How about this?
We never know what parts of our books will agitate people enough that they will send us an email. In Freakonomics, the passages that inspired the most emails were the discussions of swimming pools vs. guns and real estate agents not always acting on behalf of their clients.
In SuperFreakonomics, far and away the most common subject of emails is drunk walking vs. drunk driving. ...
The other thing about that passage that makes people angry is that they interpret our arguments as condoning drunk driving, despite the fact that we cite my own research that shows that drunk drivers are 13 times as likely to cause a fatal crash. We end by telling people to take a cab.
When we wrote that people should take a cab, however, we never actually did the calculation. Is it really true that taking a cab is the right thing to do?
According to our estimates, there are 21 billion miles driven drunk each year, resulting in 13,000 fatalities. That works out to be about one fatality for every 1.6 million miles driven drunk.
Economists typically use a value per statistical life of $6 million. So, for instance, when trying to decide whether the benefits of a government program outweigh the costs, the benefit per life saved is calculated as being worth $6 million. If one person dies for every 1.6 million miles driven drunk, and the value of a life is $6 million, then the cost in extra deaths per mile driven drunk works out to be about $3.75.
That number is an average. Obviously, it will depend on how drunk you are, and it ignores other risks associated with driving drunk like getting arrested.
How much does it cost to take a cab? According to this web site, a three-mile cab ride will cost you about $8 plus tip in most major cities. After a tip, that is about $3 per mile — not too different than the implied cost per mile of driving drunk.
Obviously, I’ve left out all sorts of other costs and benefits in this simple analysis that could tip the balance one way or another, but I suspect most people will be surprised to see how close it is to a toss-up.
via freakonomics.blogs.nytimes.com
Here is the comment that I left at the Freakonomics blog:
A well known problem with using the benefit-cost ratio is that it ignores scale. While true that the benefit-cost ratio is only 1.25 when individuals are choosing to take a cab (and likely lower since the loss of life is often imposed on others, not internalized), the net benefits should be scaled upwards by 21 billion miles. The public net benefits of taking a cab are $15.75 billion. If I've gotten the calculation correct it is hardly a "toss-up."
Update: Another problem with the benefit-cost ratio is the variation in the bottom line number based on alternative categorizations of benefits (negative costs) and costs (positive benefits). Consider the example above. It considers the cost per mile per cab but leaves out the cost per mile by private car, which is about $0.54/mile according to AAA. Suppose you count this as a benefit (cost avoided) of taking a cab. Instead of a benefit-cost ratio of 1.25 per mile you have a ratio of 1.43 per mile [=(3.75+.54)/3)]. But if you factor this in as the net cost of taking a cab then the ratio is 1.52 per mile [=3.75/(3-.054)]. Either way, the analysis above biased in favor of making the benefit-cost ratio close to one and therefore more controversial for little apparent reason (other than to push buttons).