More Evidence on Car Seats vs. Seat Belts

Things move quickly in the modern world. Within two hours of posting my academic paper on car seats vs. seat belts on the Freakonomics web page (the first time this paper had seen light of day), another economist found the paper and tested its hypotheses on a very different data set and reported back the results.

The economist is Paul Heaton, a graduate student at the University of Chicago and a former co-author of mine. (Our affiliation is largely irrelevant here — he just happened to have data handy to test my results — but I mention it in the interest of full disclosure). The data he has are from New Jersey motor vehicle crashes. The big difference between his data and mine are that he has all crashes (even if no one dies. I only had access to crashes in which someone died. This difference is important, because a concern in the fatal crash data is what economists call “sample selection.” The choices people make about what safety device to use will affect whether they die, which in turn affects which crashes I see in my data.

Heaton replicated the most basic specifications in my paper. His results are remarkably similar. He found no difference in the death rates or incapacitating injuries for children in car seats versus children using adult seat belts. Like me, he found a slight advantage for car seats in preventing non-incapacitating injuries relative to adult seat belts (car seats offered a 10% improvement for these less serious injuries in both of our samples). The only difference is that in his data the gap in injuries between car seats and seat belts is statistically significant, whereas in my data set it was not.

When I compare my findings to the existing NHTSA estimates on fatalities, I can see how our approaches differ and why we get different answers. What is more puzzling to me is why my results and Heaton’s both suggest very little injury benefit of car seats, but the medical literature often finds 70% (!!) reductions of injuries with car seats relative to seat belts. We find reductions that are an order of magnitude smaller. They use very different methods — surveying people in the weeks after crashes for instance — but still it is really a puzzle. Which is why, when you read my paper, I am extremely cautious in interpreting the injury findings.

I hope that the medical researchers, Heaton, and I can all work together to try to make some sense of the conflicting results being generated by these different methodologies to resolve this important question.

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  1. Chris Adams says:

    Having almost died at the age of 4 (in 1973) when I was thrown from the backseat into the windscreen of a car without backseat constraints, I am always interested in this question.

    Some questions:
    1. Is the law on child safety seats actually enforced?
    2. How much variation is there in the strictness of the law. I note that Australia requires harnesses (attachments from the seat to the car) for child safety seats.
    3. Obviously the weight and size of the child are important. You account for this with age, did you look at what happens if the sample is split 2-3 and 4-6?
    4. Why would any parent put their child in a car without a car seat, and what type of sample selection bias does this cause?

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  2. Holly says:

    Great paper Levitt! I will soon have my B.A. in economics, and I was able to read this paper and recall terms from an econometrics class that I have recently taken. Thank you very much for posting it! Aside from that it was interesting to see the data you collected. Based on the topic I would have suspected a different outcome, however the data speaks diferently.

    And those are some great questions Chris Adams!

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  3. Anonymous says:

    Thanks for publishing this blog. I have been reading it for awhile now and I am hooked.

    Various questions regarding automobile safety have interested me for some time, particularly those related to the general question: are SUVs or passenger cars “safer”?

    My wife is advocating we buy our 16-yr-old a passenger car because she has seen statistics that imply injuries and fatalities in accidents involving SUVs are higher than in those involving other types of cars. I, on the other hand, have a hard time believing that passengers in an SUV being driven by a responsible individual are in greater danger than those in a Corolla, for example.

    There are those who cite statistics from various government sources indicating that injury and fatality rates are higher per mile driven in SUVs than in other passenger cars. This point is made in an article found in a recent Sunday issue of the NY Times.

    But this view (based on these statistics) seems to me to have several possible flaws, among them:

    (1) the statistics cited talk about auto fatalities involving SUVs, but they aren’t clear as to whether the fatalities refer to the passengers of the SUV, or the other vehicles involved in the accident.

    (2) it’s not clear whether SUVs experience a higher rate of collision than other vehicles (It is possible that the data has “sample bias”. For example, drivers of SUVs may be more aggressive or less careful than drivers of other vehicles)

    (3) statistics regarding SUV fatalities are largely affected by the incidence of roll-overs, which strike me as operator errors. The circumstances required for a roll-over to occur seem to me to be the result of reckless driving – or – extreme actions taken in an attempt to avert some sort of impending disaster. (But even in those instances in which extreme actions are motivated by an impending danger, there is no guarantee that the same danger that provoked the extreme action causing the roll-over would not have caused some other sort of collision or accident.)

    In short, following the “Law of Proportionate Belief” put forth by Arnold Kling, I find it hard for me to believe that a cautious driver abiding speed limits and common sense (and therefore not truly subject to roll-over risk under normal circumstances) but subject to the other dangers of the road (drunk or other dangerous drivers) would be safer in a Corolla than in an SUV.

    Perhaps an interesting subject for you to explore some time.

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  4. Anonymous says:

    Unless you find significant claims that seatbelts are BETTER than car seats for children 2 and up, the cost of car seats is irrelevant.

    Sure, they cost $200. But the cost is a sunk cost when your child turns 2. So, thinking on the marginal level, if there is no difference in safety, and you already own the hunk of junk, why not just leave it strapped in?

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  5. Anonymous says:

    Is there anyway to figure out if there correlation to owning the carseat and strapping the child in. in other words if people who have a car seat are more likely to have the child strapped in, possibly caused by just seeing it there empty, or the child is more likely to stay happily strapped in because they can see above the window line and into the front seat. As for the person who above who says that the money is already spent. If there are two cars it would be nice to not have to schlep the safety seat back and forth.

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  6. Anonymous says:

    To the blogger with comments with questions about SUVs versus passenger cars, here is a link to an interesting paper by an economist at UCSD that examines many of the issues you raise in your post:

    Link to paper on SUV safety

    She finds that SUVs are safer for occupants but much more dangerous for the passengers of other cars involved in SUV accidents.

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  7. Jim Voigt says:

    So I guess I’m not an economist if I think it’s a good idea to spend the $200 on the car seat even if it only on the “chance” that it will do a better job of saving my child’s life.

    To put it another way, do economists seriously stare at the selection of car seats at Babies-R-Us and wonder to themselves whether the marginal difference in safety is really worth the money? Perhaps this just integrates a little too much reality into the otherwise wholly real-world-detatched science of economics. Ironic considering its claim to present “the world as it really is”.

    A quick technical question, though. Does your study include kids all the way down to infancy? I assume a child seat is safer than mom holding baby in her lap in the front seat.

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  8. Anonymous says:

    DOn’t you have to buy a larger seat as the child gets older? Then first seat would be a sunk cost. If the extra seat only had a marginal safety feature, then it may be a waste of money. It would be similar to paying extra for airbags or something.

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