Where did 4 & 40 come from?

I understand there is research connected to the 1&20 law, and research for the 2 year recommendation for RF. Why is 4 & 40 the bare minimum age to booster? Is there any reason that became the standard minimum? Is there a study connected to it?

Thanks!

My guess is that the average sized kid is 40 pounds at 4 years old and once upon a time, most car seats harnessed to 40 pounds.

yep... once upon a time, seats only harnessed to 40lbs... and so when you're looking at ideal at that time (also when we needed all boosters to start at 30lbs), the average age for hitting 40lbs was 4.

However, it's still a number most of us are comfortable with mostly because there still are so few seats out there that harness longer that EVERYONE can afford... but with the maestro, those ranks are closing... if it made a good booster longer (IE, went taller), we might be able to convince everyone to wait to booster.

So it's really just an outdated rule based on necessity, if I'm reading correctly. That makes sense to me.

It is also my understanding that submarining is also more likely under 40lbs

It was... but it stuck because 4 is still the minimum that kids are mature enough for a booster... and 40lbs is when they are less likely to submarine (as opposed to 4&30)

Of course, around here, we aren't ones for minimums.

yetanotherjen said:

It is also my understanding that submarining is also more likely under 40lbs

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I keep reading that here, but I've never seen a study about it, have you ever run across anything? (All I've ever seen is that small kids in boosters mostly suffer from head injuries, not as often abdominal injuries, though seatbelt syndrome from using no booster at all is way, way worse for littler kids, and possibly in inferior/improperly used boosters...?--given that over a third of boosters are used wrong, it may be just the littler kids at worse risk of submarining? Just brainstorming...don't mind me )

what if you have a child that is 40 pounds and/or 4 or close to those minumums - If they have the maturity to sit properly, is there harm in them being in a booster?

Note: I am not proposing that I move a child into a booster, just curious...

There is one study on submarining I think, I've seen it linked several times. But I'm not sure where it is...

I don't know many 4 year olds genuinely mature enough to sit properly, sadly. I much prefer 5 as a bare minimum, and that's what I tell parents-- with the information as well that it's 6 or closer to it for many kids, and even later for some.

mykidsmom said:

what if you have a child that is 40 pounds and/or 4 or close to those minumums - If they have the maturity to sit properly, is there harm in them being in a booster?

Note: I am not proposing that I move a child into a booster, just curious...

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I wonder that too. Can anyone answer that? If a child is barely 4 and 40lbs and is mature enough to sit properly for the whole trip, is it really acceptable? Or is there a real reason why it is not? Like, is there anything to show their bodies can't handle the force of a 3pt vs. a 5pt harness until they are a certain age?

(saying "4 year olds aren't mature enough" isn't an answer. Let's assume they are mature enough)

My DD is 4 & 40lbs. I have not had her ride in a booster yet because I gathered here that it's a :thumbsdown: and we have plenty of other options for her. But I'm still curious.

I thought it had to do with several things:
- maturity
- 40lbs decreasing the likelihood of submarining (but now I hear that may not be true)
- children's spines mature between the ages of 4 and 6. 4 is when their bodies begin to take on more adult-like proportions.

This study ONLY compares RF and boostered children, and then children moved to a belt too early, but here's a relevant paragraph...

http://www-nrd.nhtsa.dot.gov/pdf/nrd-01/esv/esv19/05-0330-O.pdf
"As can be seen in Figure 9, there is a
noticeable increase in MAIS 2+ injury rate if the
growing child switches from rearward-facing to
a forward-facing booster at around 3 years of
age. The injuries to the 2-4 year-olds in boosters
are mainly head injuries. Two children in frontal
impacts sustained spine fractures; one of them a
combination of fatal head and neck injuries. The
injury rate in a booster decreases somewhat
when the child grows older. At the switch to the
adult belt only, between age 7 and 10, there is a
remarkable increase in injury rate. The injuries
for these children are spread over the whole
body, with a distinct difference in abdomen
injuries, which are only seen for the belted-only
children. More than half of the MAIS 2+ injured
belted-only children aged 7-10 had AIS 2+
abdominal injuries"

So THIS study finds no abdominal injury in boostered kids. (I'm googling, I can't find the one that does...Oh, how about this one? It says the same thing... "Face and lower extremity injuries were the next most common of kids in booster seats, at 9% and 8% respectively. For injured children in seat belts, abdominal injuries and face injuries were the next most common (12% and 9%)." http://www.webmd.com/parenting/news/20091019/car-booster-seats-cut-injury-risk-for-kids

I know back in the olden days, shield boosters in the US and Canada had different weight limits (US 30-40, Canada 40-60) because smaller children were submarining out, and Canada had stricter standards, is that where that came from? If not, I hope that someone can find a link to that study about littler kids submarining

Here's some info that talks about shield boosters and mentions the 30-40lb range, and recommendations that children under 40lbs should be moved into a HBB with an internal harness. Does this site (CPSafety) belong to someone here?
http://www.cpsafety.com/articles/shieldbooster.aspx
Couple of snips, but was this because at the time ALL boosters had a 40lb min??:

American Academy of Pediatrics: Children who weigh 40 pounds or less are best protected in a seat with a full harness...
...
University of North Carolina Highway Safety Research Center: A small shield does not provide nearly as much upper body and head protection as a full harness does for the 30 to 40 pound child. Also, a small shield does not provide as much upper body and head protection as does the lap/shoulder belt used with a belt-positioning booster...a 30 to 40 pound child should remain in the full harness seat rather than being switched to a booster.
...
It has never been recommended that a child under 40 lbs use a booster (with or without a shield).

[one isue is that children can...] Submarine UNDER the shield and have spinal/neck injuries or are actually decapitated.

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