It's no wonder that critics and safety advocates worry about the trauma caused by traveling at such speeds. Douglas Smith, a University of Pennsylvania neurologist, conducted tests in 2003 to analyze how a person's head rotates while on a ride. It's the rotation of the head, along with the excessive speeding up and slowing down, that is the main cause of brain injury. He found that people's head rotation and acceleration and deceleration while on rides were not at levels that would cause brain injury. Since then, he's repeated the study, and initial data support his first findings.