Excerpts from an article in the Sept 2002 issue of "The Atlantic":
Indeed, he [Bruce Schneier] regards the national push for a high-tech salve for security anxieties as a reprise of his own early and erroneous beliefs about the transforming power of strong crypto. The new technologies have enormous capacities, but their advocates have not realized that the most critical aspect of a security measure is not how well it works but how well it fails.
[Here's an example of measuring how "good" security is by how well it fails]
… at Sea-Tac Airport, someone ran through the metal detector and disappeared onto the little subway that runs among the terminals. Although the authorities quickly identified the miscreant, a concession stand worker, they still had to empty all the terminals and re-screen everyone in the airport, including passengers who had already boarded planes. Masses of unhappy passengers stretched back hundreds of feet from the checkpoints. Planes by the dozen sat waiting at the gates.
In Seattle a single slip-up shut down the entire airport, which delayed flights across the nation. Sea-Tac had no adequate way to contain the damage from a breakdown — such as a button installed near the x-ray machines to stop the subway, so that idiots who bolt from checkpoints cannot disappear into another terminal. The shutdown would inconvenience subway riders, but not as much as being forced to go through security again after a wait of several hours. An even better idea would be to place the x-ray machines at the departure gates, as some are in Europe, in order to scan each group of passengers closely and minimize inconvenience to the whole airport if a risk is detected — or if a machine or a guard fails.