Three people were airlifted to the hospital Monday after a suspected impaired driver caused a serious wreck on Interstate 5 in Federal Way, ending with a car on fire, authorities said. State troopers and firefighters responded to the scene, in the southbound lanes of I-5 near 375th Street just north of Pierce County, at about 7:30 p.m. after receiving reports of a serious two-car crash. A preliminary investigation found that the causing driver was speeding and made an improper lane change just before colliding with the second vehicle, according to the Washington State Patrol; An 83-year-old woman and an 86-year-old man in the second vehicle were airlifted to Harborview Medical Center in Seattle. Their conditions were not immediately available. A third person in that car was not injured.