Innumeracy on the Metro
The Washington Metropolitan Area Transit Authority — “Metro”, for short — has had an ad up in its rail cars for the last couple months hyping how reliable the bus fleet is. Yes, this is part of the same series that brought down complaints of sexism, but I have a different complaint: even if you’re not a woman, Metro still thinks you’re stupid and innumerate.
In either version of the ad, one person notes to the other that “a Metro bus goes on average 8,260 miles before a breakdown.” And that seems like a pretty big number, all right, which is just what the ad wants you to think.
One way to critique this ad is by comparing to a more familiar domain; I’ve put about eight times as many miles on my current car and never had a breakdown, despite being less than scrupulous about scheduled maintenance.
But then I’m usually driving big highway miles, not all day, every day, in stop-and-go city traffic, so maybe this isn’t a fair comparison. Besides, we can’t expect every public-transit customer to be have even that much familiarity with automobiles.
There’s another way to peel back the wool this as tries to pull over our eyes, though: the numbers. Putting a big number like 8,260 in the as copy is designed to make your brain shut off and give up, but let’s push forward a bit.
WMATA runs about 160 bus lines. Let’s say that each one uses only one bus; obviously they use more but I’m trying to be generous to Metro here. Let’s further say that each one runs for five hours a day. This again seems an underestimate; five hours barely covers both rush hours, and if there’s some line that does run even less the slack will be taken up by our huge underestimates elsewhere. Finally, let’s say each bus averages eleven miles per hour, which again seems to be a low guess.
Now we have 160 bus lines with 1 bus per line, running for 5 hours per day, at 11 miles per hour. This gives us an estimate of 8,800 bus-miles per day. And remember that this is a dramatic underestimate; the real number is certainly much larger.
So what does this mean? Metro’s own ad gives an average of 8,260 bus-miles per breakdown. That is, they expect at least one bus to break down every single day, and likely more than that. Doesn’t sound so reliable now, does it?
But Metro is counting on you not realizing that this is what their ad really means. They expect you to say “big number; must be pretty good” and think no more about it. And, depressingly enough, they’re probably right. Most of the riders who see this ad simply don’t have the tools to take it apart like this.