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My wife and I have seen many parents in a rush buckle their kids in a car seat despite one of the straps being twisted (I've even done it myself a few times- don't tell my wife- because I had a hard time believing it was that big of a deal.)

I did some research and found many articles online claiming that if the seat strap of a child restraint car seat is twisted it makes it far less safe.

  • Tums 2 Tots:

    It is very possible to miss twisted straps when buckling your child in but this can be very dangerous.

  • Rear Facing Toddlers

    If the straps are twisted they can't distribute the crash forces evenly.

  • Advocate Health Care

    children can be at risk of being ejected from their seat if the strap is twisted

To me these claims seem unscientific. Has anyone done research comparing the effectiveness of a car seat when the straps are straight vs. when they have a single twist?

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    "To me these claims seem unscientific." -- I am a bit surprised, as they make perfect sense to me. Less contact area, more likely to rip / tear, more likely to jam leaving the belt slack, or defeating seat-belt tensioners... – DevSolar Jun 13 '17 at 7:16
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    They are "unscientific" in the sense that they don't provide any evidence or justification of their claims. That said, it sounds exactly like what you'd expect. Twists would ruin the force distribution and lead to stress concentrations that could break a belt easier. – JMac Jun 13 '17 at 11:06
  • @JMac: I don't really expect instructions of use to come with references to scientific studies; that doesn't imply that safety tips are made up of whole cloth. Perhaps it's a different understanding of the term "unscientific". – DevSolar Jun 14 '17 at 6:43
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    @DevSolar No, but you also shouldn't blindly trust information that websites seem to be regirgutating without support. I understand the skepticism. Based on the answers there doesn't seem to be a lot of direct evidence for these claims; just a lot of indirect evidence. It's not a bad reason to be skeptical, but the answers seem to do a good job showing why that statement may have been made on the sites. – JMac Jun 14 '17 at 9:28
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I found a study (Misuse of child restraint systems in crash situations-danger and possible consequences) that tested how serious different types of child restraint system (CRS) misuse are. They measured the impact on several different "body segments": head and face (H); neck and cervical spine (N); chest (C); abdomen and lumbar spine (A); and lastly upper and lower limbs (L).

For each of these body segments, a risk factor has been given, from 0 to 3. A score of 0 means that neither films nor dummy measurements have shown an additional risk of severe injury. When a 1 is given, it means that, in the misuse situation, a slight effect has been observed but that the situation remains acceptable. If a body segment received a score of 2, it is because a clear influence of the misuse on the child dummy kinematics has been observed on the dummy measurements or on the films and that it means a higher risk of receiving injuries for a child in that situation. And finally, when the situation was critical, a score of 3 was given.

Two of the tests involved twisted straps/seatbelts:

Forward facing CRS Harness
Group 1 - (9-18kg)
Dummy attachment

                            H   N   C   A   L
Harness twisted - 2 turns
            on each strap   0   0   2   0   0 

Booster cushion
Group 2 (15 – 25 kg)

                             H   N   C   A   L
Seatbelt twisted – 2 turns   0   0   2   1   0 

However, there is another danger—a twist can prevent the belt from being properly tightened, causing slack (as this study points out). Slack, in general, is very dangerous, since the child can be ejected from the carseat and hit their head (the first study has numbers for this too).

  • Thank you so much. I was wondering more about the situation that normally occurs which is that one of the straps is twisted once and a parent in a rush doesn't bother fixing it. I'd imagine that if they were that twisted any parent would fix it. – Eliyahu Jun 14 '17 at 3:05
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I found evidence that appropriate installation of car seats is better than inappropriate installation. I did not find any data that addresses your claim about twisted belts specifically.

I did a rather frustrating search of the scientific literature. Your claim is fairly specific. A lot of the literature talks about inappropriately installed car seats, but they don't usually go into detail on exactly how each car seat in their data set was inappropriately installed. I frequently ran across the claim that inappropriate use of safety seats is both extremely common and dangerous [1,2, and others]. When I followed the sources cited to back up these claims, I ran into paywalls despite my university credentials.

This paper pointed out a confounding factor in studies that compare injury rates with and without proper car seat use:

Because sampling was based on the medical treatment obtained after the crash and thus the likelihood of an injury, subjects least likely to be injured were underrepresented in the study sample.

In other words, if you strap your child in properly, you might avoid the hospital altogether, and therefore avoid being in their study. That paper used statistical techniques to try and control for this effect. These two papers collected data, but declined to make comparisons. Is suspect that their relatively small sample combined with this confounding effect were why they declined to make comparisons. All three papers state that proper safety use is important.

This report from the American National Transport Safety Board has a ton of data about child safety seats, their effectiveness, and their misuse. This figure from page 69 compares accident rates in properly and improperly used seats. The children in the improperly installed car seats were more likely to be severely hurt or killed. In the properly installed car seats, none of them died. In the high severity accidents 5 of 16 children, 31%, in the improperly installed car seats were ejected from their seats and died. I do not think these children had twisted straps.

Proper vs Improper safety seats

Although this data supports a broader version of your claim, it is not particularly high quality. The effects of car seat installation on injury rates was not the primary objective. The data here has a small sample size, because it is a subset of a larger broader data set. I suspect that the assessments of accident and injury severity are difficult to accurately measure.

This quote supports the claim indirectly. "Many restrained children survived very high-speed crashes without injury to the neck or other parts, with deceleration injuries confined to bruising from belt loadings." A seat belt is supposed to spread the force of the crash over its area. I would think that a twisted belt has lower area, and therefore higher pressure, and probably worse bruising.


Speculation:

Despite the lack of specific evidence, the claim seems plausible. In adults seat belt injuries are fairly common even when seat belts are worn properly. All of the force of the crash is concentrated in the area of the seat belt. (Seat belt injuries are far less severe than injuries from being thrown around a car unrestrained. Wear your seat belt.) Many of the properly restrained children in the NTSB report were still hurt. All seat belts, for adults and children, are designed to be used untwisted. My hypothesis is that twisting the seat belt can concentrate the same amount of force over a smaller area, and presumably result in more severe seat belt injuries.

If my hypothesis is correct a twisted restraint belt ought to be somewhat worse than a untwisted one, but still way better than no restraint at all. The effect of untwisting a restraint is small compared to the effect of wearing one. In any scientific study large effect sizes are easier to measure and prove. The studies reviewed are able to find an effect despite their issues with confounding factors, imprecise measures of severity, and problematic reporting methods. The size of their effect is presumably bigger than the noise. For a small effect such as a twisted seat belt, the study might have to be huge.

However, many these limitations on studies don't apply to laboratory tests or simulations. It is possible that someone experimented with twisting the seat belt on a crash dummy. These people could have, but didn't. It is also possible that someone used a virtual crash dummy to examine the same question. I am out of time to research this question, but if another SE user has time, I suggest starting your search there.

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