Ice and snow made conditions on the M8 treacherous near Harthill
Ice and snow have left Scotland's roads in their worst condition for 20 years, the transport minister has said.
Temperatures dropped as low as -16C and several areas of the country were hit by heavy snowfalls.
The runway at Edinburgh Airport was closed until early afternoon due to snow and a plane slid off the runway at Prestwick Airport.
Several roads were closed because of snow and a bus overturned in Perthshire. No-one was seriously hurt.
Fourteen gritters were among vehicles which crashed on Highland roads.
Transport Secretary Stewart Stevenson told the BBC: "When you have weather conditions of -16C, when even salted water will start to freeze, you are not able to keep the roads open continuously.
"With the very heavy snow fall we have the kind of conditions where you should only be travelling when it is absolutely necessary."
Peebles in the Scottish Borders suffered heavy snow falls
Temperatures fell below freezing in Glasgow, Edinburgh and most other towns and cities overnight, but plunged to -16C at Tulloch Bridge in the western Highlands and to -12C at Tyndrum.
The Edinburgh area experienced particularly heavy snow falls early on Wednesday morning, and there have been problems on the M8, M74, A1 and A9. West Lothian Council said that all schools in its area had been closed.
A Ryanair flight from Dublin came off the runway at Prestwick and ran on to the grass after apparently hitting ice. No-one was hurt.
And a Pegasus Travel single-deck service bus travelling on the Dunning to Auchterarder Road in Perthshire overturned at about 1400 GMT.
The incident happened on a stretch of the road near the turn off to Jeanfield Farm.
There were four passengers and the driver on board the Mercedes bus at the time. Police said all were "walking wounded", with one man receiving treatment for a cut to his head. No other vehicles were involved.
What I think people should do is look at the weather forecast, listen to the radio bulletins, perhaps look at the Traffic Scotland website
All of Scotland's police forces warned motorists to take extra care, particularly on untreated roads and pavements.
In the north east, which had been badly affected on Tuesday, police again urged drivers to stay off the roads unless their journey was absolutely necessary.
The A96 was described as passable with care, while Northern Constabulary advised drivers in the Aviemore area to avoid going out as conditions were "treacherous" on the A9 between the Slochd and Drumossie.
More than 70 schools in the Highlands were closed because of the weather.
Duncan MacNeill, who is in charge of looking after major roads for Transport Scotland, rejected suggestions that not enough was being done in order to keep the roads clear, particularly Scotland's busiest road, the M8.
He said contractors working for Transport Scotland had spread more than 250 tonnes of salt on the motorway over the previous 24 hours.
Mr MacNeill added: "What often happens is that if snow falls after the salt has been spread, it takes some time for that salt to work its way through the snow.
"Sometimes what happens is you get a larger amount of snow than you predicted in certain areas, but I can assure you that we have got tonnes of salt on the strategic road network and I reckon we have got about 40,000 tonnes of salt in storage. That would last a long, long time.
"What I think people should do is look at the weather forecast, listen to the radio bulletins, perhaps look at the Traffic Scotland website, which shows you areas where there are problems, and consider taking alternative ways to travel.
"The other thing is to keep away from the peak times when there is most likely to be the most congestion."
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