At least one person has died and the A23 has been closed in both directions just north of Pyecombe after an accident.
The crash happened at about 4.30am this morning (Monday 11 January).
Five vehicles are reported to have crashed, with at least one overturning. Queues had already formed in the wind and the rain by 6am.
The roads were said to be waterlogged in places with plenty of surface water making driving conditions difficult.
The Highways Agency did not say when the road would reopen but there were indications that it would be shut for much of the morning.
It said that the A23 London Road was closed both ways, with queueing traffic due to a serious multi-vehicle accident between the A273 at Pyecombe and the A281 Henfield turn-off.
A diversion was in place via Sayers Common, the agency said. Queues formed on the A27 Brighton bypass after the A23 was closed from Patcham to allow recovery work to take place and crash investigators to do their job.
The route round the back of the Devil’s Dyke quickly became congested with others opting for the Shoreham flyover before heading north.