Light Vehicle Sales report from the Bureau of Economic Analysis adjusted for population. This data series stretches back to January 1976. Since that first data point, the Civilian Noninstitutional Population Age 16 and Over (i.e., driving age not in the military or an inmate) has risen about 65%.
The moving-average for the per-capita series peaked in 1986. Twenty-nine-plus years later, it is now down 25.9% from that 1986 peak.