Stuck in the Sand: Methodology
Environmental Working Group (EWG) developed its Persian Gulf oil dependency index by analyzing the U.S. Department of Transportation's 2001 National Household Travel Survey (NHTS). From the NHTS survey, EWG obtained data for the 50 largest metropolitan statistical areas including the number of drivers in each area, total miles driven and the number of households in each area. The data was obtained from 141,860 surveys in the 50 cities. The sample size ranged from 465 people in Oklahoma City to 26,477 in New York. A metropolitan statistical area (MSA) is defined as an area that includes at least one urbanized core area of 50,000 or more people "together with adjacent communities having a high degree of economic and social integration with that core" (Census 2005).
From these data, EWG calculated the mean number of miles traveled per driver in each MSA. EWG then multiplied the number of miles traveled per driver by the U.S. Department of Transportation's (DoT) 2003 reported average fuel efficiency for cars and light trucks of 20.3 miles per gallon (DoT VMT 2003). The result gave us the total amount of gasoline consumed by the average person in each urban area. According to data from the U.S. Department of Transportation, the mix of vehicles and their fuel efficiency is approximately the same across metro areas nationwide (DoT Fuel Efficiency).
To determine geographically where our gasoline originates, we consulted the U.S. Department of Energy's Energy Information Administration (EIA) which reported that each 42-gallon barrel of crude oil produces an average of 19.9 gallons of gasoline (EIA Ask an Expert 2005). About 95 percent of our gasoline is refined in the United States from domestic or imported crude oil. Only about five percent of our gasoline is imported in the form of already-refined gasoline (EIA Net Imports 2003). The U.S. exports less than five percent of the gasoline produced from domestic crude oil (EIA Exports 2003).
EWG estimated that because about 16 percent of our crude oil came from the Persian Gulf in 2003 (2,425,000 barrels per day out of 15,333,000), 16 percent of our gasoline came from the Gulf (EIA Net Imports 2003, EIA PAD 2003). We adjusted this figure to 15 percent of our gasoline to account for the fact that most of our already-refined gasoline that is imported comes from outside the Gulf. We then multiplied the average gallons of gas consumed per driver in each metro area by 15 percent to determine the portion of Persian Gulf gasoline consumed by the average metro area driver.
The EIA reports that it does not track the origin of American gasoline. The EIA adds that "even if we knew at which company's refinery the gasoline was produced, the source of the crude oil used at that refinery may vary on a day-to-day basis. Most refiners use a mix of crude oils from various domestic and foreign sources" (EIA Primer on Gasoline 2005). Therefore, EWG estimated that the mix of gasoline from the Persian Gulf, the U.S., and other sources is consistent across the United States. In any event, every region of the nation depends on the inflow of Persian Gulf oil, either because the region uses the oil directly or because the use of Gulf oil in one region means that more oil is available from other sources in another region.
To determine how much money each metro area resident sent to the Persian Gulf, we multiplied the portion of Persian Gulf gasoline used by each resident by the average U.S. price of gasoline as of August 15, 2005 ($2.55 per gallon) and then multiplied the product by 55 percent, the amount of each dollar paid for gasoline attributable to crude oil as opposed to taxes, distribution, marketing and refining (EIA Gasoline Update 2005).
EWG obtained transit ridership data from several tables listed by the American Public Transportation Association (APTA Ridership Report 2003, APTA Factbook 2005).