The combination unit was traversing a straight section of roadway, on an approximate 4 percent upgrade, when the tractor began to jackknife. As the power unit rotated clockwise, it left the roadway to the right and traveled several feet before the driver corrected the jackknife. The unit was redirected toward the travel lanes, however, before it re-entered the roadway, it rotated counterclockwise, and the driver lost control. The power unit crossed into the eastbound lane where it was struck in the right side by an eastbound van. Although a saddle tank on the power unit ruptured, there was no fire.
Source: wiktionary