Hundreds of people were trapped in their cars in freezing conditions on the A31 in Hampshire overnight.
Police declared a major incident and the military, coastguard and fire service helped to rescue motorists.
The dual carriageway through the New Forest was blocked from late on Thursday after two lorries jack-knifed.
Traffic began slowly moving shortly after 05:00 GMT. Police advised motorists to avoid the route due to abandoned vehicles.BBC reporter Duncan Kennedy, who was in the tailbacks overnight, said the carriageway in both directions was like a “snow-encrusted pavement” as temperatures dipped as low as -10C (14F).
Insp Andy Tester of Hampshire Constabulary said it had been “very challenging” with traffic blocked for 12 miles (20km) from the Ashley Heath roundabout near Bournemouth to the M27 near Southampton.
“It was a very high volume of vehicles and we have to prioritise the most vulnerable people.”
He said the road is passable but urged drivers to avoid the route as an operation to recover abandoned vehicles was underway.
Hampshire Fire and Rescue Service said it had five teams in 4×4 vehicles helping police provide food, drink and blankets to people stuck in the queues.A rest centre was set up at Copythorne Scout Hut with Hampshire County Council staff providing supplies for stranded drivers.
Defence minister and Bournemouth East MP, Tobias Ellwood, who was stuck on the A31 until 03:00, criticised emergency planners for not asking for military assistance until a late stage.
“We have to understand that when we get weather like this which is a massive shock to the system, there is an expectation and understanding that everyone chips in.
“To have decision-makers at the top of the tree being so delayed and not prompt enough in making the decisions, that is frustrating to the personnel wanting to get out and help – they can only be mobilised if they get the green light.”
A Hampshire Constabulary spokesperson said the situation had been “particularly challenging” with a lack of viable diversion routes.
“As a result of this and the number of people impacted, the current response was judged not sufficient by our Gold Commander and a major incident was called at 8.30pm..
“This enabled us, in line with the plans we rehearse, to take a lead and bolster the multi-agency response including immediately engaging with the military who were supporting us in our Command suite and working alongside us”.
An amber warning of wind and snow remains in place with the authorities warning of long delays and cancellations of public transport.