The two main roads between England and South Wales were closed after sheets of ice up to a metre square fell from overhead sign gantries and smashed the windscreens of five vehicles.
Meanwhile 200 motorists were helped to safety after being stranded overnight when snow fell a foot deep in Devon.
They were helped from their cars by police, Army and civilian teams on the A38 and A380 south of Exeter and the A386 near Tavistock.
Thousands of schools across the country were forced to close while travellers faced airport hold-ups and train and bus service delays.
The two Severn bridges were closed for "safety reasons in the present weather conditions", the Highways Agency said.
A spokeswoman said: "There is more traffic on the road today compared to yesterday so we are hoping to get the message out so people won't attempt to head to the crossing at the moment. We are hoping to keep the disruption to a minimum."
Both the M48 Severn Bridge, which is 43 years old, and the M4 Second Severn Crossing, which opened in 1996, were shut.
One lane in both directions on the M48 bridge reopened after nearly six hours, with a speed limit of 50mph, while the other bridge remained fully closed until further notice.
Jim Clune, general manager of Severn River Crossing PLC, said sheets of ice up to a metre square fell from overhead gantries and smashed the windscreens of five vehicles.
He said: "It's very much a safety hazard and of course these gantries span all carriageways on the motorways."
There were no reports of injuries or accidents. The incident was "very unusual" and may have been caused by thawing, he added.
The stranded motorists in Devon were helped by police and Army teams who used 4x4s to reach trapped drivers, with other motorists choosing to make their own way to safety.
Around a million children enjoyed an early start to the weekend as up to 4,000 primaries and secondaries were shut.
In the West, more than 1,000 schools were closed, while similar numbers were shut in the Thames Valley and in a combination of eastern England plus counties to the north of London.
Wales was also badly hit, with more than 200 closed.
Luton Airport was closed for a number of hours and when flights resumed they remained subject to delay and/or cancellation. Services at Bristol were also suspended for a large part of the day, with many flights cancelled.
First Great Western train services between Bristol Temple Meads and Severn Beach were suspended, with no replacement transport available, because of poor rail conditions.
The AA said that by the end of today it expected to have attended a total of more than 70,000 breakdowns since Monday.
More than 6,100 call-outs were received today by 12.30pm, and the workload throughout the day was double the norm for a Friday in February.
The severe weather conditions left 21,000 homes across the West Country without power.
Western Power Distribution said Devon was worst hit, with 12,000 residents without electricity.
Properties in Cornwall and Somerset were also affected, with 8,000 homes in the Taunton area cut off.
Forecaster Rachel Vince, of MeteoGroup, the weather division of the Press Association, said the main hazard tonight would be widespread icy conditions.
"There may be snow flurries in some areas, but nothing like last night, and it will get colder, with temperatures falling to -3C (27F) or -6C (21F), or locally -10C (14F).
"Tomorrow will be a cold day, and there should be some sunshine around, then Sunday sees another weather feature coming in from the west, and there may be some snow, turning to rain in most areas.
"Indications are that Monday's rush hour will not be as problematic as last Monday's, but it is something forecasters are keeping a close eye on, and people should be careful to check their local forecasts and traffic reports."
Councils have been perilously close to running out of salt and grit, with some forced to ration its use.
Transport Minister Lord Adonis said the Government was taking steps to ensure councils were able to get the salt they needed.
"We are working to co-ordinate with local authorities to see that local authorities which face an acute situation are prioritised for new supplies," he told BBC Radio 4's The World At One.
"We essentially have been liaising between local authorities to ensure that those that are short of supplies get them from those who have got more."
He said ministers were looking to see whether they needed to bring in additional supplies from abroad.
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