It is illegal to drive a motor vehicle if it is loaded with cargo or people and interferes with driving safely or obstructs your view in anyway, according to California Motor Vehicle Code 21700. The law requires you to be able to see your mirrors and be able to see out of each window.
If you are stopped, you can be ticketed and the fines can be severe. Overloaded vehicles cause hundreds of accidents every year. If you are involved in an accident with a vehicle that was overloaded with people or things, you may be able to be compensated for your injuries. Drivers who overload their vehicles could be guilty of negligence and face criminal charges. It would be wise to contact an experienced Personal Injury attorney and find out what your rights are under California law.