Heavy snow closed one of the main routes into Devon, the A38, for the third winter in a row.
Devon County Council put its severe weather response plan in place on the A38 at Haldon Hill and the neighbouring A380 at Telegraph Hill at 0100 GMT.
Gritters were stuck behind hundreds of stranded cars and a council spokesman said it was a "major incident".
Both roads later reopened, but traffic could only move slowly. Up to 4in (10cm) of snow fell in parts overnight.
Although traffic on the A38 is flowing again, conditions are extremely difficult and vehicles are moving very slowly.
Incident commander for the response plan, Sgt Mike Rose from Devon and Cornwall Police, said the snow ploughing designed to alleviate problems on Haldon Hill had worked well with temporary road closures in place early.
"But obviously the amount of snowfall and the area over which it was spread far surpassed anything anticipated with the Haldon Hill plan, so we've had to extend our operations a great deal and there's a lot of work to do," he told BBC News.
"There's been a lot of snow - certainly more than I've ever seen before anywhere up here."
Snow that had been turning to slush has started to freeze again in places, making conditions even more difficult.
Snow ploughs worked for hours to clear the A361 North Devon Link road which was described as treacherous.
A petrol tanker had to be recovered after it jack-knifed in icy slush on the A386 slip road at Crownhill in Plymouth.
Earlier, Devon and Cornwall Police urged people to delay their journeys until conditions improved.
They said the road network was treacherous throughout Devon, with vehicles getting into difficulty as soon as they reached any kind of incline.
Insp David Eldridge said no journey was essential enough for anyone to be out on the roads in Devon on Monday morning.
Hugh Griffiths, from Devon County Council's control room, admitted the primary network had been "in a bad way" earlier.
The runway at Exeter Airport has been closed and flights disrupted.
Flybe has cancelled all its domestic flights until at least 1500 GMT, and travellers have been advised to contact their airlines.
The heavy snow has now cleared the South West, but the Met Office said there was still a risk of wintry showers with temperatures expected to fall to -4C overnight.
The weather has affected most parts of the county, with bus services suspended or severely restricted.
"We can completely ignore timetables today," a Plymouth CityBus spokesman said.
Most schools have already closed for the Christmas holidays, but many out-of-school clubs which were due to open have been forced to close because of the weather.
People with outpatient appointments at North Devon Hospital and the Royal Devon and Exeter (RD&E) Hospital have been asked not to attend.
RD&E spokesman Jeff Chinnock told BBC News patients would be contacted with new appointments as soon as possible.
One of Devon's two air ambulances has been grounded for the past three days because of weather conditions at Eaglescott Airfield, Burrington.