FTL: "FHWA defines a roadway departure (RwD) crash as a crash which occurs after a vehicle crosses an edge line or a center line, or otherwise leaves the traveled way. Another term our partners often use is lane departure, which is synonymous with RwD, since both include head-on collisions when a vehicle enters an opposing lane of traffic."

Road departure fatalities are high because of head-on collisions, not because there is an epidemic of people crashing into trees along the side of the road. If you follow the links on the cited page, they clearly show that head-on crashes result in more fatalities than tree+utility pole crashes.