Thousands of motorists on many roads in Wales experienced gridlock as a second day of heavy snow created travel chaos and closed more than 600 schools.
The unforecast snow also hit buses and trains and delayed up to 2,000 rugby fans heading to Edinburgh for Saturday's Six Nations game.
Ambulance chiefs said services were at full stretch and 999 calls should only be made for life or death emergencies.
Cardiff, Carmarthenshire, the valleys, Swansea, and Powys were worst affected.
People were stuck for up to 5 hours on the A449 at the Coldra roundabout near Newport.
At 2000 GMT police said the traffic was starting to ease, but two lorries which had broken down were causing problems on another part of the road.
The Welsh Ambulance Service said staff were battling snow and ice to reach calls amid a mounting number of calls.
Chief executive Alan Murray said: "The weather is delaying our responses across Wales and it is also increasing the number of calls we are getting.
"Responses have been particularly difficult in the south and this pattern will continue across the country as the snow moves north.
"We would ask for public understanding in these difficult circumstances and would assure everyone that the service is doing its best to manage."
St John Cymru Wales' specialist vehicles were brought in to support ambulances.
Earlier in the day the M4 was closed between junctions 47 and 49, Swansea West to Pont Abraham and later police closed the A48 between Carmarthen and Cross Hands.
Drivers were asked to leave their cars on the side of the road and head to a rest centre in Cross Hands or Carmarthen.
People with young children, or elderly or poorly people in their cars were asked to stay in their vehicles and call officers.
N-ice house: Gareth and Mike sent this shot of their igloo in Pentrebach
Police and ambulance workers used 4x4 vehicles to try to help people stuck on the road, with concerns about freezing overnight conditions.
Roads in many areas, such as Newtown in Powys, were only passable with care and police asked motorists to make only essential journeys.
The second band of wintry weather in two days surprised forecasters who had predicted no more snow after Thursday's downfall.
But on Friday they forecast up to 10cm (4in) of snow, with reports from mid Wales of much more.
BBC Wales meteorologist Derek Brockway said: "The forecast has gone badly wrong.
"The low pressure over north-west France moved much further north giving us snow on what should have been a dry day.
"There's more rain and heavy snow for some areas to come, clearing tomorrow and turning milder on Sunday.
"However the thaw could create localised flooding," he added.
At Cardiff International Airport, between 1,500 - 2,000 rugby fans were delayed for several hours waiting for flights to Edinburgh for Saturday's Six Nations clash with Scotland to take off.
There was some disruption on the railways
A flight to Cork was diverted to Bristol and passengers were forced to travel there. The airport said there would be delays throughout the afternoon and evening.
Overnight temperatures fell to -3C in places and drivers were warned to beware widespread icy patches as temperatures drop.
Many routes throughout the country had bumper-to-bumper traffic.
In south Wales, the A4061 Bwlch mountain road was closed, as was Rhigos Road at Hirwaun and the A4233 Station Road at Maerdy and Aberdare.
On the railways, Arriva Trains Wales said most valleys routes were still running except Merthyr Tydfil, where there was a points failure at Abercynon.
There were cancellations on routes west of Swansea, as well as on the Heart of Wales line.
The Bridgend Designer Outlet shopping centre closed in the afternoon, staff at Carmarthenshire Council were advised to leave work if they had concerns about being able to get home, while all Newport and Caerphilly council offices closed from lunchtime.
The Welsh Assembly Government said its staff were expected in work "unless severe disruption to roads and public transport prevent them from doing so safely."
But it said staff living in badly affected areas should consider whether to leave early.
There are regular weather and traffic updates on BBC RadioWales on 93 - 96&104 FM; 882 & 657 AM or