2010 UCLA Study and Map by Prof. Mike Manville and Prof. Donald Shoup (UCLA), reported on by the LA Streets blog. No link to the original study / seems hard to find even on the UCLA profiles of the researchers.
Total Vehicles In Urbanized Area: 6,433,000
Urbanized Area: (sq. mile): 2,980
Vehicles per Person: 0.54
State of California Traffic Census for Highways/Interstates