"As we've discussed, gasoline prices are just part of the story. The lack of growth in miles driven over the last 5+ years is probably also due to the lingering effects of the great recession (high unemployment rate and lack of wage growth), the aging of the overall population (over 55 drivers drive fewer miles) and changing driving habits of young drivers."
http://www.fhwa.dot.gov/policyinformation/nhts.cfm
Why Young People Are Driving Less and What It Means for Transportation Policy http://www.uspirg.org/reports/usp/transportation-and-new-gen...
Data has shown that between 55+ retiring and younger potential drivers preferring not to drive, vehicle sales have plateaued. If self-driving cars take off in the next ~5 years, that'll be the death of a large part of the auto industry.
End result - car sales stagnate.