An overturned beer lorry on the A27 eastbound caused major delays for motorists.
Both lanes between Hollingbury and Falmer were initally closed by emergency services, who arrived at the scene around 11:30am.
Crews from East Sussex Fire and Rescue Service managed to shift the lorry so that lane two could be opened.
The lorry's load was removed and a recovery truck was brought in to clear the obstruction.
Traffic was diverted around the scene to Coldean Lane but some drivers reversed to travel the wrong way up a nearby slip road, which was not sanctioned by police.
Inspector Phil Nicholas, of Sussex Police's road policing unit, said: "Full closure of the eastbound carriageway lasted some 45 minutes at most.
"Just as we advise drivers to take warm clothing and hot drinks with them during the winter, it is sensible to ensure you have water and the means to keep cool when the weather is hot.
"We regret delays caused by this incident, but thank motorists who showed patience while everything was being done to get traffic flowing again."
The cause of the incident is being investigated and anyone who saw what happened should call Sussex Police on 101 or email firstname.lastname@example.org, quoting serial 543 of 18/7.