The National Transportation Safety Board determined that a truck driver’s fatigue and methamphetamine use and failure to respond to slow moving traffic within a work zone resulted in a 2015 multi-vehicle crash near Chattanooga, Tenn., that killed six people and injured four.

The Cool Running Express tractor-trailer was traveling northbound on Interstate 75 on June 25, 2015 when traffic slowed while entering a marked work zone. The driver did not slow with the traffic and hit the rear of a Toyota Prius, starting a chain of crashes that involved seven other vehicles and a total of 18 people.

The NTSB found that although the truck driver had an opportunity for overnight rest before the crash, he had likely gone without sustained rest for 40 hours prior to the accident. It was also discovered that pre-employment screening failed to include previous accidents when Cool Runnings Express was hiring the driver, limiting the company’s ability to assess his safety performance and potential risk.

“Ending impairment in transportation and reducing fatigue-related accidents are on the NTSB’s Most Wanted List because fatigue and impairment have led to so many tragic outcomes – not only in commercial trucking but in all modes of transportation,” said NTSB Chairman Christopher A. Hart. “Our investigation reveals how this driver’s choices and actions, in the days and hours before the crash, led to the crash and loss of life.”

The truck driver’s post-accident drug test revealed methamphetamine use. The NTSB found no prescription for its use and concluded that the driver illegally used the drug prior to the crash and the drug degraded his performance.

The NTSB’s report also highlights that the number of trucks involved in fatal crashes in work zones is disproportionately higher than the number of other vehicles involved in fatal crashes in work zones. Speeding, distraction, and impairment are key factors in these crashes.

The NTSB reiterateed its Sept. 2015 recommendation to the Federal Highway Administration to amend guidance on the use of supplemental traffic control strategies and devices for work zone projects.

“Implementation of our recommendations made today can lead to a prospective employer having access to a driver’s full driving history, including involvement in previous crashes, incidents of speeding, or operating while impaired by alcohol or other drugs,” said Hart.

“Access to this history will help make pre-employment screening more effective, help keep high-risk drivers off the roads, and thereby possibly prevent this type of accident from happening again.”