EMPLOYEE WAS RIDING IN A PASSENGER VAN GOING TO THE PIT TO THE PRODUCTION EQUIPMENT HE WAS OPERATING. HE WAS SEATED IN THE LAST SEAT OVER BACK TIRE OF THE VAN WHEN IT HIT A POTHOLE AND EMPLOY EE WHO WAS WEARING HIS SEATBELT STRUCK HIS HEAD ON THE CEILING OF THE VAN. VAN SPEED WAS 5 TO 10MPH.