It all depends on the accident.
Young man where I use to live, left a football game, headed home with 3 girls in the car. He was the only one buckled up. Wet roads, car hydroplaned into ditch and flipped. Broken collar bone was the worse injury to the 3 girls. Young man died.
That happens but it's the exception. Most of the time the one's belted in have the least injuries. Later model cars are built to protect the passengers. Staying in the seat will give you the most protection.
Not disagreeing, but seat belts are not all they are cracked up to be. Airbags do more good than seat belts.
A few months ago, 2 trucks head on collision, just down the road from where I live. Man wearing seat belt died, man not restrained minor injuries. It happens more than people think.
Studies have proven that drivers, restrained by a seat belt, take more chances, drive faster, and dont pay attention to their driving as much as non-restrained drivers. The feeling/thought that they are safer, because they are buckled up, makes them less attentive and take more chances.
Cut a lady out of a new F150 Saturday morning. Side swiped by a small SUV type vehicle, lady was not buckled up, but side airbags deployed and she had minor injuries. Older lady, couldn't get over truck console and couldn't get drivers door open. 2 vehicle were about 200 yds apart and I dont know anything about the other vehicle other than it hit a tree head on. Not sure about injuries.
I recall hearing of a state trooper comment in all his years of arriving on accident scenes he never unbuckled a dead person.
That's because first responders, firemen, or the coroner are the ones that remove the bodies from the vehicles.