light on the issue, a group of Lower Manhattan residents is preparing to sue the New York Police Department over its security plan for the World Trade Center, saying that the plan will leave the center in “fortresslike isolation” and the area around it “as impervious to traffic as the Berlin Wall.”