Evacuation Procedures for the Twin Towers had been Improved
The upgrades in facilities and evacuation drills combined with the low occupancy rate at the time of the attack allowed the vast majority of civilians who were not trapped above the crash zones to escape.
There is also isolated anecdotal evidence that the evacuation drills became more frequent after the attack.
How could they let this happen? They knew this building was a target. Over the past few weeks we'd been evacuated a number of times, which is unusual. I think they had an inkling something was going on. 1