2003 GMC Sierra 1500 Pickup Owner's Manual - Page 13

Sitting in a reclined position when your vehicle is in motion can be dangerous. Even if you buckle up, your safety belts can't do their job when you're reclined like this. The shoulder belt can't do itsjob. In a crash, you could go intoit, receiving neck or other injuries. The lap belt can't do its job either. In a crash the belt couldgo up over your abdomen. The belt forces would be there, notat your pelvic bones. This could cause serious internal injuries. But don't have a seatback reclined if your vehicle is moving. For proper protection when the vehicle is in motion, have the seatback upright.Then sit well back in the seat and wear your safety belt properly. 1 -6