As you operate your car, you need to push down on the pedals, which also pushes back against your buttocks and lower back. Since you're applying pressure with your foot, it's actually causing more pressure on your buttocks than if you were simply sitting there passively. For this reason, you should stay out of the driver's seat for at least a few weeks after your BBL.
How long you are off work depends on your occupation. If you do clerical work (i.e. stockbroker, teacher, or programmer), you can return to work when you feel up to it. This usually takes about two to three days. Do not go to work for three weeks if you do manual labor (i.e. entertainer, truck driver or personal trainer). Regardless of your employment, do not lift anything weighing more than five pounds for three weeks.