Fire crews are due to practise dealing with a major crash in the Southwick Tunnel on Friday night.
Crews from East and West Sussex will use a routine closure of the tunnel for maintenance as an opportunity for a training exercise.
The simulated collisions and mock rescues are intended to test the emergency plans and procedures for dealing with a major incident in the 535-yard tunnel on the A27.
Traffic on the Brighton bypass will be diverted at the Hangleton and Portslade turn-off and via the Holmbush roundabout in Shoreham from 8pm on Friday to 6am on Saturday.
The exercise is scheduled to start at 9.30pm and last for about two hours.
Fire crews from Brighton, Hove, Shoreham, Lancing and Worthing will take part in the exercise.
They will respond to a 999 call reporting a serious incident deep inside the tunnel, including multiple vehicle collisions and fire.
To make the scene as realistic as possible scrap vehicles will be strategically placed along the road inside the tunnel.
And smoke machines will create a smoke-logged atmosphere in which firefighters wearing breathing apparatus will carry out their search and rescue operation.
The East Sussex Fire and Rescue Service borough commander for Brighton and Hove Keith Ring said: “With the tunnel closed for routine maintenance it provides an ideal opportunity for firefighters from both services, working alongside partners from South East Coast Ambulance Service and Sussex Police, to test themselves in what should be an extremely realistic simulation.
“I hope no one taking part will ever be tested in a similar situation for real.
“However, it’s important for all of the services to work together to plan and prepare for as many potential emergency situations as possible.
“The experience gained from exercises such as these can make a massive difference in a real life situation.”
Roy Barraclough, deputy district commander for Worthing and Adur at West Sussex Fire and Rescue Service, said: “This kind of joint training opportunity is invaluable for the fire service so we can put our emergency plans to the test.
“Thankfully, this kind of incident is extremely rare.
“However, it’s really important that we plan and prepare so we have the skills and resources to respond should an emergency like this occur.
“Members of the Tunnels Operations Team (Balfour Beatty-Mott Macdonald) will also be present to observe and advise.
“Any lessons we learn from this exercise will, of course, be shared with other services and be incorporated into our pre-determined response plans.”