Obey California law and you won't have to worry about someone sneaking up behind you on the highway. The state requires that vehicles have at least two rear view mirrors and bans hanging items on the rear view mirror that could limit your vision.
Visibility, Number and Location of Mirrors
The California Vehicle Code requires that rear view mirrors be placed so that you have a rear view of the highway for at least 200 feet behind your car. At least two rear view mirrors meeting this visibility requirement are required in all vehicles that are subject to registration in California, except motorcycles. One of the mirrors must be placed on the car’s left side. Rear view mirrors are required on both sides of a vehicle if it is loaded or towing another vehicle so that the driver’s view of the rear is blocked, unless it is a passenger car where the driver’s view is blocked by passengers. Additionally, buses and trolley coaches are required to have both left and right side rear view mirrors.