A tractor-trailer jack-knifed and is blocking part of Interstate 24 at the state Route 840 interchange westbound in Rutherford County, said Sheriff Robert Arnold. The truck caused a pileup with 15 cars that has now been cleared. "Secondary roads are extremely hazardous and slick," said Sheriff Arnold. use extreme caution if you have to drive tonight." Sheriff's deputies have worked about 12 crashes and six motorists' assists since the snow began about 4 p.m.