I remember when I first heard of the campaign for Mandatory Seatbelt use, It was shortly after I was in an accident. From this accident I got my jaw broken in three places a piece of metal rod stuck through my leg and many cuts and scrapes but the rescue workers on the scene said I was lucky. They also said they thought if I had been strapped into the seatbelt I would have been hurt worse if not killed... When I saw the car I had been in, I had to agree with them.
I have been a Firefighter and Medical First Responder for almost 9 years now and over the years I have worked many automobile wrecks. In most of these it was unclear whether the seatbelt was or would have been a help or hindrance. However there have been a few where it was clear the seatbelt caused or would have caused problems.
I would like mention a couple that come to mind... One incident involved a Volkswagen that was hit in the side by a much larger vehicle. The driver was not wearing his seatbelt and received a broken hip and several minor cuts and scrapes. The impact on the drivers side had threw him into the passengers seat, the top of the door frame on the drivers side was pressed down into the steering wheel and the door handle was against the gear shift lever that is between the front seats... How bad do you think the drivers injuries would have been had he been belted into that seat? ... Another incident involved only one car... A woman was driving a midsize GM car (can't recall the exact type) down a step grade with rock face on one side of the road and a drop off on the other... She had a tire blowout on the front of her car causing her to hit the rock face pretty hard... She was wearing her seatbelt and received no injuries from this, but something had caused a fire to break out in the engine compartment. Her seat belt had jammed and she was trapped in the car. When we arrived on the scene she had her door open and the cab (passenger compartment) had already begun to fill with toxic smoke... We first thought she could not get out because she was panicking, but we could not get the belt to release either. We had to cut the seatbelt to get the woman out... and she was transported to the hospital where she was treated for smoke inhalation.
I am not saying that a seatbelt is never helpful, I am just saying in some cases it is harmful (possibly even Deadly). This should not be a law it should be a choice.