Hundreds of drivers are spending the night in their cars after becoming stranded due to heavy snow.
Mountain rescue teams have been sent to help those stuck on the M74 in Dumfries and Galloway after severe weather led to closures at Millbank and Beattock.
Traffic Scotland says gritters are clearing routes, but are having to work around a number of jack-knifed lorries.
Snow has also caused problems in Northern Ireland, forcing schools to close and disrupting public transport.
An amber Met Office warning for snow and ice is in place in south-west Scotland, extending to parts of the central belt, and parts of Northern Ireland until 08:00 on Wednesday.
Forecasters warn of “heavy squally showers of snow” which may cause power cuts and disruption on the roads, with up to 25cm (9.8ins) of snow possible on high ground, and up to 10cm (4ins) at lower levels.
Much of England and Wales is covered by a less severe yellow warning for snow and ice on Wednesday and for snow and strong winds in some places until the end of the week.
Steve Harris became stranded on the M74 at 17:30 GMT and was still stuck between junctions 11 and 12 more than eight hours later.
“I’ve not moved really in the last four hours,” he told BBC Radio 5 live at about 03:00 GMT.
“I’ve managed to get out of my car a couple of times to stretch my legs. The snow is probably between four and six inches deep around the cars.”
Another driver, Danielle, told the station earlier that she had been stranded for more than five hours.
“I stopped at 19:30 GMT and since then I’ve moved about two miles.
“I just can’t understand how you can be stationary for five and a quarter hours.”
James Coles, from Moffat Mountain Rescue, said his teams were taking fuel and food to those who need it.
“We’re going back and forth between the cars checking on the welfare of people and seeing if there’s anybody who needs to be taken out of their car, or needs any advice,” he added.
He said traffic was moving very slowly at 03:00 GMT, but there were still problems due “a significant number of jack-knifed lorries”.
Earlier, police in Dumfries and Galloway warned against “all but essential travel”.
Dumfries & Galloway Virtual Operations Support Team – which was set-up to respond the weather issues in the region – said: “We are working to clear the stuck vehicles and clear the crashes.
“Please remain in your vehicle and signal the mountain rescue team or police if you need any help or contact us via 999.”
Scottish Fire and Rescue’s station manager in Dumfries and Galloway said motorists had been driving “too close behind me” on his way to a crash, despite “blizzard conditions”.