A Met Office yellow warning for ice and snow is in place for Cornwall and parts of west Devon.
Schools are shut and some roads are blocked after heavy snow fell in parts of Cornwall.
More than 80 schools are closed or opening late and drivers have been urged to take extra care.
Devon and Cornwall Police said the A30 and A39 near Carland Cross, near Truro, were both blocked, with reports of cars and lorries stuck in the snow.
A yellow warning for snow for Cornwall and parts of west Devon, and ice in all of Devon, is in place until 10:00 GMT.
Image source, Nicolle Mitchell
Erica, from Four Lanes, near Pool, told BBC Radio Cornwall snow started in the area at about 01:00 GMT and left cars trapped after “they were covered in snow in about an hour”.
BBC Radio Cornwall’s Emma Clements said St Dennis had experienced “probably around 10cm [4ins]”.
She said from one of the village’s greens “which is quite near the main road between Newquay and St Austell, which runs right through the village” it had started becoming slushy by about 08:00.
Image source, @sweatpeafreesia/Twitter
Cornwall Fire and Rescue Service tweeted it had received reports of vehicles getting stuck on the A39 at Trispen and the A391 at Bugle amid “hazardous” driving conditions.
It said: “If necessary, pull over if it is unsafe to continue.”
Police tweeted that highways staff were aware of problems caused by snow.
“Please allow extra time for any journey and drive with care,” the force said.
The Met Office warning said people should expect rain, sleet and snow, followed by clearing skies leading to icy stretches and some disruption to travel.
Cornwall Council said it had deployed gritters on main routes, but there was still the potential for ice on roads.
Devon County Council said its gritters had been out overnight, treating more than 2,000 miles (3,220km) of the county’s main routes.
About 14 miles (23km) of cycle routes around Exeter have also been treated.
Devon’s volunteer snow wardens have also been advised to be prepared to treat priority routes in their local communities this week.
No schools in Devon have been closed, but some school routes are not operating, the county council said on its website.
⚠️ Yellow weather warning updated⚠️
Snow and ice across Cornwall and parts of west Devon
Now – 1000 Today
Latest info 👉 https://t.co/QwDLMfRBfs
Stay #WeatherAware⚠️ pic.twitter.com/i43kzgj08e
Follow BBC News South West on Twitter, Facebook and Instagram. Send your story ideas to [email protected].