May

13

2009

Hey, it's Walk/Bike to Work Day tomorrow. And in honor of that, here's a little math problem to keep you occupied, courtesy of *Guesstimation: Solving the World's Problems on the Back of a Cocktail Napkin* by Lawrence Weinstein and John Adam.

"What are the relative costs of fuel (per kilometer or per mile) of New York City bicycle rickshaws (human-pedaled taxis) and of automobiles?"

Courtesy ARoberts

You might need a few hints.

- How far does a bicycle rickshaw travel in one day?
- What does it cost to fuel (i.e. feed) the bicyclist?
- How far does a car travel on a gallon of gasoline?

What'd you get? Post your answer as a comment. Once a few folks post answers, I'll post the one from the book, as well as the "work."

OK, so how would you even start to figure it out?

Here's a cool calculator that will show you how far your car can travel on a gallon of gas. I put in a bunch of different makes/models of cars, of different ages. For the sake of easy math, I'm going to say, on average, a car can go 20 miles on a gallon of gas. (Yes, I'm sure your car does better. And this is, after all, a guesstimate.)

According to Twin Cities gas prices, a gallon of gasoline today will run you about $2.20.

So $2.20/gal divided by 20 mpg = $0.11/mile (or $0.07/km).

When I carry passengers on my bike, I can go a maximum speed of maybe 15 miles an hour, but I'm much slower on hills, maybe 7 miles an hour, and I probably average somewhere around 11 miles an hour. But I'm carrying two kids, not two adults, so I should pick a number at the low end of that range. And, of course, a pedicab is not always moving. So for the sake of easy math, I'll just say an average of maybe 5 miles per hour. Assume an 8-hour working day, and that gives you 5 mph x 8 hrs/day, for a total of 40 miles per day.

Calculating fuel for the cyclist is the tricky bit. The CIA's World Factbook estimates the US per capita GDP at $48,000. That works out to $132/day. And this 2006 Forbes article says Americans spend about 13% of their income on food. So we spend maybe $17/day on food.

$17.00/day divided by 40 miles/day = $0.42/mi or $0.27/km.

The car uses more energy per mile, but gasoline is a cheaper fuel than food. However, the fuel costs for the car depend on the distance traveled. If you double the distance traveled by car, you double the fuel cost. With a pedicab, the fuel cost is the same, per day, no matter how far you go.

And, of course, you're probably eating today whether or not you're pedaling a rickshaw. So get out there and ride! Don't let that fuel go to waste!

Guess what? This article that I just read has motivated me to hop on my neglected bike and pedal to the metal, as they say. I am sure that my workplace will enjoy and/or appreciate all of the work that I will be doing, and I might get some positive reinforcment with my fellow collegues! However, I regret to inform you that I was a little concerned with the content portion of this article, as I believe more information is necessary to have a more substantial impact of the denizens of society. I was, however, plesantly surprised at the amount of money per mile a cyclist pays. Thank you, and have a nice day.

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