The collision, involving an HGV is between Fishbourne and Warblington, the fire service said.
Crews from Chichester, Bognor, Hampshire, Havant and Cosham fire station are on the scene alongside the police and ambulance service.
One person from the HGV was rescued using small tools with the driver of other vehicle self-released prior to the fire service arriving, a spokesman said.
He added: “Crews are now working to contain a diesel leak using environmental packs.”
Other major roads in the county are largely clear of traffic with no significant delays.
On the railway network Services between Worthing and Littlehampton/Barnham remain disrupted due to a broken down engineering train, Southern Rail said.
A spokesman added: “We can run services in the Brighton/London-bound direction, but Littlehampton/Barnham-bound services will be cancelled or diverted.”