A 20 window tint is fairly dark and can make driving at night very difficult or even dangerous. It blocks out a good percent of light and is illegal in many states. However, some states do allow darker tinting on back or side windows than on front windows.
Before tinting vehicle windows, it is best to check the laws in the state you live in. However, it is possible to receive a ticket in another state while traveling if the tinting laws differ. According to Officer Forums, reasons tinting windows dark is often illegal is to protect pedestrians, to ensure the driver's safety and to allow police officers to see inside the vehicle.