Can I Have a Diet Coke With That Ice Cream Sundae?

Essentially that's what UC Davis, Yale, and MIT researchers found among California families who purchase very fuel efficient vehicles—they also pair them with gas hogs. If your family owns two vehicles, do you meet the profile?

3 minute read

October 3, 2017, 6:00 AM PDT

By Irvin Dawid


4-Wheeling

Louis Rivieccio / Flickr

The researchers found the pairing of the opposite types of vehicles by studying California Department of Motor Vehicle trends for two-car households over several years, report Peter Fimrite and Michael Cabanatuan for the San Francisco Chronicle on Sept. 27.

If people buy a more fuel-efficient car, down the road when they replace one of their cars, the car they buy is going to be less fuel-efficient,” said lead author David Rapson, an associate professor of economics at the UC Davis Department of Economics and co-director of the Davis Energy Economics Program. “So the effect of fuel economy standards is reduced.”

The other finding was the rebound effect (also referred to as the energy efficiency paradox) that often comes with the purchase of fuel-efficient vehicles—they are driven more often than had they been less fuel efficient.

These tendencies reduce by up to 60 percent the expected future gas savings from increased fuel economy in two-car households, said the study, which was funded by the California Air Resources Board (CARB). 

As someone who has dedicated his career to trying to figure out solutions to climate change, this is troubling,” Rapson said. It means “there is a strong force that we didn’t know about before that is going to erode the benefit of forcing people to buy more fuel-efficient cars.”

The study, released Sept. 19, uses a psychological term to describe this pairing in its title: "Attribute Substitution in Household Vehicle Portfolios [pdf]".

The results don’t mean humans are naturally greedy or hedonistic, Rapson said. In fact, the behavior of car buyers fit “attribution substitution” patterns displayed in previous studies. For instance, households with two children of the same gender are more likely than families with a boy and a girl to have a third child.

Law of unintended consequences

Rapson concludes that the tendency to pair the gas guzzler with the gas sipper, and the tendency to drive the latter more often than one otherwise would, suggests that CARB needs to consider these human tendencies when setting standards in their clean vehicles programs. 

“Unintended consequences like this need to be taken into account when making policy,” he said. “On average, fuel economy standards are putting more fuel-efficient cars in households. That can be good if it reduces gasoline use. But if it causes people to buy a bigger, less fuel-efficient second car to compensate, this unintended effect will erode the intended goals of the policy."

Price signals needed

The findings points to the limits of reducing greenhouse gas emissions by regulations alone. "We don’t suggest getting rid of fuel economy standards and doing nothing else,” Rapson told the UC Davis News..

“But our conservation policies need to be effective. A more promising solution is to put a price on carbon, such as what California is doing with cap and trade. It produces economic incentives that evidence shows will reduce carbon emissions."

Cap and trade adds about 12 cents to a gallon of gas in California, according to Ross Brown of the Legislative Analyst's Office.

Another pricing option would be to either supplement or replace the current Clean Vehicle Rebate Project with a "feebate" that would not only incentivize zero-emission vehicles, but also place a surcharge on gas hogs, so consumers might consider the fuel economy of that "second vehicle."

Wednesday, September 27, 2017 in San Francisco Chronicle

portrait of professional woman

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching. Mary G., Urban Planner

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching.

Mary G., Urban Planner

Cover CM Credits, Earn Certificates, Push Your Career Forward

Aerial view of town of Wailuku in Maui, Hawaii with mountains in background against cloudy sunset sky.

Maui's Vacation Rental Debate Turns Ugly

Verbal attacks, misinformation campaigns and fistfights plague a high-stakes debate to convert thousands of vacation rentals into long-term housing.

July 1, 2025 - Honolulu Civil Beat

Logo for Planetizen Federal Action Tracker with black and white image of U.S. Capitol with water ripple overlay.

Planetizen Federal Action Tracker

A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

July 9, 2025 - Diana Ionescu

Map of Haussmann's redesign of Paris in the 1850s through 1870s under Napoleon III.

In Urban Planning, AI Prompting Could be the New Design Thinking

Creativity has long been key to great urban design. What if we see AI as our new creative partner?

June 30, 2025 - Tom Sanchez

Red and white "Wildfire Evacuation Route" sign on signpost.

Cal Fire Chatbot Fails to Answer Basic Questions

An AI chatbot designed to provide information about wildfires can’t answer questions about evacuation orders, among other problems.

7 hours ago - The Markup

Protester at Echo Park Lake, Los Angeles holding sign that says "Housing is a human right"

What Happens if Trump Kills Section 8?

The Trump admin aims to slash federal rental aid by nearly half and shift distribution to states. Experts warn this could spike homelessness and destabilize communities nationwide.

July 10 - Shelterforce Magazine

Aerial of rainbow painted crosswalks at large intersection in Castro District, Sna Francisco, California.

Sean Duffy Targets Rainbow Crosswalks in Road Safety Efforts

Despite evidence that colorful crosswalks actually improve intersection safety — and the lack of almost any crosswalks at all on the nation’s most dangerous arterial roads — U.S. Transportation Secretary Duffy is calling on states to remove them.

July 10 - Streetsblog USA

Urban Design for Planners 1: Software Tools

This six-course series explores essential urban design concepts using open source software and equips planners with the tools they need to participate fully in the urban design process.

Planning for Universal Design

Learn the tools for implementing Universal Design in planning regulations.

Home and Land Services Coordinator

Appalachian Highlands Housing Partners

Associate/Senior Planner

Gallatin County Department of Planning & Community Development

Senior Planner

Heyer Gruel & Associates PA