The IIHS booster seat fit study – the “best” fitting booster seat?


As most of you are now aware, the IIHS has released the findings of their “fit” study on the effectiveness of Belt Positioning Booster (BPB) seats.  In this study, they did not actually crash test any of the various boosters seats – they just evaluated how the different boosters positioned the lap/soulder seatbelt on the 6 year old Hybrid III crash test dummy.  While I applaud the Institute for taking this initiative it is important that we, as parents, CPS technicians,  and advocates learn to assess what constitutes a “good belt fit” and what doesn’t.  Memorizing a list of “Best Bets” and “Not Recommended” choices isn’t very helpful overall, although I do think that some of those manufacturers who received several poor ratings for their products might want to seriously reevalute their designs.  Honestly, we’ve been saying for years that most combo and 3-in-1 seats do NOT do a very good job of positioning the seatbelt properly when the seat is used in booster mode.   Maybe now we’ll finally start to see some improvements in these suboptimal designs.  We can hope, right?         

Let’s try to tear ourselves away from those lists and talk about how to assess a good belt fit with a BPB.  We can call it CarseatBlog’s “6 Step Plan to Maximize Booster Seat Effectiveness”.    

Step #1:  Assess.  Is the child actually ready to use a BPB safely and effectively?  This is a tough one and parents/caregivers really have to make that assessment carefully.  I could write an entire thesis on this one point but for the sake of time and blog space, I’ll spare you today.       

Step #2:  With the child sitting all the way back in the booster, assess whether or not the legs are long enough for the knees to bend comfortably over the edge of the booster cushion.  If the booster is too deep from front to back for the child’s legs, do not use this particular BPB.  Look for a different booster that isn’t as long from back to front.  Some boosters are much better suited to older kids (7+)  with longer legs because of how “deep” they are.  Putting a child in a BPB when they can’t bend their knees comfortably over the edge is similar to putting a child in just an adult seatbelt when they can’t bend their knees comfortably over the edge of the vehicle seat.  Both will promote slouching that can compromise belt fit and increase the risk of serious injury in a crash.   

Optimal booster seat depth prevents slouching

Optimal booster seat depth prevents slouching

Step #3:  Assess the fit of the lap portion of the lap/shoulder belt.  It should be low and touching the top of the thighs.  It should be no where near the abdomen.  Remember, it’s a lap belt not a belly belt.  The strong hip bones can take the force of a crash.  However, if the lap belt is riding too high and doesn’t contact the hip bones, it’s going to tear through the soft abdominal area until it reaches the spinal cord way in back.  

Optimal lap belt fit low, touching tops of thighs

Optimal lap belt fit low, touching tops of thighs

Step #4:  Assess the fit of the shoulder belt.  It should be positioned so that it *makes contact with the child’s chest* and the belt should cross at the collar bone.  The shoulder belt should not be touching the neck and it shouldn’t be so low that it’s crossing near the edge of the shoulder either.   Some backless boosters (but not all) come with a little strap and a clip that can be used to help lower the shoulder belt if it’s still too close to the child’s neck even when the child is sitting in the booster.  Personally, I recommend against using a backless booster if the child is still too small to properly fit in the shoulder belt even when sitting on the booster.  In this case, the child would be better off using a highback booster (HBB) which has a built-in shoulder belt guide.         

Optimal shoulder belt fit
Optimal shoulder belt fit

Step #5:  Assess whether or not the child can stay in the proper (crash-ready) position even when sleeping.  Most kids (and even a lot of adults) simply cannot remain upright while asleep without leaning against something.  A highback booster should provide some level of sleep support but the amount of support can vary widely depending on the model.  Some highback boosters offer almost no support for sleeping kids while others provide a great deal of support.  Also consider whether the child will need a little bit of recline in order to stay upright while asleep.  Does the booster manufacturer allow the product to be used in a slightly reclined position (if that’s even possible depending on the design)?  Most BPBs  with an adjustable back will only allow the booster to be used in a slightly reclined position if you can adjust the vehicle seatback to match the recline.  In other words, the back and the base of the booster are usually required to be flush with the vehicle seat.   Only a few select BPB models can truly claim to have a recline option.

Step #6:  Assess if the child has sufficient protection in a side impact crash.  If the child has a seated height of less than 31″ tall (measured from bottom of tush to top of head), which is the seated height of the 5th percentile female dummy,  then you cannot assume that the vehicle’s side curtain airbags will be able to protect the child’s head in a side impact crash.  Maybe it will but the truth is that many vehicle curtain airbags do not deploy down far enough to protect the heads of child occupants.  Therefore, if the child has a seated height of less than 31″ tall when sitting in the BPB – a highback booster that offers decent side impact protection (i.e., deep head wings, energy absorbing foam, rigid structure) could provide much needed additional protection in a SI crash even if the vehicle is equipped with side curtain airbags.   

Alright, now let’s recap.  How do we determine which booster seats are likely to provide good crash protection for specific children in specific vehicles?  First we assess, then we assess, assess and assess.  Then we assess and assess some more.   And if one booster seat doesn’t seem to meet all the child’s needs then you keep searching until you find one that does.  If you use a different car – you have to reevalute.  When the child grows larger and taller – reevaluate.  It’s highly unlikely that any one booster seat is going to provide the best fit for a child when they’re 5 years old and also when they’re 10.  You wouldn’t expect a 10 year old to ride the same bike that he/she rode when they were 5, would you?       

Going back to the fit study for a moment – we know that it can’t gauge the true effectiveness of all BPB models for all children (in the entire weight and height ranges of the individual products), in all vehicles.  Although I do think the IIHS was probably able to pinpoint some suboptimal designs and hopefully that will make a difference in the future.      

“No matter how a booster did in our evaluations, parents still need to see how it fits their child in their car,” Lund advises.  He urges them “not to rush to buy a new booster if theirs isn’t among the top seats. Check how it fits and remember, it’s better for children to ride restrained in any booster than to let them ride unbuckled.”

Absolutely.  I don’t think anyone would argue with Mr.  Lund’s comment there.  But I think he does a disservice to parents and children everywhere by suggesting that they shouldn’t be overly concerned if their booster seat isn’t doing what it’s supposed to do.  Sure, something is always better than nothing but we really shouldn’t tolerate mediocrity when we’re talking about products that we rely on to protect our children under the worst possible circumstances.


  1. Sylvia Giussani February 4, 2010
  2. CPSDarren December 23, 2009
  3. Kat October 3, 2008
  4. skaterbabscpst October 2, 2008
  5. Nisi October 2, 2008
  6. murphydog77 October 2, 2008