Fourteen cars and a semi-truck tied up the highway in six separate crashes.
The first crash started when a vehicle going too fast for conditions spun out on southbound U.S. 31 at about 7:30 a.m. The other vehicles piled up behind it and the semi jack-knifed.
Five people suffered non-life-threatening injuries. They were taken to area hospitals.
Police said they will likely be issuing citations in the crashes, but the investigation is not yet complete.