The first crash involved six vehicles and happened around 5:45 a.m. on N.C. Highway 147 South near Chapel Hill Street.
Emergency crews took three people to Duke Hospital's emergency room with non-life-threatening injuries, police said.
Pre-school teacher Hali Garrett said she heard the crash.
"I woke up about 5:40 (a.m.) hearing a succession of about five collisions, but it was strange because they seemed to be a few minutes apart," she said. "Between the last few crashes, it sounded like people were honking, perhaps trying to warn other drivers of the collisions."
The right two lanes of N.C. 147 South were blocked, but they reopened by 7:45 a.m. Sand trucks were called to the area to help take care of the icy spots.
The second crash happened on U.S. Highway 15-501 between Cornwallis Road and Martin Luther King Jr. Parkway. No other details were available.