The north-south A470 is closed between Rhayader and Doldowlod, Powys, after a crash.
Poor driving conditions also closed routes including the A542 Horseshoe Pass near Llangollen, Clwyd, and the A4233 and A4061 around Merthyr Tydfil, mid-Glamorgan.
An AA spokesman said: "Drivers should not be lulled into a false sense of security by the break in the heavy snow - ice is far more dangerous to drive on than snow."
A teenage boy died and his mother was seriously injured when they were hit by a lorry after leaving a car that had crashed on the A1 in North Yorkshire near Scotch Corner on Wednesday night.
A 42-year-old man died further along the road when his car crashed with a lorry south of Cockburnspath in Berwickshire.
In Aberdeenshire, a man was treated in hospital after being injured when the weight of snow caused a cattle shed to collapse on Thursday morning.
Meanwhile, a search has begun in Glen Affric in the Highlands for a 40-year-old man and a boy, 15, who have not been heard from for two days.
Widespread school closures have continued, with about 950 shut in Wales and 700 in Scotland.
About 2,000 closed across northern England, as well as hundreds in the Midlands and more than 1,000 in the South West, along with the University of Bath.
The rest of southern England was badly hit, with more than 900 shut in Bedfordshire, Hertfordshire and Buckinghamshire, 250 in Oxfordshire, 380 in Surrey and 600 in the Solent.
Nearly 600 were shut in East Anglia, with a further 500 in Kent and 200 in Essex.
The Federation of Small Businesses has criticised head teachers for closing schools too readily, saying lost working days affect many companies.
Overnight temperatures plunged to a low of -18C (0F) in Woodford in Greater Manchester and Benson in Oxfordshire. Monitors recorded -13C (9F) in Manchester, -6C (21F) in Glasgow and - 3C (27F) in central London.
Up to 5cm (2ins) of snow is forecast for eastern England during Thursday, and snow showers are also possible in Aberdeenshire, the Midlands, the South East, Pembrokeshire and Cornwall. Stronger winds could lead to some drifting.
Maximum temperatures should remain around freezing nationwide, while overnight it could drop to -20C.
BBC weather forecaster Liam Dutton said: "Ice will be the main hazard for most people today. Side roads will be a nightmare."
Sporting fixtures continue to fall foul of the weather, with race meetings at Kempton and Lingfield called off and the weekend's racing, rugby and football fixtures under threat.
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