Snow and ice have made driving conditions difficult in some parts of the north and west of Northern Ireland.
There are reports of heavy snow in County Londonderry, including Tobermore, Draperstown and Maghera.
The Roads Service said it used snow ploughs to keep some routes clear and has gritted the main roads, but some minor roads are extremely icy.
Part of the road between Warrenpoint and Rostrevor in County Down has collapsed, according to Translink.
The public transport company said the collapse has affected its Ulsterbus Service 39 between Newry and Kilkeel, which will be diverted through Mayobridge and Hilltown.
Police have asked drivers intending to use the Rostrevor Road to take an alternative route.
Motorists contacted the BBC on Wednesday morning to say the A6 road between Derry and Dungiven was "treacherous" with compacted snow and ice.
A small number of schools in counties Londonderry and Tyrone have also been affected by the icy weather and will remain closed on Wednesday.
Aidan O'Callaghan from the Roads Service told BBC Radio Ulster: "Overnight it seemed to be a story of two halves. Down in Belfast there is no sign of snow, but up in the north west and the west of the country there was a significant snowfall yesterday (Tuesday) evening and overnight.
"Our staff and manpower has been deployed almost continuously since yesterday afternoon to spread salt and keep the roads clear and the snow plough has been in action overnight too."
He added staff were still working to clear "isolated pockets" where there is still "compacted snow".
Stenaline's 10:30 GMT ferry service from Belfast to Liverpool has been cancelled due to adverse weather conditions.
Some Ulsterbus services in Derry, Strabane, Coleraine, Omagh and Larne are subject to changes and cancellations.
Passengers should check Translink's Ulsterbus website for the latest travel updates.