Freakonomics did a good Ted Talk about why we probably don't need car seats for kids who are 7: https://www.ted.com/talks/steven_levitt_surprising_stats_abo...

And, did a follow up on exactly what you're talking about, although points out that their results differ from some other analysis: https://freakonomics.com/2005/07/09/more-evidence-on-car-sea...