The tint applied by car manufacturers at the factory must allow at least 70 percent of visible light through. This regulation from the National Highway Traffic Safety Administration applies to any window required for driving visibility, including the windshield and windows immediately to the right and left of the driver.
Manufacturers, recognizing that each state has its own laws, often leave the other automotive windows at the same lightly-tinted, 70-percent level. The NHTSA guidelines for trucks and sport-utility vehicles allow manufacturers to tint back and rear windows to any darkness, according to the New York Times.
Arguments for lighter tinting include maximizing what a driver can see at all ambient light levels, and law enforcement personnel feel safer when they are able to see into a stopped vehicle. On the other hand, drivers enjoy darker tints that provide privacy, lower the vehicle temperature and protect skin and upholstery from ultraviolet light damage.