Although we do not yet have comprehensive enough data to confirm how much safer it is to keep kids harnessed, it just makes sense. Afterall, it has been a standard safety measure for grown adults in the racing industry, for pilots, astronauts, etc. People can and have been killed or severely injured in crashes at just 12 MPH. If a child still fits a harness seat according to the manufacturer's recommendations, then that child should not yet switch to a booster. A booster is only safer than a harness if the harness cannot be used correctly and then only if the booster will be used correctly. (If a child no longer fits a combination seat's harness limits, then he/she must use it as a booster at least until you have a more appropriate harness)
Crash test comparisons....
FRONTAL impact: harnessed vs. booster
SIDE impact: harnessed vs. booster
Remember that each "graduation" in seat type is actually a demotion in safety. If a child in a booster still finds that the shoulder belt crosses the neck/face rather than lying flat on the collar bone, then this child needs a high weight harness seat. If a child cannot or will not use a booster correctly for the entire length of every ride, then that child needs a high weight harness seat.
IS YOUR CHILD READY FOR A BOOSTER
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Don't you want to keep your young child harnessed at least until their skeletal structure is developed? Remember: bones do not finish developing until puberty! Brain scan imagery reveals that kids do not develop the cognitive synapses associated with the attention span necessary for correct booster use until about 5 years old.... It's just not worth the risk to diminish your young child's chance of survivial by switching to a booster prematurely when there are options out there that will reduce the risk of injury by staying harnessed until kiddo meets both physical & maturity requirements for a booster. |
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Cervical spine developmental comparison : 2 year old (left) vs. 6 year old (right)

Thoracic spine development comparison : 2 year old (left) vs. 6 year old (right)

Lumbar spine developmental comparison : 2 year old (left) vs. 6 year old (right)
Tip: Switchable/Automatic Locking Retractor seatbelts can help remind wiggly kids to remain properly positioned in a booster, but some kids are still able to squirm out of position. If your vehicle does not have S/ALR seatbelts, it would be safest to keep a young child in an extended weight harness alternative or the Recaro Young Start booster that has built-in shoulder belt lock-offs.
4 years and (not or) 40 pounds is the minimum recommendation for "graduating" a child into a booster
Video Demonstrates What can Happen to a Child in a Crash Without a Booster Seat
Never use a lap-only belt with a booster
SIDE IMPACT CRASH TEST VIDEO COMPARISON
WITHOUT ADEQUATE HEAD PROTECTION
Although these are German crash tests with German booster seats, the laws of physics do not change -- if the booster does not contain the child's head the child will suffer severe/fatal injuries.
(more support for highback booster seats)
Tip: Backless boosters offer no side impact protection, so I suggest using these only in newer vehicles that have side curtain airbags and/or center shoulder belt with headrest. Dedicated boosters that have deep head wings filled with EPS/EPP foam (like in bicycle helmets) for side impact protection offer more ideal optimal protection, especially when the vehicle lacks side curtain airbags and/or center shoulder belt.
IS YOUR CHILD READY TO STOP USING A BOOSTER
- Does the child sit all the way back against the auto seat?
- Do the child's knees bend comfortably at the edge of the auto seat?
- Does the belt cross the shoulder between the neck and arm?
- Is the lap belt as low as possible, touching the thighs?
- Can the child stay seated like this for the whole trip?
If you answer "no" to any of these questions, your child needs a booster seat for optimal crash protection!
When the seatbelt is improperly positioned your child may suffer head injury (including permanently disabling brain trauma, paralyzing spinal injury & death) or Seatbelt Syndrome.... Backless boosters must never be used in vehicles without a built-in headrest!
Note: A parent and good friend of mine recently expressed concern that her child's head could be severed from the body so she tucked the shoulder belt under the armrest. This is a false fear! Children's heads are not severed from the body because of the shoulder belt. While every buckled occupant may experience rope burn type injuries on the neck during a crash, this is minor in comparision: children are much more likely to suffer internal decapitation, skull facture + brain damage, paralysis, etc. from not using the shoulder belt. Some crashes are simply unsurvivable for any vehicle occupant, but using boosters properly with the shoulder belt across the collar bone enhances the possibility of survival by well over 50%