Raleigh, N.C. — Multiple wrecks were reported Monday morning, when dense fog caused low visibility and WRAL viewers said they encountered icy patches on roads north of the Triangle.

Fog reduced visibility to between zero and a quarter-mile in some areas, WRAL meteorologist Nate Johnson said. North of Durham, temperatures dipped below freezing, allowing moisture to freeze, mostly on bridges and overpasses.

At noon, visibility in Raleigh was still only about three-quarters of a mile. Most of the fog had lifted by 3 p.m. and some parts of the Triangle saw a bit of sun Monday afternoon, Johnson said.

A tractor-trailer overturned on U.S. Highway 401 South, near Perry Creek Road, Raleigh police spokesman Jim Sughrue said. Traffic was detoured onto Ligon Mill Road for about two hours. Sughrue said there were no serious injuries.

Earlier, heavy fog and a wreck slowed traffic along Interstate 40 East, between mile markers 274 and 276 in Durham County, transportation officials said.

A wreck also shut down I-40 West, at the U.S. Highway 70 Bypass, near the Wake-Johnston county line during part of the morning rush hour.

WRAL viewers reported that ice was making roads in northern Granville County dangerous and that trucks were putting down salt. Schools in Mecklenburg County, Va., were put on two-hour delay to the fog and potential for icy bridges.

"With that thick fog which is widespread across the entire viewing area, you want to give yourself plenty of extra time this morning," WRAL traffic reporter Brian Shrader said.

In thick fog, drivers should slow down, use their low-beam lights, and leave plenty of distance between vehicles.

Johnson said dense fog and clouds were keeping temperatures down. In Boone, where skies had cleared by mid-day, temperatures had risen into the upper 50s. But in the Raleigh area, the mercury was having trouble rising beyond about 45 degrees.