California Motor Vehicle Code 21700
Obstruction to Driving
Driving your vehicle with an obstructed view is both illegal and dangerous. If you have your car so loaded that you can't see out the rear and side windows, you could be placing yourself and others in harms way.
Some cars and trucks are so loaded with kids, moving boxes, and junk that the driver cannot see properly. If the driver gets into an accident, he or she could be charged with criminal penalties and face civil penalties as well. It's never a good idea to drive when you can't see out the car windows.
If you are moving, or some other activity that requires carrying many belongings in your car, make sure you can see properly. You should at least be able to see all of your mirrors, see out the rear window, and see out the passenger side front and rear windows. When stacking boxes in the car, remember to leave room for visibility once the doors are closed.
Sometimes drivers will overload their cars with passengers instead of boxes. It is also illegal to have so many people in the car that it obstructs your view and your ability to drive. If you are going to an event that involves a large number of people, you should take two cars or rent a van large enough to transport everyone safely. You cannot depend on your passengers to move their heads in time for you to see oncoming cars.
If you are pulled over for driving a car that is overloaded, you will receive a ticket and have points added to your license. You could receive fines up to $1000 for each offense. It would be much cheaper to rent a moving van than it would be to pay the fines for overloading your car. Weigh your options before packing your car full of boxes.
When in doubt, it is always best to err on the side of caution. If you can't see out every window, and see all mirrors, then you are probably overloaded.