While tramming 2yd LHD out of the mine, employee got to the spiral between the 650 and 500 level. As employee came around the corner, saw the 18 ton haul truck rolling backwards at an unusual rate of speed. Attempted to reverse LHD and braced for impact. Haul truck hit LHD at the bucket end and bounced it off the rib. Employee was wearing EE's seatbelt.