This sounds like something extremely unusual and therefore perhaps not cause for great concern. Still very tragic though. Does anyone know common it is for a child to be strangled in a car by a seatbelt?
it really isn't THAT common... most kids know not to play with their seatbelts.
Still, this is also more common when you combine a child with low impulse control or with parents who didn't train him well at using his booster (not saying they are at fault, just saying they didn't know to train him from doing this as they weren't aware of the danger) with locking shoulderbelt retractors.
seatbelts that only have lightweight locking latchplates only have the shoulderbelt lock in the event of a crash or stopping as one would in a crash, which requires a lot of braking.
Shoulderbelts with ALR retractors, which are great for installing forward facing carseats, on the other hand, lock anytime the belt is pulled out all the way.
So, if the belt is not locked when the child is put in the seat and the child has not been trained or simply (due to immaturity or special needs) does not have the impulse control to leave the belt alone, this is when the danger is very real.
Should the child quietly (so as not detected by a parent who is keeping eyes on the road and hands on the wheel) pull the belt out all the way, then, worse still, wrap it around his neck, activating the locking mechanism, it becomes something that can be lethal to the child.
It's why, until I'm sure beyond any doubt or impulse that my child would not do that EVER... I will lock those types of belts on young booster riders.
I will, on rearfacing restraints (as this type of thing can also happen if a baby/toddler uses the foot to pull the belt... and it can do something less bad than strangling... it could bind around the foot/leg or an arm/wrist, which is still very dangerous) always (and I do advise others to do this) lock the shoulderbelt, even if they use a locking clip or built in lockoffs to prevent the tipping (I simply advise not retracting it to the same degree of snugness you would if not using the locking clip/lockoffs... aka... after you have gotten your knee/hand out of the seat... pull all the way out.... let it retract on it's own... so there's still a tiny bit of slack that would normally be too much, but, since the locking isn't for holding the seat in tight, it's fine for preventing baby/toddler getting injured).
It's why I don't discourage and actually support and encourage my boys, who are now old enough that I would say neither of them needs to for themselves) lock the belts so that their sister (or others, if they aren't in a car where I'm there and know that the other kids won't do it) won't try to bug them by pulling at the belts (or just play with them for another reason).
Now, Ruthie is getting more mature.... I'm not having to lock dh's passenger seat's belt when she rides with him on occasion (she used to pull it to herself with her foot and try to play with it, so I buckled it and locked it...all but the driver's seat will lock, so I just do NOT put her behind the driver's seat... even when buckled, there is belt you can reach)
I think when she's 5, I will be able to not do that, and remind my dh to do that.... If it were a bigger car, I wouldn't have to bother... but in that tiny space, she can reach it, so it's an extra thing to train her about.... and she doesn't have the best impulse control when that behavior is something a certain 8yr old (though he got the message on it not being funny when I told him what all I could happen) thought was funny last year!!