Researchers find consumers compensate for fuel-efficient car by buying bigger second vehicle; losing 60% of fuel economy

Via GCC: http://www.greencarcongress.com/2017/09/20170927-soda.html

An analysis by a team from the University of California, Davis, MIT and Yale suggests that households that buy a fuel-efficient vehicle tend to compensate for that purchase by buying a bigger, more powerful second vehicle. . . .

The researchers explored the phenomenon of attribute substitution in the context of multi-vehicle households. Attribute substitution is the trading off of the particular attributes of one product with the attributes of another—ordering a side of fries with a diet soda, for example, or decreasing the size of other televisions in a home after buying a larger screen version. . . .

Broadly, they found evidence that households substitute attributes across the vehicle portfolio. Increases in the fuel economy of the kept car reduced the probability the household purchases a car in the lower quartile of gallons per mile (the highest fuel economy quartile); rather such increases raised the probability the household buys a car in the upper quartile (the lowest fuel economy quartile).

• Attribute substitution erodes over 60% of the fuel savings from the fuel economy increase of the kept vehicle on net after accounting for all of these factors. As a specific example, consider a 10 percent increase in fuel economy from the average vehicle in our sample. Given the average miles driven (688 per vehicle in our sample), this 10 percent fuel economy increase would directly lead to a 69 gallon decrease in annual fuel consumption. However, due to attribute substitution the next vehicle the household purchases will be less fuel efficient than it otherwise would have been. This decrease in fuel economy of the newly-purchased vehicle reduces the fuel savings from our thought experiment to 40 gallons, holding usage of the two vehicles constant. But we also find significant changes in usage patterns that further reduce the net fuel savings.
  
  Mileage of the kept car increases significantly. A large fraction of this increase is due to shifts in miles traveled from the now less fuel efficient purchased vehicle; however, we also find a net increase in overall mileage across the two-vehicle portfolio. Accounting for all of the changes, the net savings of the exogenous increase in fuel economy falls from the naive estimate of 68 gallons to 24 or 27 gallons, depending on whether the initial vehicle was the most fuel efficient vehicle in the household.
  
  —Archsmith et al.

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I think that's a side-effect of other primary reasons. People buy a fuel-efficient car as the primary car because they want to keep expenses down. But that little car simply can't do everything.

Right now, my second vehicle is a minivan, so that I can take 6-8 people along for a ride occasionally. When you have kids, they tend to have friends, and it's nice to be able to offer to take all the kids yourself so that your neighbors don't have to drive their kids separately. Everybody has a minivan, and we take turns offering to drive them all. Saves both fuel and everybody's time when people take turns carpooling this way. When the in-laws come to visit, we can all travel in a single vehicle to go out to eat together. It's actually more efficient when that somewhat gas-guzzling minivan prevents the use of a second vehicle.

A lot of households (outside of cities) have a truck as a second vehicle. I used to, and I really miss the ability to go pick up random crap (yes, including fertilizer), throwing it in the back, and not worrying about getting the interior of the vehicle all dirty. That was more important when I was younger, pre-kids, and saving money by buying furniture and appliances from local classifieds (the print version of craigslist, for you young people), garage sales, etc.

The key point here is that "they bought a car less efficient that they otherwise would have."
Maybe. That is an assumption, likely colored by the national trend to buy SUVs

Our situation is the reverse. When purchasing our LEAF in 2012, we kept our 2002 minivan for the occasional long drive to visit relatives in Southern California. The LEAF is our everyday driver and we continue to be very happy with our decision to go electric.

I suppose the researchers want me to be disappointed by this 60% "erosion", but I'm a "glass is 40% full" kind of guy