Highways bosses say the roads have been gritted but drivers still need to be sensible.
There's an Amber Met Office weather warning for snow from 2pm-9pm which could see up to 10 centimetres of the white stuff falling.
Police are warning routes such as the A30, A38 and M5 could be affected
Devon County Council say plans are in place to reach communities if they become cut off by snow.
Emergency services are being based around Haldon and Telegraph Hill.
Extra tractors and ploughs are also being located on the North Devon Link Road.
Devon and Somerset Fire crews are urging people to take extra care on the roads as stopping distances can increase ten fold in icy conditions.
A spokesperson said: "In severe and wintry weather it's even more important to plan your journey.
"Check the weather forecast in advance and plan your journey.
"In bad weather, major roads are more likely to be cleared and gritted, still allow plenty of time for potential hold-ups."
Click play to hear Hugh Griffith, Devon's Highways Operation Manager
Councillor Stuart Hughes, Devon County Council Cabinet Member for Highway Management, said: "Not all of the county will see snow this afternoon, and the south of the county is less at risk than other parts of Devon.
"But where we do see snow, if it does fall as forecast, then people could have difficulty travelling this afternoon, particularly if their route takes them over higher ground.
"The biggest risk will be between 3pm and 8pm, but there could be some showers beyond that which could potentially leave communities cut off tomorrow. We would urge everyone to plan their journey home and please take extra, however you’re travelling.
"Highways England’s gritting teams are also working around the clock to treat major trunk roads and keep them free from disruption.
Chris Regan, Highways England’s Head of Service Delivery in the South West, said: "Gritters are out treating our routes around the clock but it is still important to drive to the conditions when snow is forecast.
"If you need to travel in the afternoon, make sure you keep your distance and reduce your speed because, even in conditions that seem normal and the snow is not settling, it can be slippery if ice patches have formed, or where fresh salt has not been worked into the carriageway.
"Drivers should plan their journeys, monitor weather reports and pack a snow kit of blankets, food, water and a shovel if they really need to travel. We would however encourage drivers across the region to think about how they might return home this afternoon before the snow affects their journey."
Chief Inspector Adrian Leisk, Head of Roads Policing for the Alliance, said: "We are aware of the upgraded weather warning for this afternoon and tomorrow, which is an amber warning for snow encompassing parts of Devon.
"It is vitally important that the public take these warnings serious. It is in place from 2pm today meaning that it will impact rush hour traffic, therefore we advise that people give serious consideration to their journey home from work and the school runs; we recommend that people do not travel unless essential.
"The biggest reoccurring error that we see from road users is that during times of severe weather, they do not alter their driving habits.
"Please can all road users reduce their speed, give extra breaking distance between them and the vehicle in front, and if you must travel, leave both extra time for your journey and prepare for the worst - ensure you have a full tank, food and water in the vehicle, shovel and stop signs, a full charged phone and warm clothing; all the things to prepare you in-case of a breakdown."
David Peake is Somerset's Highways Manager
Across Somerset The Breeze understands there's more than 60 staff on call at the moment to help with the roads.
They're on standby for continuous gritting if needed - and can also operate snowploughs if required.
Somerset has 2,000 people receiving care at home in the county. The County Council doesn’t provide the care but works closely with providers to make sure they can support people when if disruptive weather comes.
A spokesman said: "We have been in regular contact with providers actively checking on those who are most vulnerable and working to make sure they have the help they need.
"Take extra care and look out for others during the cold and icy weather.
"Look out for friends and family who may be vulnerable to the cold and ensure they have access to warm food and drinks and are managing to heat their homes adequately.
"Perhaps discuss with friends and neighbours about clearing snow and ice from the front of homes and public walkways nearby to try and minimise the risk of falls, if you feel safe to do so"