Both directions of U.S. 36 reopened Friday after being closed for two hours because of six accidents involving 39 vehicles, including a semitrailer, the Boulder County Sheriff’s Office said. The biggest accident was a 12-vehicle chain-reaction crash at about 6:45 a.m. that forced the closure of eastbound U.S. 36 at Table Mesa in Boulder. The accident was close to the McCaslin Boulevard exit. Two other crashes forced westbound U.S. 36 to be closed at Flatirons Crossing in Broomfield but it reopened at 9 a.m. Eastbound traffic reopened about 9:45 a.m.