One man has died after a lorry crashed through the central reservation of the A27.
The route is shut between the Fishbourne roundabout in Chichester and the Emsworth junction in Emsworth in both directions.
A West Sussex Police spokeswoman said a vehicle travelling westbound had crashed into the central reservation and ended up in the eastbound carriageway.
They added that no other vehicles were involved.
A spokesman later added that a male fatality had been reported and the next of kin would be informed.
Highways England have said a lorry went through the reservation.
Police and other emergency services are at the scene carrying out an investigation.
Highways England has said the route is not expected to be cleared until at least 3am.
Drivers are advised to avoid the route, and a diversion is in place via the A259.
Drivers stuck in traffic have been tweeting about the delays.
Kevin Farrell tweeted: ‘Left work for my usual 25 minute commute at 4:45. It’s 7:10 now and I’m at a standstill and still miles from home. #a27 #traffic’.
Highways England have also said there has been a second collision on the A27 eastbound between the junctions with the A2030 and the A3(M).
One lane is currently closed.
The route is expected to clear by 9.30pm.