why seatbelts aren’t X-shaped/haven’t changed at all
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They have. Some of the early ones used to be simple lap belts (at least in the US). Now an over the shoulder harness is pretty much mandatory.
There was a fair amount of resistance to the initial mandating of seat belts (at least in the US) and simplicity and ease of use (consider the average IQ) was needed.
Harness type seat belts arguably provide better protection in high speed crashes but most of that advantage has been mitigated by the increasing sophistication of air bags. So the benefits are now fairly limited in all but the most extreme cases – which is why these types of seatbelts are required mostly for racing and high speed track use.
They are also very difficult and inconvenient to use properly. A proper harness has to be VERY tight – more so than an average person would be comfortable with. It takes a long time to put on (20-30 seconds perhaps) and severely restricts movement (as it should to function) It is more than likely that having them installed in a car would overall REDUCE the use of seatbelts which is contrary to public safety.
They are like that for infants, as their smaller bodies are more likely to slip out of a simple three-point seatbelt in the event of a crash. Car seats for infants and young children usually have a five-point harness system.
For adults and larger children a three-point belt is more likely to be sufficient. Of course, a harness would be even better but at some point it’s a compromise between convenience and pure safety. A three-point seat belt that goes over the lap and torso can be secured with a single, quick action, and compared to a harness it is more comfortable and doesn’t restrict motion as much. This also makes it more likely that people will actually use them, and a seatbelt you use is always safer than one you don’t.
Because one of the challenges of seat belts is actually getting people to use them.
3 point seat belts along with air bags is about as far as we have reached, in terms of people actually using the system as well as the system’s ability to actually protect people. Anything less would be less effective (duh), while anything more carries the risk that people would avoid taking the additional steps, which would also result in less protection.
Before airbags were common, a common feature of cars was a shoulder belt that stayed attached but automatically moved on and off the occupant when the car was started and stopped, respectively. (see, for example, https://www.youtube.com/watch?v=nvS0hhVFdZE ) Aside from the fact that that system has safety issues of its own, it was found that, because the shoulder part of the seat belt was automatic, people would often skip the step of attaching the lap portion of the belt, which is unsafe.
So back to your suggestion of an X shaped belt, or a 5 point belt, etc. The reality is that people aren’t going to take the time to completely secure those belts. Not when the existing system only takes like 1-2 seconds.
3 point plus an airbag is better for you if you don’t intend on wearing a helmet plus HANS device every time you get in a car. The problem with 4/5/6-point harnesses is that you run an increased risk of internal decapitation because you hold the torso rigidly against the seat back but the head is still free to move forward. This is how Dale Earnhardt Sr died.
3-point allows your torso to pivot around the shoulder belt and move with your head into the airbag.
You put small children into a 5-point because they can’t take it off & don’t get airbags. A seat belt is worthless if not worn properly, and little kids will squirm themselves out of it. You put really small kids in a rear-facing seat to keep their head & torso aligned in the event of a heavy deceleration, like hitting something.
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