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New Study Reveals Impact of Auto Roof Racks on Fuel Economy

June 10, 2016

A recent study by NREL research engineer Yuche Chen and researcher Alan Meier at Lawrence Berkeley National Laboratory (LBNL) examined the national impact of roof racks on fuel consumption. In this first-of-its-kind study, Chen and Meier determined that in 2015, the popular vehicle add-on accounted for national light-duty fuel consumption that amounts to roughly 100 million gallons of gasoline a year.

The study was published in Energy Policy and projects that if consumers and industry do nothing to change current roof rack technologies, and how people use them, the fuel consumption penalty that comes from having roof racks could increase to 300 million gallons of gasoline a year by 2040. They estimated that the use of roof racks will increase by about 200% in the United States by 2040. In addition to their projections for wider use, Chen and Meier calculated how effective various policy measures and new technology designs could be at mitigating the fuel consumption impact. They also factored in projections of increased vehicle electrification and hydrogen fuel cell technologies, based on estimates from the U.S. Energy Information Administration (EIA).

"One of the main findings from the study was that removing empty roof racks when they are not in use would account for the largest reduction in projected fuel consumption related to roof racks," said Chen, explaining that vehicles with empty roof racks were seen to travel longer distances than vehicles with loaded roof racks. Removing roof racks when they're not in use, combined with new industry standards that would optimize the aerodynamic design of such technologies, could reduce national roof rack-related fuel consumption to 25 million gallons of gasoline a year.

Chen, who joined NREL's simulation, testing, and integration group for transportation just a year ago, has known Meier for a long time. The two researchers began looking into the impact of roof racks when Chen was a Ph.D. student at the University of California, Davis, and Meier was a mentor of his. The idea -- to closely examine the impact of roof racks on fuel consumption--sprung from the researchers' observation on how many roof racks they saw while doing field work on the road.

"That led us to think about how some people don't consider how this equipment has an impact on fuel efficiency," Chen explained. Chen and Meier found some studies that examined the fuel impact of roof racks at a per-vehicle level.  "None of those studies examined the aggregated impact that auto roof racks have on fuel consumption at a national level," said Chen.

While roof racks require vehicles to use more energy due to their aerodynamic drag, many other vehicle add-ons, Chen warns, will raise similar questions. For example, as more connected driving technologies increase -- which could have significant fuel efficiency gains -- the advanced computer systems that new vehicles rely on will also require more energy. Sound systems, television screens, GPS systems, and other gadgets that drivers add to their vehicles all require additional energy. Such after-market accessories aren't captured in vehicle fuel economy certification procedures. Expanding on the roof rack study, Chen hopes to examine other types of vehicle add-ons.

To collect their data, Chen and Meier used a bottom-up approach through online forums and crowdsourcing for data collection sources. Roof rack usage rate estimates were based on nationwide video surveys, conducted by authors and workers recruited by Amazon Mechanical Turk. They also created an energy inventory model that included rack usage rates, vehicle stock, expected vehicle miles traveled through 2040 based on EIA's 2015 Annual Energy Outlook, and vehicle-level roof rack fuel consumption penalties. From there, the researchers conducted a sensitivity analysis to consider how the national fuel consumption impact would change with certain types of inputs, such as driving on the highway or driving with different roof rack designs.

Finally, the researchers analyzed the potential impact that changes in policy, technology, and behavior could have on roof rack-related fuel consumption. If a policy required energy labeling of roof rack technologies, for example, significant savings in energy consumption could result from more efficient designs and changes in consumer behavior.

Already, Chen said, the study has sparked a fair amount of buzz. "We have gotten a lot of interest from the general public," he said. "I'm receiving emails from people asking questions about the research, and what they can do to change their behavior to reduce immediate fuel consumption and ultimately climate change."

Generating public interest, Chen added, is one of his main priorities from the research in order to influence policy that may reduce roof rack fuel penalties. He also hopes to connect with the auto roof rack industry to further investigate how the study can impact deployment of more efficient technologies.

To learn more, download the recently published paper, "Fuel Consumption Impacts of Auto Roof Racks." Learn more about NREL's transportation research in systems analysis and integration.

—Wayne Hicks