gwern comments on Wear a Helmet While Driving a Car - Less Wrong

47 Post author: James_Miller 30 July 2015 04:36PM

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Comment author: gwern 17 January 2016 02:44:37AM *  4 points [-]

How much do these helmets reduce risk and how much do they cost to buy & use? That's tricky to answer, but maybe some bounds will be helpful.

\$685 in present value is roughly equivalent to \$27 spent each year over the next 50 years, discounting at 3% (sum(sapply(1:50, function(t) { 27 * 0.97^t }))). So even something which reduced your car TBI risk to zero would not be worth paying more than \$27 each year for the rest of your life. The mentioned helmets all sound like they only reduce acceleration or energy somewhat, and Crasche is quoted by Dorikka as estimating a 25% reduction in impact (which translates to an unknown reduction in TBI risk); another quote claims seatbelts reduce death/injury by ~50%. Let's be extremely optimistic and go with the latter, that our TBI risk falls by 50% using a Crasche helmet. Then the gain from a Crasche helmet is 685 * 0.5 = \$342.

The cost of the Crasche helmet is ~\$30, leaving \$312. Let's assume there's never any replacements and we just need to consider the annual hassle of wearing it. Working backwards again, that leaves room for an annual cost of no more than ~\$12.3, which is small. I drive maybe thrice a week, so the per-trip cost of use needs to be <12.3 / (52*3) or <\$0.07. Alternatively, if we don't think the Crasche helmet is remotely sufficient, a much better helmet might cost more like \$100 up front, leaving \$242, leaving \$9.5 for annual expenses, or <\$0.06.

Personally, while I don't mind driving with a helmet as much as \$1 an hour (and so extremely high annually) like some people claim, I think I would mind a nickel's worth each trip, which defeats Crasche even with extremely optimistic assumptions on efficacy. If we wanted to make Crasche cost-effective, we could argue that \$100k/year is a better value, which will double estimates of benefit; or we could try to expand our sources of harm to include TBIs from other sources like falls (although that would also increase the cost of usage: it's one thing to only need to wear it in your car where no one will see, another thing to walk around routinely wearing it); or we could deny discounting, which increases the loss considerably and helps overcome the fixed present cost of buying the helmet. But to be fair, we'd also want to reduce the efficacy of Crasche to much less than 50%, take into account that we're wearing seat belts while a large fraction of TBI cases likely were not, consider the advent of self-driving cars in the next 15 years reducing human error rates, and overall, I'm not seeing much that looks like it could make a driving helmet the sort of slamdunk case that one can make for, say, vitamin D. As far as car safety goes, a helmet feels like it's going to be inferior to stuff like getting a dashcam, upgrading to an electric car with its bigger crumple zones & higher mechanical reliability (I understand the Tesla cars may be the safest ones on the market right now), saving up for self-driving cars, making a habit of reinflating tires more regularly, taking a defensive driving class, etc - to say nothing of much larger risks like falling getting out of the shower. (I installed some anti-slip treads after I did just that; feels like money well-spent every time I step out.)

The same also applies to walking helmets: falls are highly concentrated in the elderly and very young (while car TBIs are more evenly distributed) and you would need to wear a helmet almost 24/7 once you've guarded against ice and shower falls (increasing costs much more over car TBIs). So race-car drivers, football players*, people at much higher than usual risks? Probably, maybe. Regular people? Not really.

* although I would say after reading through all the football studies and these TBI studies, I would not, hypothetically, let my children ever play contact football and especially not highschool football. If they want to play sports, there must be safer ones they could try. Like BASE jumping.

Comment author: gwern 24 August 2016 12:47:20AM 1 point [-]

"Long-Term Outcomes Associated with Traumatic Brain Injury in Childhood and Adolescence: A Nationwide Swedish Cohort Study of a Wide Range of Medical and Social Outcomes", Sariaslan et al 2016, is a population registry study which reports within-family correlations adjust for education about various negative outcomes with 1 or more diagnosed TBIs representing 9.1% of the population (their twin sample was too small); within-family studies control for tons of the usual confounders (and indeed the correlations are smaller than if you had used the general population) and are probably close to the causal effect (possibly underestimating it since so many TBIs go unreported).

