(CNN) Passengers later said that they had been fishtailing for miles, according to court documents.

Some said their Greyhound bus driver had red eyes when she was taking tickets in New York. One man said he saw the driver doze off behind the wheel.

Another driver on the same road saw the bus swerving and remembers thinking, "They are going to kill somebody," the documents show.

At 1:33 a.m. on October 9, 2013, a Greyhound bus headed to Cleveland from New York slammed into the back of a tractor-trailer on I-80 in Pennsylvania.

The force of the crash tossed one passenger from the bus onto the road and forced the back wheels of the bus to lift off the ground.

One woman died and dozens more were hurt.

The passengers believe the driver fell asleep behind the wheel, and a CNN investigation reveals Greyhound has not been enforcing one of its own rules meant to keep passengers safe.

The Greyhound bus slammed into the back of a tractor-trailer.

'I have never screamed so much in my life'

Elora Lencoski was 18 years old at the time, an aspiring opera singer with a promising career.

She was one of just four opera singers admitted to the prestigious Cleveland Institute of Music that fall and she had won a trip to meet her idol, the famous soprano Renee Fleming, in New York. She and her friend decided to take Greyhound. They were returning from their trip when the crash happened.

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The last thing Lencoski says she remembers is telling her friend "good night."

"I remember waking up in the hospital screaming. I have never screamed so much in my life," she said. "I kept forgetting where I was and I would just scream and not stop because the pain was so intense."

Lencoski suffered a brain injury and a badly hurt leg. But the most devastating injury: vertebrae in her neck that shattered, affecting her voice box, and seriously diminishing her ability to sing, her lawsuit says.

Lencoski is suing Greyhound. Her lawyer said Greyhound's driver was too tired to drive.

One person died and dozens were injured in the crash.

Greyhound's own rules not enforced

CNN has found that Greyhound, despite saying safety is a priority, does not enforce its own rule related to driver fatigue.

Internal Greyhound documents show drivers are supposed to stop about every 150 miles, get out and walk around the bus, check the tires and stretch, to mitigate fatigue.

But CNN discovered that the so-called rule, known as rule G-40, is treated as a guideline. It's not enforced.

"Remember Rule G-40: Stop approximately every 150 miles to check tires and walk around the bus for a safety stop at roadside rests. Use these stops to refresh yourself and stay completely alert by using these short-term alertness management actions," a 2012 Greyhound information bulletin states.

Those actions include "get out, stretch your muscles and get fresh air" as well as "some quick physical activity."

A copy of the driver rule book in the court file says "drivers must check the tires, lugs and wheels approximately every 150 miles, employing procedures determined by company directives."

A review of Greyhound's posted routes, many of them overnight, shows there are several across the United States where Greyhound has no required stops for more than 150 miles, therefore the onus is on the driver to make 150-mile safety stops.

For example, those routes include Phoenix to Lordsburg, New Mexico; New York to Washington; Los Angeles to Quartzsite, Arizona; Atlanta to Charlotte, North Carolina; New York to Boston; Chicago to Memphis, Tennessee; and New York to Cleveland -- the same route where the 2013 crash happened.

"They don't enforce their 150 miles safety rule because it costs them money," said Lencoski's attorney, Jon Ostroff.

Ostroff also represents 22 other passengers who were on the bus that night.

"I think Greyhound was more at fault for this crash than the driver who fell asleep behind the wheel because they allowed a dangerous driver to drive," he said.

Greyhound is the largest bus company in North America.

A pattern of driver fatigue

Greyhound is the largest bus company in North America. Eighteen million passengers ride Greyhound buses some 5.5 billion miles every year across the country. Most trips are routine, but when a major accident happens, it sheds a light on a longstanding issue in the passenger bus industry: driver fatigue.

A government study from 2012 found 37% of all passenger bus crashes were due to driver fatigue.

Of the 184 fatal crashes involving a passenger bus from 2010 to 2014, five were due to driver fatigue, according to the National Highway Traffic Safety Administration.

Greyhound has a "satisfactory rating" from the Department of Transportation.

Greyhound's most recent major accident happened in January near San Jose, killing two people. A California Highway Patrol officer said the driver had mentioned being fatigued before the crash, which is still under investigation. It's unclear if he followed the 150-mile rule.

"I would never allow anyone in my family or anyone I know and love to ride on a Greyhound bus," Ostroff said.

Another passenger who was riding the bus with Lencoski lost his leg and sued separately. The jury in his case found that "Greyhound demonstrated reckless indifference to the safety" of passengers and drivers. The jury also faulted the company for "giving contradictory language in their rules and training" related to "fatigue level," and not "enforcing their rules."

He was awarded $23 million, and another $4 million in punitive damages. In addition, the jury tacked on $150 to send a message that Greyhound wasn't following its 150-mile rule.

Greyhound has a "satisfactory rating" from the Department of Transportation.

Long rides, limited breaks

Greyhound declined repeated requests for an interview and did not answer specific questions for this story. In an email, Greyhound said it has an "excellent safety record" and continues "to improve our safety programs."

But the company's CEO, David Leach, in a deposition obtained by CNN, makes it clear that it's up to drivers to determine if they're tired.

"Would you agree that if she's feeling tired before she gets on the bus, she shouldn't even be driving?" the lawyer asks in the deposition.

"Well, I don't know that either. I mean, it's why we have best judgment. I think she has to use her best judgment. And, you know, if she's feeling fatigue or feels like she shouldn't be driving that coach, she has an obligation to tell us that. Absolutely," Leach replies.

Asked how Greyhound enforces the 150-mile rule, Leach says they rely on the drivers to do so.

"I'm just saying they're professional motor coach operators ... and so they, they do their job ... well, I'm expecting our drivers to stop when they're supposed to."

"But you don't have any way currently to enforce that safety stop. You've already testified to that, right?" the lawyer asks.

"Right," Leach says.

Leach also acknowledged that some of the longer routes could have drivers going twice as far as 150 miles without stopping.

"So if they don't feel they need to stop, it's OK with you, as the CEO, that they drive 333 miles without a break?" the lawyer asks.

"It would be fine with me," Leach responds.

In the 2013 accident, the driver, Sabrina Anderson, never took a break.

She was 178 miles into the trip when she crashed.

Both Greyhound and Anderson deny that fatigue was the cause of the incident, saying one of Anderson's legs went numb before she passed out. In her deposition, she says she doesn't remember the moment of impact.

Anderson declined to speak with CNN.

Deborah Hersman, former chairwoman of the National Transportation Safety Board and now president and CEO of the National Safety Council, said overnight motor coach crashes were one of the issues that concerned the NTSB when she was at the helm.

"The problem is that there's a tremendous risk, because the driver has to stay awake when there's tremendous pressure biologically to sleep," Hersman said, especially between 2 a.m. and 5 a.m.

In addition, studies have shown that overnight workers get less sleep and less restful sleep during the day, she said. Combine that with the mundane task of sitting behind a wheel.

"Sitting there and doing the same thing, there's not a lot of stimulation," she said.

She said some charter bus companies have stopped offering overnight routes because of the risk and the number of fatigue-related crashes, but most of those were private charter bus companies. "I would never pull an all-nighter with my family in the car, knowing what I know about fatigue and night risks, never."

Lencoski, once a rising star, said the nightmare stole her identity, her self-esteem and her only dream.

"I would take scars on my face if I could have a voice back," she said. "I don't care how I look, especially now, I hate looking at myself. It's like I don't know me anymore."

"There is no dream I had greater than singing on stage. ... If someone in my life dies, I am still here, but if I can't sing, am I really still here?"