Minneapolis, Minnesota (CNN) --
For 13-year-old Arrianna Merritt, the words mother and hero go hand in hand.
Three years ago Sunday Arrianna watched her mother Kim Dahl drive a school bus loaded with children across the Mississippi River when the bridge began to buckle up and down. Thirteen people were killed in the bridge collapse in Minneapolis, Minnesota.
What Arrianna remembers most was the choice her mother made that day after the bus became tilted upward on a steep incline.
"Either you let us free fall -- or you hold that brake to the ground and you stay where you are."
Dahl held the brake with her foot until she could reach for the parking brake.
Millions of viewers saw the jaw-dropping images of the school bus as it teetered on the edge of a broken stretch of highway, with nothing but a guard rail separating it from a deadly fall.
Arrianna sat in the bus with her brother, 50 other children and eight adults.
The bus "went up, and then it went down," Dahl recalls, adding it soon turned into free fall and she was "hanging on for dear life." "I thought 'this is it,' I'm not getting out, and I'm going to die."
With the parking brake on, Dahl kept the bus in place while everyone made it out the rear emergency door safely. Arrianna, 10 at the time, and her then-5-year-old brother David, were the last ones and had refused to get off with the others.
"We didn't want to leave our mom," David, now 8, remembers.
"I'm like 'get your butts off the bus,'" Dahl said. They eventually did.
Dahl says she managed to break free herself after a jolt on the bridge seemed to release her seatbelt, which had been stuck.
Since that day, those on the bridge and their loved ones have never been the same.
This particular mother-daughter pair say they've always had a tight bond but Arrianna says she and her mother have grown even closer since their ordeal.
"We've always been close, but I think after the accident we got closer because we shared the same experience and we know what its like," Arrianna said.
The physical reminders of that day often overshadow the emotional ones.
"It does hurt to see my mom in pain because she cant do the things that she used to love to do."
Dahl broke her back in the fall and that's been the primary cause of complications since.
"It's like a new me that I'm living with," Dahl said. "It's almost like your freedom is taken away... I can't go grocery shopping by myself because I can't lift the milk up and put it in the cart."
"My kids and my husband -- I couldn't ask for a better family," Dahl said. "I mean my kids step up and do a lot of the work that I can't do or help me do things that I can't do. They're pretty good about saying 'yea.'"
"I don't know if life will ever be normal like it was before July of '07 and prior, but we've made the best for it," said Dave Dahl. "We've adjusted our lives, we've changed our lifestyles. But we've just got to be positive."
Dave Dahl reminds himself that things could have been worse. "Our family unit here in this house is five people," he says. "Within an instant it could have been two."
Arrianna in particular has taken on a number of duties, like cleaning the bathrooms and doing the laundry, that she says has taught her how to be "a grown-up."
"Most of the time when my mom is in the hospital or getting surgeries, I have to step in and be the surrogate mother to my younger siblings," Arrianna explains. The family also includes another daughter, Brianna, age 11.
"And my dad is working a lot of the time so he kind of doesn't have much time."
"It kind of teaches me to grow up some and how to be a grownup and what it's like."
She adds it can get difficult once in awhile, but these days she simply reminds herself who she's doing it for.
"Now I kind of understand why I have to stand in and why sometimes I have to be the adult."
Kim Dahl deals with PTSD, as well, and still sees a therapist twice a month. And while Arrianna no longer has therapy sessions of her own, occasionally she still struggles.
Kim Dahl is among 122 victims and their families who've filed suit against URS Corp., an engineering and construction firm hired by Minnesota DOT to conduct engineering analysis on the bridge before it collapsed. A federal investigative report blamed the collapse on support plates that were too thin, and increased weight on the bridge from construction equipment.
The amount of punitive damages plaintiffs are seeking against URS is "hard to say," said Dahl's attorney Bill Harper, who estimated it could be as much as $100 million.
"URS was hired by the state to make sure the integrity of the bridge was safe for the public," Harper said Wednesday. "They did not do what they promised to do -- and as a result -- that bridge collapsed."
Attorneys for URS said the company's "work was unrelated to the causes of the bridge collapse. ... URS did not design the bridge, was never asked to verify the bridge's original design, and did not know the bridge's original design was defective."
Hennepin County District Judge Deborah Hedlund is expected to rule on punitive damages by mid-October.
On Tuesday Kim Dahl and Arrianna visited the site of the collapse -- where the new I-35 West bridge stands. The sound of a jackhammer in the distance is enough to remind Arrianna of the construction noise taking place on the bridge at the time of the accident. It brings her to tears.
Thankfully mom is nearby.
"She is kind of like my counselor because she listens," Arrianna said. She no longer needs a therapist, but it's these rare moments when mom comes in handy.
You need to move on, and you need to be strong and live your life to the fullest.
--Arrianna Merritt, bridge collapse survivor
And occasionally, Arrianna says, their roles are reversed. She recalls hearing her mother sometimes say things about how life would be easier if none of this had ever happened.
"You need to move on, and you need to be strong and live your life to the fullest."
Those are words the family is living by.
Kim Dahl has had to give up what her husband Dave Dahl would call the "wilder motor sports," and the five of them have taken on a new hobby: road trips.
Kim Dahl says she'll never be able to drive a bus again -- something she misses more than anything. She had the same bus routes for years and knew the kids and their parents well.
But the family camper may be the next best thing. So far, the've have made it as far as Florida to the east and Colorado to the west.
"My mom could never afford it...we never went anywhere," Kim Dahl recalls of her childhood. "And we went on [our] first trip and it was just awesome. To learn stuff and to see stuff was just amazing."
As Dave Dahl describes it, there's "no stress, there's no itinerary, nothing, we just go."
"Wherever we get is where we get," he adds. "We kind of have a goal of where we like to be in the end."
Given the path Kim Dahl and her loved ones have traveled since 2007, that phrase could be the family motto too.