A 25mpg, single occupancy car going the same Oakland to LA distance would use about 15 gallons of gas. Or about 300 lbs of CO2 (*depends on ethanol content)
So a fully occupied PC-12 releases less greenhouse gases per person for the same trip.
The average car will burn about three gallons of gasoline an hour cruising on the highway, so about 60lbs of CO2.
If you assume highway cruising to be 60mph, that's only 20mpg. That seems a low average for modern cars.
The average MPG of a new car is only about 25, and the average age of cars actually on the road is over a decade. And 60MPH is a slower highway cruising speed than I've seen anywhere outside of big cities or congested areas. 75+MPH is far more common.