Ronald Louis of Haiti's Civil Protection Office says the truck's brakes failed Saturday afternoon as it drove on a hilly road outside the western town of Petit-Goave.
Seventeen people died at the scene, many of them crushed. Three more died at a hospital. The forty who were injured were taken to hospitals in the area.
Fatal traffic accidents are common in impoverished Haiti, where police seldom enforce traffic laws and many vehicles are overloaded with passengers.