Study finds that increased vehicle travel and decreased occupancy have undercut the impact of improving fuel economy over last 40 years
A new study by Dr. Michael Sivak, Director, Sustainable Worldwide Transportation, University of Michigan Transportation Research Institute (UMTRI), has found that from 1970-2010, an increase in vehicle distance travelled in the US, coupled with a decrease in the number of occupants in the vehicles, combined to undercut the impact of advances in vehicle fuel economy during that period.
From 1970 to 2010, vehicle distance travelled in the US increased by 155% (from 1.674 trillion km to 4.260 trillion km); however, because vehicle load (i.e., occupants carried) decreased by 27% (from 1.9 to 1.38 persons), the occupant distance travelled increased by 84% (from 3.182 to 5.867 trillion km). Sivak found that while the vehicle fuel economy of the entire light-duty fleet improved by 40% (from 13 mpg US to 21.6 mpg US, or from 18.1 l/100km to 10.9 l/100km), because of the decrease in vehicle load, the occupant fuel economy only improved by 17% (from 24.8 mpg US to 29.8 mpg US, or 9.5 to 7.9 l/100km).
As a consequence of the changes in vehicle fuel economy, vehicle distance travelled, and vehicle load, the total amount of fuel used increased by 53% (from 303 to 463 billion liters).—Sivak 2013
|According to the 2011 American Community Survey by the US Census Bureau, 79.9% of workers not working at home drove to work alone.|
The study includes a brief discussion on the effects of potential future changes in vehicle fuel economy, vehicle distance travelled, and vehicle load on the amount of fuel used for personal transportation.
Sivak notes that:
Nominal reductions in fuel consumption due to improved fuel economy need to be adjusted downward due to the rebound effect. (EPA recommends a 10% decrement.) In other words, a 20% increase in fuel economy would result in a net 18% reduction in fuel consumption.
Because changes in fuel economy take a long time to percolate through the entire fleet, an 18% reduction in fuel used by vehicles purchased in a given year (due to a 20% improvement in their fuel economy) would result in only about a 1% reduction of the fuel used by the entire fleet.
(This has led some researchers to argue that the policy emphasis should be on reducing vehicle distance traveled through an increased fuel tax—with fleet-wide effect.)
Because the amount of fuel consumed is directly proportional to vehicle distance traveled (holding everything else constant), a 20% decrease in vehicle distance traveled would translate into a 20% reduction in fuel used.
Increasing occupancy (e.g., through ride sharing) reduces vehicle distance traveled, and also slightly worsens fuel economy due to the extra weight. In a scenario of increasing vehicle load by 20% from the current 1.38 to 1.66 occupants, vehicle distance travelled would be reduced by about 15%, while vehicle fuel economy would worsen by about 1%. Combining these two effects yields a reduction of about 14% in the amount of fuel used.
The study was supported by Sustainable Worldwide Transportation. The current members include Autoliv Electronics, Bridgestone Americas Tire Operations, China FAW Group, General Motors, Honda R&D Americas, Meritor WABCO, Michelin Americas Research, Nissan Technical Center North America, Renault, Saudi Aramco, Toyota Motor Engineering and Manufacturing North America, and Volkswagen Group of North America.
Michael Sivak (2013) Effects Of Vehicle Fuel Economy, Distance Travelled, And Vehicle Load On The Amount Of Fuel Used For Personal Transportation In The US: 1970-2010 (UMTRI-2013-10)