People have been urged not to travel as an amber alert for snow remains in place for much of Scotland.
About 1,000 vehicles were stuck on the M80 motorway overnight, with 300 still there on Thursday morning.
Some motorists spent 18 hours on the road before the carriageway was finally cleared on Thursday afternoon.
However, Police Scotland have advised people not to travel during the amber warning period, which runs until 10:00 on Friday morning.
Emergency crews battled throughout Thursday to clear the M80 outside Glasgow, a job made more difficult by the number of abandoned vehicles.
Transport Minister Humza Yousaf tweeted that all but 10 cars had been removed from the road by about 15:00, and it reopened fully after 17:00.
However, with conditions still difficult and a Met Office amber weather warning still in place, motorists have again been urged not to travel unless absolutely necessary.
A red warning of potential risk to life expired at 10:00 on Thursday, but roads across the country remain treacherous.
Traffic Scotland said "numerous routes" across the country were "experiencing snow blowing off the fields reducing visibility and leaving drifts".
The vast majority of ScotRail services are still cancelled, but the rail operator is now running a "very small number" of trains in the central belt.
None will run after 19:00, and the firm warned that there will be no services in the worst-hit areas on Friday morning while lines are tested.
Most buses in the central belt have also been cancelled and Glasgow Subway services will end at 19:00, but Edinburgh trams are running.
About 200 people spent the night at Glasgow Airport, where all flights have been cancelled for the rest of the day.
The airport said it hoped to reopen on Friday morning, depending on the weather. However, it said there would be continued disruption and urged passengers to only travel after checking on the status of their flight.
Some flights went ahead at Edinburgh Airport on Thursday, but these have now finished and bosses urged passengers not to come to the airport, saying airlines were "currently assessing their plans for tomorrow".
More than 1,600 schools across the country are closed - with the vast majority expected to also be shut on Friday. Parents should check their local council websites for details.
Elsewhere, the Scottish Conservatives have cancelled their conference, which had been due to open in Aberdeen on Friday.
Speaking at Holyrood, First Minister Nicola Sturgeon delivered a blunt warning to haulage firms, saying there were far more lorries on the roads than there should have been during the worst of the wintry conditions, and paid tribute to the work of the emergency services.
She said: "I saw some branded HGVs in pictures yesterday and given the branding on them I would struggle to say that their transport was unavoidable."
Ms Sturgeon stressed: "The advice today for the general public remains do not travel unless it is unavoidable.
"And let me be very clear what that means. While everything possible will be done to keep roads clear and open, if you do travel during this period you do face significant risk of encountering blocked roads and possibly becoming stranded."
However, Brian Kenny of the Road Haulage Association told BBC Scotland's Newsdrive programme that the first minister had "missed the point".
He said: "Most of these HGVs were trying to get back to a safe haven."
Fresh heavy snow fell in many places overnight, with the Met Office saying some areas of Scotland saw up to 30cm (11.8 inches) of snow fall and up to 40cm (15.7 inches) in "a few places" by mid-morning.
Emergency services spent the night battling to keep vehicles moving on the roads, with a particular focus on the M80, which runs between Glasgow and Stirling.
The road became blocked on Wednesday afternoon and for much of the night, with an estimated 1,000 vehicles at a standstill at its height as eight-mile tailbacks built up in both directions.
Some drivers abandoned their vehicles on the road overnight, making it harder to remove them.
Police spoke to trapped motorists to ensure they were OK and to distribute water and snacks, with a plan implemented to allow agencies to go in and recover stranded vehicles and treat the road.
Lesley Forster told BBC Scotland she had managed to get home after spending a "horrific" 18 hours on the M80 near Cumbernauld.
She left Edinburgh at 14:00 on Wednesday and got back to Mollinsburn, north of Glasgow, at 08:00.
'Had nothing with me'
Ms Forster said: "I had nothing with me. I had to use the water I had with me to put in to my window wipers. I was stuck for 18 hours and have never been so frightened in my life. It was freezing cold. I had a total meltdown.
"There were no snow ploughs or anything. A young lad came out of his house with a sled filled with water and a few bits of food - all he had in the house - and handed it out. I am so grateful to him.
"Once it started to get light and you could see everything round about you and you have got the reality of it, and were thinking: 'You are going to come out of this alive".
Local residents delivered food and drinks to many trapped motorists, with Mountain Rescue personnel and other emergency service crews also involved.
Among those stranded was a bus full of passengers.
Stein Connolly at the Transport Scotland control room said that while problems on the M80 had been cleared, issues should persist into the weekend.
He told BBC Radio Scotland's Newsdrive programme that he had "never been as close to the Met Office in my life", saying that "I don't think it is going to end, not right away".
Mr Connelly added: "We have got the amber warning until tomorrow but there is still yellow warnings right into the weekend.
"So we are still going to have some challenges, some cold temperatures and some snow, but if we can maintain the roads the way they are now, hopefully going into a yellow we will be in a better place and we will be able to maintain that."
Police Scotland told motorists to avoid the worst-hit areas, and to stay in their vehicles if they do become stuck.
Supt Helen Harrison said: "We do not want anyone to put themselves or others at risk in these conditions.
"If you can postpone your journey in affected areas until after the amber warning ceases, I would urge you to do so."
Meanwhile, friends Leanne McKillop and Yvonne Davies were among those who spent the night at Glasgow Airport, where they said they have had nothing to eat and nothing to sleep on.
Ms McKillop told BBC Breakfast: "We have been stuck here since 10am yesterday morning. The airlines basically just walked away saying it was to do with the airport.
"There are people sleeping on the floor, there were not enough blankets. There was bedding for about 100 people and there are about 500 people here.
"Our flight is meant to be going at 12:30 today but we don't know - it is a case of wait and see."
This was the first time a red alert for snow has ever been issued in Scotland, and only the second time anywhere in the UK.
Where to find the latest weather and travel information
Up to date forecasts can be found on the BBC Weather website
In times of severe disruption you can also follow the BBC Scotland severe weather Twitter list of key sources.
We have compiled a list of where you can find information on school closures.
Below are a number of other traffic information sources.
What the weather warning colours mean
- Yellow: Severe weather expected. Yellow means you should plan ahead thinking about possible travel delays, or the disruption of your day-to-day activities.
- Amber: Be prepared for disruption. There is an increased likelihood of bad weather affecting you, which could potentially disrupt your plans and possibly cause travel delays, road and rail closures, interruption to power and the potential risk to life and property.
- Red: Extreme weather is expected. Red means you should take action now to keep yourself and others safe from the impact of the weather. Widespread damage, travel and power disruption and risk to life is likely. You must avoid dangerous areas and follow the advice of the emergency services and local authorities.
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