An associate professor at Embry-Riddle Aeronautical University referred to turbulence as "the last of the unanticipated threats," regarding air travel. According to the National Transportation Safety Board, 63 people have been seriously injured by turbulence in flights over the United States since 2007. Many of those injured are flight attendants and other crew members who were unbuckled at the time of injury.
A safety expert and current senior instructor at the University of Southern California pointed out the problem faced by pilots. Passengers and crew would be safest if they remained seated with their seat belts fastened throughout the flight. As that is not possible, a pilot must decide when the seat-belt sign should be lit and when it should be turned off. If passengers are asked to remain in their seats for too long, they will likely begin to ignore the sign.
Flight attendants face an elevated risk of being injured by turbulence because of the amount of time they spend unbuckled. These injuries are part of why air transportation workers face an elevated risk of injury. According to the Bureau of Labor and Statistics, workers in that industry suffer a higher rate of injury than those in all industries except construction, health care and public safety. After one particularly serious incident, Delta Airlines changed its policy from asking flight attendants to be seated immediately when turbulence is indicated to requiring them to be seated with seat belts and shoulder harnesses secured.
Avoiding turbulence is not always possible. Airlines have their planes fly around thunderstorms and areas of obvious turbulence, but clear air turbulence is more difficult to detect. The equipment capable of spotting such turbulence is expensive and bulky. At this point, airlines simply use word of mouth to detect and avoid turbulence. Pilots and air traffic controllers pass along information regarding where they struck rough patches. For those already injured, that information comes too late.
Source: USA Today, "Danger of turbulence remains safety threat to air travel," by Bart Jansen, 10 September 2012