There are delays of five minutes with queues increasing on the A270 Lewes Road northbound in Bevendean, the AA said.
The average speed is reported to be 10mph.
There are no major delays being reported on the county’s rail network but planned engineering work is taking place.
A Southern Rail spokesman said: “Please follow Government travel advice for your tier when travelling this Christmas, with those in tier 4 making essential journeys only.
“Engineering work will be taking place across our network on all days. For more info [see] http://southernrailway.com/Christmas”