Airport Perimeter Screening - Reliability = 0% 

Airport perimeters are protected by a fence in all cases, cameras and other security devices at larger airports, and sporadic security patrols by airport police officers. 

Terrorists will attempt to enter the air transportation system at the weakest point.  The smallest, least secure airports are sometimes protected by a fence only six feet high, with no barbed wire on top.   Even larger airports face significant perimeter security challenges simply due to the size to the perimeter.  An average perimeter at a mid size U.S. airport, is 15 miles long.

Some airports, such as New York’s La Guardia airport, are on a waterfront and the waterfront is easily approached by small boat.  Well after new, post 9/11 security measures were put into effect, a group of lost boaters ran aground on the secure area of La Guardia.  Even though they were not only not trying to hide, but actually trying to find help, they wandered the secure grounds of this major U.S. airport, near runways and taxiways, for hours, before finding a guard shack, where they reported in.  No one knew they were there.

It is not difficult to defeat a fence or security cameras, especially after dark, to gain access to a commercial airliner.  But, terrorists wouldn’t even have to bother.  Vendor trucks carrying airplane meals, water, construction equipment, supplies, newspapers, communications equipment, and all manner of items needed by airports to function, are routinely admitted to the airport perimeter with only a cursory inspection, if that.

An admitted Al Qaeda operative, legitimately employed as an airport vendor’s truck driver has already been convicted after he admitted to using his truck driver cover to observe security weaknesses and strategies at the airports he serviced after 9/11.

Given that the airport perimeter screening system already allowed an Al Qaeda operative to routinely access a major airport to gather intelligence, the reliability of airport perimeter screening in preventing armed terrorist access is, obviously, zero.