If the World Trade Center attack had occurred at a busier time, it would have taken occupants four hours to get down the stairs--hours they didn't have. The solution: emergency elevators. Surprising? They exist in the U.S. only in Las Vegas's Stratosphere Tower, but experts expect they'll eventually be a skyscraper requirement. To make elevators fire-safe, shafts are sheathed in concrete, studded with heat sensors, and pressurized to keep out smoke. Cars have heat- and water-resistant electronics. Not in the plans: The Freedom Tower won't have truly fire-safe elevators, but there will be lifts for rescuers and disabled occupants to use in an emergency.