A vehicle leaving the Northern State Parkway careened off an exit ramp, crashed through road signs and ignited a house fire early Monday in Dix Hills, authorities said. Firefighters from at least 15 departments responded to the scene on Deforest Road North after receiving a call at 5:49 a.m., authorities said. The location, at Exit 42N for Deer Park Road, is separated by a small wooded area from the Dix Hills Fire Department.