TBI is common enough that the effects are large on a population-wide basis:

We found that the crude population contributions of TBI explained approximately...2%–6% of the population differences in the outcomes. The strongest population attributable risks were found for the severe outcomes, including psychiatric inpatient hospitalisation (PAF = 5.5%; 4.9%–6.1%), premature mortality (PAF = 4.7%; 2.9%–6.5%), and disability pension (PAF = 4.6%; 3.8%–5.3%).

Interestingly, the effects of TBI get worse with age: the youngest age bracket (despite having lots of falls) has the smallest RRs while they increase especially for adolescents. (This was the opposite of what I expected.)

They find RRs of:

  1. disability pension: 1.49

    • Population rate: 3.9%
    • Lifetime cost: Sariaslan gives a quick opportunity cost estimate of "US$1.3 million per person" from the inability to work & earn a normal income over the next 30 years

  2. Psychiatric visit: 1.31

    • Population: 14.2%
    • Lifetime cost: ?

  3. Psychiatric hospitalization: 1.57

    • Population: 5.7%
    • Lifetime cost: ?

  4. Premature mortality (death age <41): 1.40

    • Population: 0.8%
    • Lifetime cost: tricky since it's not quite a RR on all-cause mortality: the age 41 is when they stopped following individuals in their sample. If we assume that the mortality RR remained indefinitely, then using my Gompertz curve functions, a RR of 1.40 for a 30yo would represent a loss of 3.09 years over the next ~50 years, for a 10yo a loss of 3.147 years over the next ~70 years etc. So ignoring discounting, ~\$150k from the mortality alone.

  5. Highschool dropout: 1.28

    • Population: 8.9%
    • Lifetime cost: somewhere upwards of $250k in the USA, estimates as high as $1m.

  6. On means-tested welfare benefits: 1.19

    • Population: 11.5%
    • Lifetime cost: Sweden is famous for spending a lot on welfare, but the welfare & public spending are quite diverse, this doesn't specify how long each individual is on welfare or whether it's household or individuals. http://ec.europa.eu/social/BlobServlet?docId=9044&langId=en suggests that a prevalence of 11.5% must refer to household welfare, and estimates that in 2008 the average number of months (from 1990 to 2008) was 6.1 months at 43987 SEK per month or \$5.2k or \$31.72k total on average.

So the undiscounted cost of 1 nonfatal TBI for a 10yo (why 10yo? so they have time to drop out of highschool) in Swedish America would be something like:

(0.039*1.49 -0.039)*1300000 + ? + ? (0.008*1.40 - 0.008)*3.147*50000 + (0.089*1.28 - 0.089)*250000 + (0.115*1.19 - 0.115)*31720 = >\$32,269

For the 30yo

(0.039*1.49 -0.039)*1300000 + ? + ? (0.008*1.40 - 0.008)*3.09*50000 + (0.115*1.19 - 0.115)*31720 = >\$26,030.482

Unfortunately, hard to see how to do much about this. The top category of TBI is normals falls, cars, then assaults, then miscellaneous: https://www.cdc.gov/traumaticbraininjury/pdf/blue_book.docx

An estimated average of 595,095 are fall-related TBIs, 292,202 motor vehicle traffic TBIs, 279,882 struck by or against events, and 169,625 assault-related TBIs occurred annually....Struck by or struck against events are those in which a person was struck unintentionally by another person or an object, such as falling debris or a ball in sports, or that someone struck against an object, such as a wall or another person. For this report, only unintentional and undetermined struck by or struck against events were included. Struck by or struck against events related to assaults (for example, being struck by a fist) are in the assault category. Struck by or struck against events were only reported for all ages because small sample sizes precluded reporting them for all three data sources (NHAMCS, NHDS and NVSS)

For 30yos, cars are 40% of TBIs, falls are 2.7% of TBIs, assault is 19% of TBIs, struck by by or against is 0.7%, and the ominous 'other' is 37%. So while we can probably ignore the 'assault' due simply to demographics (I suspect most readers here have low risk of crime), and car travel represents a fairly focused area of intervention, the 'other' can't be fixed except by always wearing a helmet, which would be difficult if only for social reasons.

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