Seven on your Side Investigates found that over the past two decades, new safety features have been implemented in new construction -- while one major recommendation still hasn't been put into place.
"I could not save my son on 9/11, but I hope that I could help save other peoples' sons and daughters," mom Sally Regenhard said.
Her 28-year-old son Christian died in the attacks at the World Trade Center, after graduating from the FDNY Academy just a few weeks earlier.
"He was a fabulous person, a beautiful human being," she said. "He was someone who really joined the fire department for a short period of time."
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She turned her grief into activism.
"I wanted to fight back," she said. "I wanted to get accountability and responsibility."
She created a non-profit called the Skyscraper Safety Campaign, and one of the goals was to improve building codes and emergency response.
"People need to feel that they're safe in the buildings they live in, work in, or try to save people in," she said.
She didn't do it alone, getting help help from other 9/11 families and fire safety experts.
Glenn Corbett, a fire safety professor at John Jay College of Criminal Justice, worked with Regenhard on pushing for changes.
"We did get some improvements," he said. "It's a step forward, but it's not as far as we need to go for this."
Their activism helped make taller builders safer.
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New buildings more than 420 feet tall need an additional stairwell, emergency lighting, and impact resistant walls in elevators and stairwells.
They also have enhances fire proofing and improved radio communication.
"I was told, 'You'll never do it,'" Regenhard said. "But that was the greatest thing to say to me. There's no way I would accept that."
But there is something that hasn't changed -- the width of the stairwells in tall office buildings.
They were 44 inches wide in the World Trade Center towers on 9/11, and for the most part, the building code remains the same today in regards to egress capacity.
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That's despite the fact that a federal investigation showed if the towers had been full that day, it would've taken more than three hours to evacuate and the stairwell capacity would "not have been sufficient."
"It's something we think is critically needed, and we never got it after 9/11," Corbett said.
The new World Trade Center complex has extra wide stairwells, a 20% increase, and the developer built everything beyond current code requirements.
"Design is great, sexy buildings, that's fine," said Bill Dacunto, Executive Vice President of Operations for Silverstein Properties. "But life, safety, and security is the most important thing for any building anywhere."
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