Is it safe to recline the seatback while the vehicle is in motion?

No — sitting in a reclined position when the vehicle is in motion is dangerous even if you are buckled up. A reclined shoulder belt cannot properly restrain you in a crash, and the lap belt could slide up over the abdomen, potentially causing serious internal injuries. For proper protection, keep the seatback upright while the vehicle is moving and sit well back in the seat with the safety belt properly worn. (Page 17)