Snow settling on the surface of the A27 has led to lengthy tailbacks from before the Southwick tunnel east-bound and east of Brighton west-bound.
Driving conditions are worse on the A270 Old Shoreham Road and treacherous on side roads.
Freezing temperatures are expected overnight, turning many roads to ice sheets.
With further heavy snow falling and forecast, Sussex Police has re-issued its travel advice given earlier today.
The force is now recommending that drivers should not set out on journeys unless absolutely necessary.
As darkness has fallen, even the main trunk roads have started to accumulate snow in places and ice is becoming widespread.
Chief Superintendent Paul Morrison, head of operations for Sussex Police, said: “As night falls and temperatures drop, driving conditions in many areas, especially rural routes, are treacherous.
“With more snow forecast by the Met Office, I would suggest that you should not be driving unless it is absolutely essential.
“At best, your trip could be long and stressful.
“At the worst you could find yourself involved in an incident that may or may not be of your making.
“If you must set out, carry blankets, food and a flask of hot drink in case you get stuck and have a shovel or spade to help with digging out your vehicle or clearing a path in snow or ice.
“Remember that conditions may be worse en route than they are where you set out from.
“Make sure that your windscreen and windows are clear and fully defrosted and that your screen wash is topped up and not frozen.
“Don’t leave your car to defrost on its own – opportunist thieves aren’t deterred by the weather.
“Leave more time for your journey and drive slowly and carefully to take account of icy conditions and leave a good distance between your car and any vehicles in front of you.
“Be aware that pedestrians may be walking on the side of the road because of icy pavements.”
