Snow: Drivers urged to take 'extreme care' in icy conditions
Drivers should continue to take "extreme care" after overnight snowfall and a widespread frost, Northern Ireland's roads authority has said.
Roads were salted on Thursday night and Friday morning.
A warning of snow and ice has been issued by the Met Office from 17:00 GMT on Friday until 11:00 on Saturday.
The Met Office added that ice is likely to form on untreated roads and pavements, while freezing fog is also to be expected overnight.
The A5 Omagh to Ballygawley road in County Tyrone was closed due to snow and ice earlier on Friday but has since reopened.
Cathal Brown, a senior engineer with the Department of Regional Development, said another busy period was expected on Friday night.
"We've been in action since yesterday afternoon, through the night and into this morning. Then tonight is expected to be equally demanding," he said.
"Our main focus has been ensuring that the Glenshane [Pass in County Londonderry] remained open."
Mr Brown added that roads in the Sperrin mountains in counties Tyrone and Londonderry and the north Antrim hills are of particular concern, and urged drivers to take extreme care on the roads overnight.
"Up round the Sperrins, around Dungiven, Claudy and across to Donemana - that is an area that we're putting extra resources into," he said.
In Beragh in County Tyrone, a Transport NI gritter left the road and fell in to a ditch.
Mr Brown said that he understood the driver of the vehicle was not badly hurt.
"Thankfully the driver is OK," he said. "I don't have the specific details of the accident, other than to say it was a relatively narrow road. The gritter met another vehicle and unfortunately got caught in the verge and overturned."
In Londonderry, road gritters had to be withdrawn from service after they came under attack from stone throwers.
Mr Brown described the attack as "totally unacceptable".
"Gritter drivers are out to protect and assist the general public," he said.
"The drivers are out on frozen roads in hazardous conditions late at night. The last thing they need is missiles, there's no sense in it and I'd urge anyone involved in it to ensure it stops."
Sinn Féin councillor Kevin Campbell said a group of about 25 young people, aged "between 10 and about 15" were responsible for the attack in the Creggan area.
He said: "They were throwing snowballs at windows and when the gritter came in, they attacked it, first with snowballs, then stones.
"We had a similar incident a number years ago, when the community and voluntary sector had to step in to escort the gritters."
Mr Campbell added that parents on the estate need to take greater responsibility for their children.
"It beggars belief. This was happening at half past eleven at night. These children have school the next day. There's a missing link with parenting here.
"The gritter is vital to the area of Creggan. Creggan sits on a hill and when the snow comes, we get hit very hard, so that service is vital."
He said that in the interests of health and safety, the gritter withdrew from the area.
Near Ballymena, County Antrim, Carnalbanagh Primary School was closed on Friday, while St Joseph's Primary School in Galbally, County Tyrone was also shut.
In County Down, Carrowreagh Road in Dundonald and Edenticullo Road in Hillsborough have been closed due to icy conditions.