Millions of Americans purchase gasoline each day and make choices about how much to drive, if at all. A basic premise, found in any high school economics textbook, suggests that a rise in the price of something increases the demand for its substitutes. So we might expect the use of public transit to be correlated with gas prices: When gasoline gets more expensive, car travel does too; so, shouldn’t consumers opt for a cheaper alternative, such as public transit? The data shown in the graph, however, don’t reveal anything so clear: When gas prices increase, we might expect a corresponding decrease in vehicle miles traveled and an increase in public transit usage. But that isn’t the case.
The graph tracks the percent change in the price of gas, public transit ridership, and vehicle miles traveled over the past five years. When gas prices have increased, so have public transit ridership and vehicle miles traveled: Americans were both driving more and making greater use of public transit. Seasonally adjusted data on gas prices aren’t available, so that lack of adjustment may explain some of this variation. Consider that Americans travel more in the spring and summer, which is shown by the tendency of all three indicators to spike in March and for transit ridership and vehicle miles both to increase throughout the summer months.
What happens when gas prices decrease? During those times, the relationship between gas price, vehicle miles, and transit use is even less clear, serving as a reminder of the other factors at play in the everyday decisions of Americans. For example, in December 2014, gas prices decreased over 13% while public transit ridership and vehicle miles traveled continued to increase. The price of gas fluctuates often, but the daily obligations of Americans are pretty steady: Commuting to work or school, visiting family, or shipping goods may not change significantly. The convenience of travel options, availability of infrastructure, employment and income levels, and seasonal opportunities are far more influential on transportation habits than the price of gas alone.
How this graph was created: Search for “conventional gas price” and choose the monthly series. From the “Edit Graph” tab / “Add Line” option, search for “public transit” and add the “not seasonally adjusted” series. Add the third line by searching for “vehicle miles” and choosing “Vehicle Miles Traveled (not seasonally adjusted).” From the “Edit Line” option, change the units to “Percent Change” and click “Copy to all.” Adjust the view to the five years starting with January 1, 2012.
Suggested by Maria Hyrc.