WASHINGTON (WUSA9) — A year-long WUSA9 Special Assignment Unit investigation exposed how decades of government inaction allowed what some call a design flaw to kill thousands of people in tractor-trailer accidents every year.

WUSA9 followed the journey of two mothers who lost their children in violent truck underride accidents. Their mission was to change the law and stop underride accidents.

Now, their campaign continues on Capitol Hill.

Big Rigs, Big Risks

Our investigation began last summer documenting the heartbreaking stories of families devastated by violent truck underride accidents.

Rear and side underride collisions are one of the most horrific accidents on our highways.

They happen when a passenger vehicle slides under the rear or side of a tractor trailer in a crash often crushing or decapitating the people inside.

Five years ago, Marianne Karth lost two of her daughters, 17-year-old AnnaLeah Karth and her 13-year-old sister Mary in a rear underride accident.

RELATED: Mothers fight for tougher tractor trailer laws after daughters die in underride crashes

Lois Durso’s heartache comes from a violent side underride accident that killed 26-year-old daughter Roya years earlier.

"And the [trailer's] rear wheel trapped [the BMW], and dragged it for a while, all the while it was crushing her," said Durso.

RELATED: Truck Trailer rear guard rules have huge holes, safety experts say

By law, tractor trailers are required to have rear guards and research shows they help chances of survival during an accident.

Insurance Institute for Highway Safety, a non-profit organization, says the 20-year-old regulations are outdated and the rear guards don’t always hold up.

There are no law requiring side guards of any kind.

RELATED: Why don't tractor trailers have side guards?

"I mean people are dying and they don’t understand the risk," said Durso.

Karth and Durso took their grief, heavy hearts and headed to Capitol Hill. They lobbied lawmakers for new laws requiring side guards and stronger rear guards on all tractor trailers.

RELATED: How much is your life worth?

In December, their hard-work paid off and series of reporting paid off with bi-partisan legislation the Stop Underrides Act of 2017.

The bill now sits in the Commerce, Science and Transportation Committee and the mother's new mission is to pull back all that Capitol Hill Red Tape.

Groups that represent the trucking industry told WUSA9 in December, requiring rear and side guards on tractor trailers would not be easy.

"There are a lot of operations issues that you need to consider," said Jeff Simms, President of the Truck Trailer Manufacturers Association.

But the moms are ready for the fight. Karth, now lives in North Carolina, and Durso, based in Florida, returned to DC to get the Underride Act, moving again.

Karth said it's the duo’s fifteenth trip to Washington since March of last year. And that doesn't include the trips to trucking conferences, meetings with national safety groups and crash tests.

All that travel, paid right out of their own pocket.

“So we know it’s going to continue unless something is done and I hate to use this word but it’s true, it’s been a massacre out there," said Durso.

In fact, the number of people who die in accidents with large trucks is at a 10 year high.

How many of those are underride accidents?

No one knows for sure and that’s part of the problem. The government has no standardized reporting for underride accidents.

The WUSA9 Special Assignment Unit is keeping a count based on published reports. So far this year there have been at least 25 underride accidents in at least 20 different states causing 20 deaths and 11 serious injuries.

That includes Reza Zati of Jessup, Maryland and 30-year-old Christopher Padilla of Northern Virginia. Zati died in February and Padilla died on Thanksgiving day.

RELATED: Va. family grieves after man killed in Thanksgiving underride crash

Police said Padilla was driving at a high rate of speed when he slammed his car into the back of a tractor trailer on the beltway, dying at the scene.

Jayne Mansfield

Vehicles crashing under tractor trailers has happening since the 1950’s when cars first started sharing the highways with big trucks.

One of the earliest underride accidents may be the most famous. Hollywood starlet, Jayne Mansfield died in a gruesome underride accident outside New Orleans in 1967.

RELATED: 50 years after a Hollywood starlet dies, danger remains on the roadways

Mansfield and Hargitay
US actress Jayne Mansfield and her husband US actor Mickey Hargitay pose during the 17th Cannes Film Festival in Cannes, on May 4, 1966. / AFP / - (Photo credit should read -/AFP/Getty Images)
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Her death is what first motivated advocates to push for stronger underride guards on the back of trucks. In fact, some people still call underride bars “Mansfield Bars."

RELATED: Moms start online petition to 'end deadly underride' deaths

Karth and Durso want to know why 50 years later these frightening and deadly accidents are still taking the most precious of lives.

"This is grief driven," Durso said. "We both feel the weight and the burden of the underride tragedy, we have felt the pain of it first hand and we want to make sure it doesn't’t happen to other families."

Stop Underrides 12.6 by WUSA9-TV on Scribd