a contributing factor.

 When you come down that hill and you feel your bottom parts, you know, come off that seat, well, that's your negative-g forces at work right there,  he said.

G-force is the force of gravity or acceleration on the body. Negative g-force is what roller coaster riders feel as they go down a hill.

 Those forces are part of the ride, part of the thrill, part of taking you to the edge and, hopefully, bringing you back safely,  Martin said.

Martin said he is not familiar with the exact g-forces the Texas Giant exerts. But based on his experience, the g-force, depending on how strong, essentially multiplies a person's weight, putting pressure on the restraint, he said. For example, if someone weighing 300 pounds was on a steep drop with a negative 3G, it could put as much as 900 pounds of pressure on the lap-bar restraint.

But Martin doesn't believe weight was the only factor in the fatal accident. Based on his previous cases, many things likely went horribly wrong, causing the Dallas mother to fall to her death, he said.

Martin said most rides are designed for an adult who weighs 180 pounds.

The International Association of Amusement Parks and Attractions told NBC 5,  Restraint design is typically based upon a 95 percentile physical profile to comfortably accommodate the vast majority of a ride's population segment. 

Tate said she doesn't think she fits into that 95 percent.

 I'm too big for this ride,  she said.  I can't do it. Something in my head said get off. 

The IAAPA, the largest international trade association for fixed amusement parks, said amusement parks are overwhelmingly safe. The likelihood of sustaining at a permanent amusement park ride in the United States an injury serious enough to require overnight hospitalization or treatment is one in 24 million, the association said. The chance of being fatally injured on an amusement ride is one in 750 million.

According to injury reports filed by Six Flags Over Texas with the Texas Department of Insurance, there have been 14 injuries on the Texas Giant between April 2008 and April 2013. Most were minor and were caused by the ride jostling the passenger.

Texas is one of at least 17 states that does not have a state agency responsible for inspecting amusement park rides, according to a survey by NBC News. Texas also has no law granting the state power to investigate in the event of an accident.

Instead, Six Flags is conducting its own investigation into last week's fatality.

Tate said she's relieved she decided not to ride the Texas Giant.

 I could have been that woman,  she said.