I walk a lot. My commute has six miles of walking round-trip. My wife and I walk everywhere we can around town and as a way to share time and thoughts. I estimate I spend about 23,000 minutes per year walking, which is about an order of magnitude more time than the average American spends doing so. Interestingly, it is nearly equal to the amount of time the average American spends in a car. (Source: Chu, Table 2; hat tip: Julian Jamison.)

Thus, all other things equal, I have a ten-fold greater chance of finding stuff on the sidewalk than does the average American. One thing I find is cash, about $20 per year. I know it isn’t much, but putting the above information together with population statistics I can estimate how much cash is lost and found on the sidewalks and streets of America per year. It’s approximately half a billion dollars.

The calculation is as follows. Based on my rate of good fortune, the average American finds $2 cash per year while walking (a factor of ten less than me, commensurate with the ratio of average walking time to mine). There are about 300 million Americans. The product of $2 and 300 million is $600 million.

However, I walk in an area of above-average wealth. There may be more cash about to be lost and found than in the average city or town (hat tip: Alison Pollock). A rough estimate is that my rate of finding cash is higher than average by a factor of 1.14, which is the ratio of the median household income in my metropolitan area ($52,800) to the U.S. median household income ($46,300). Applying this adjustment reduces the total from $600 million to $526 million (= $600M/1.14). Let’s call it a half a billion dollars in round figures.

Though it is a rough calculation, it is a plausible order of magnitude. That is, it is not obviously wrong. It passes the laugh test. Of course the calculation itself is laughably simple. One thing it neglects is the fact that I walk in urban and near-urban environments. Actually I’m not sure in which direction this affects the results. The opportunity for lost money is greater but the higher population density suggests I’d be less likely to find it. I also did not take into consideration the time of day individuals walk. Perhaps it is relevant that I do so during commute times. But again, money may be more likely lost at commute times (I find a lot near coffee shops), but there are also more people out and about to find it. Finally, it could also be that I just tend to keep my gaze downward more than most.

Having contemplated other methods of estimation and found them intractable or full of assumptions and their potential biases, I’m fairly satisfied with my approach and results. Do you believe it, at least to an order of magnitude approximation? Why or why not?