This is only evidence of my naivete, but I am absolutely astounded by the statistics in that paper. The levels of inappropriately restrained kids are just startling.
I can understand how some well-intentioned parents could nonetheless get confused by their instruction manuals and make common mistakes, such as "borrowing" LATCH anchors in the center seating position when not permitted, using the wrong set of shoulder slots for RF vs. FF, etc.
But to allow a child to ride totally unrestrained? Or a tiny tot in just a booster seat and/or in the front seat? What are people thinking?
The data for the paper were collected 2007-2009 so in some ways it is not surprising that there weren't more 1-3 year olds rear-facing. Our own pediatrician, who is otherwise excellent, was still instructing us on the "1 and 20" rule at that time.
Americans don't like to talk about race and class but the disparities in the paper are also noteworthy. The regional differences are interesting, too, with kids in the Northeast 2-3 times more likely to be appropriately restrained than in the South or Midwest.