海角大神

Auto companies聽have made great strides in improving engine efficiency in recent decades. But those improvements haven鈥檛 done much to improve the fuel economy of America鈥檚 passenger car fleet.聽Instead, consumers have 鈥渟pent鈥澛爉ost of those efficiency improvements on bigger, faster cars.

MIT economist Christopher Knittel has carefully quantified these tradeoffs in a聽recent paper in the American Economic Review (; earlier ungated version ).聽As noted by of MIT鈥檚 News Office:聽

[B]etween聽1980 and 2006, the average gas mileage of vehicles sold in the United States increased by slightly more than 15 percent 鈥� a relatively modest improvement. But during that time, Knittel聽has found, the average curb weight of those vehicles increased 26 percent, while their horsepower rose 107 percent. All factors being equal, fuel economy actually increased by 60 percent between 1980 and 2006, as Knittel shows in a new research paper, 鈥淎utomobiles on Steroids,鈥� just published in the American Economic Review.

Thus if Americans today were driving cars of the same size and power that were typical in 1980, the country鈥檚 fleet of autos would have jumped from an average of about 23 miles per gallon (mpg) to roughly 37 mpg, well above the current average of around 27 mpg. Instead, Knittel says, 鈥淢ost of that technological progress has gone into [compensating for] weight and horsepower.鈥�

This is a fine聽example of a very common聽phenomenon: consumers often 鈥渟pend鈥� technological improvements in聽ways that partially offset the direct effect of the improvement. If you make engines more efficient, consumers purchase heavier cars. If you increase fuel economy, consumers drive more. If you give hikers cell phones, they go to riskier places. If you make low-fat cookies, people eat more. And on and on. People really do respond to incentives.