Sidney Mae Olson’s spirit is propelling her parents forward.

On May 9, the 5-year-old was struck and killed by a tractor trailer while she was crossing an intersection in downtown Andover with her mother, 3-year-old brother, and young cousin.

“We feel like every day our mission is to live her spirit,’’ her father, Eric Olson, said Monday. “The second part is that you’ve got an entire community that’s been impacted by this, and they want to help.’’

The Olson family is hoping to use their unquantifiable loss as the impetus for life-saving changes in traffic patterns, speed enforcement, and urban planning, with a goal of keeping others from having to endure the lifelong burden they now carry.

“I went to work and came home and [Sidney] was gone,’’ he recalled, while his wife, Mary Beth Ellis, was in town with the children.

“It’s a different kind of grief because she saw it firsthand,’’ he said. “We’re both struggling. We have really, really hard, terribly hard moments.’’

The family was walking to an art class on Main Street and was in the crosswalk on Elm Street around 5:15 p.m. when the tragedy occurred, according to authorities and the Olsons.

The driver of the truck, a 2015 Freightliner owned by Sysco Systems, remained at the scene, officials said.

The truck was stopped at a red light and hit Sidney when it “started moving again,’’ David Procopio, a spokesperson for the State Police, said last month.

Essex District Attorney Paul F. Tucker’s office, along with State Police and Andover police, are continuing their investigation, a spokesman for Tucker said by e-mail Monday.

“Cases like this are very technical in nature and take much more time to resolve than we would hope,’’ said Glen Johnson. “We need to take the time to do it right.’’

Olson said his family will never forget their little girl.

“My wife and I feel an incredible void that we will never replace. It’s just in everything that we do,’’ he said. “Every moment of every day, it feels like there’s a piece missing. It might sound cliche, but it feels like a piece of me died that moment.’’

The couple has sought therapy for support after the crash and is meeting with other parents who have lost their children suddenly, he said.

“Having those people around us is critical,’’ he said.

Their spirits are also buoyed by Sidney’s younger brother, Ellis, who “tells us in our darkest moments that Sid’s happy,’' Olson said.

“He talks about her every day,’’ he said. “He’s like her, I think. He’s got this very sort of positive, smiley attitude about life, and so he’s helping us go.’’

Having lived in Colorado and outside Amsterdam for many years, the family is pushing for the traffic pattern at the intersection where Sidney lost her life to be redesigned.

One change that can be made immediately, Olson said, is to relocate the stop line where the truck came to a halt to improve driver visibility.

“We have moments of hope where we feel like we can make an impact that she’d be proud of,’’ Olson said. “There’s common-sense change we can make right now in our communities that can make a big difference.’’

The family also wants to change how the traffic lights operate in the Elm Square intersection. When the Olsons entered the intersection, they had the walk sign, the family has said. But the lights allowed the truck to drive into the intersection at the same time, they said.

At a town forum slated for Thursday, Olson said he plans to speak and will make these suggestions:

Eliminate turns through intersections when pedestrians have a walk sign.

Post police details at Elm Square and “other busy intersections at peak traffic times.’’

“Improve awareness and enforcement of speed limits on Elm, High, and Central streets with portable speed bumps, signage and radar signs.’’

The meeting will be held from 7 p.m. to 9 p.m. at WWI Memorial Auditorium at Doherty Middle School, according to the town website.

Olson said other long-term goals for Massachusetts as a whole include using video cameras to enforce speed and traffic rules and designing streets and roads with a focus on pedestrian and bike traffic.

The campaign to improve safety for pedestrians and bicyclists is giving the family a sense of purpose through their grief, Olson said.

“It’s like I can just feel her energy in everything we do. I’m very much not the same person I was . . . before she was born. You transform as a parent, as everyone knows. She is part of us,’’ he said. “I think that’s why we’re so focused on the stuff we’re focused on. We don’t want to lose that. We want to carry that forward and make it bigger and make her part of more people.’’

John R. Ellement can be reached at john.ellement@globe.com. Follow him on Twitter @JREbosglobe.