"As you get ready to hit the road this summer, with the kids loaded inside and the bikes strapped to the roof of your car, you may want to stop and consider that the roof rack on your car may be costing you as much as 25 percent more in gas.
"In the first study of its kind, Lawrence Berkeley National Laboratory (Berkeley Lab) researcher Alan Meier, working with Yuche Chen of the National Renewable Energy Laboratory, have estimated the fuel consumption penalty of this popular and fast-growing vehicle add-on.
...
"For example, roof rack usage rate estimates were based on nationwide highway video surveys conducted by the authors and workers recruited through Amazon Mechanical Turk."
http://universityofcalifornia.edu/news/roof-racks-drag-fuel-economy
Author: Julie Chao, Lawrence Berkeley National Laboratory
Date: April 25, 2016
...
"For example, roof rack usage rate estimates were based on nationwide highway video surveys conducted by the authors and workers recruited through Amazon Mechanical Turk."
http://universityofcalifornia.edu/news/roof-racks-drag-fuel-economy
Author: Julie Chao, Lawrence Berkeley National Laboratory
Date: April 25, 2016