Here are some ideas about how you can stage the important central accident scene.
Character 3 and two friends are driving along (stage this with two characters standing in a spotlight with the third character leaning into the space between their heads and shoulders). They are laughing and shouting. The 'car' stops and the two friends exit, leaving character 3 alone. The action freezes, and the lights go up on the other side of stage.
Character 1 is shouting off-stage, having an argument with her husband. She crosses the stage, grabbing her car keys as she goes.
Character 2 is at home, when the sound of a crash outside is heard.
He goes to the 'window' and looks out. He mimes picking up a phone, and gives details of the scene he can see outside. He says he's a doctor and will go to help. Blackout.
When the lights come on again, the sound of loud music is heard and two bodies are seen, lying awkwardly - one on the ground and the other on the chairs (as if in a car). Character 2 approaches the bodies and leans into the 'car' to turn off the radio. Silence.
Character 2 looks at the two bodies, and bends down to check for signs of life. He starts making the accident victims more comfortable. Character 3 starts moaning, character 2 tells him that the ambulance will soon be here. Freeze.
Using physical theatre, characters 1 and 3 are drivers of cars made up of the other actors. Each 'vehicle' faces the audience, and each is lit by a spotlight, with the rest of the stage in darkness. The actors making up the vehicles can make the sound of the cars, gear-changing etc.
Character 1 is angry and driving badly. She talks about her husband and their argument this morning. She makes angry comments about other drivers on the road. She jumps a red light, saying 'No-one will be around this time of the morning'.
Character 3 is playing music (perhaps rap style performed by a member of the cast forming his car.) He sees character 1 crossing his path, and shouts, 'I can't stop!' Character 1 shouts, 'Oh no! Another car - I'm gonna hit him!'. Sound effects can be used here to create the skidding noises, engine noises and sound of breaking glass. The lights go out. Silence. General lights come back up to reveal two bodies lying on the 'road'.
Voices are heard saying things such as: 'She drove straight through the red light!' 'The other driver didn't have a chance!' 'She was going too fast!' 'Are they still alive?' Everything sounds confused and chaotic. Eventually a siren is heard as the lights fade.
Deciding on the ending is important too. Do you want to finish using the same sequence as at the start? This could focus the audience's mind on the characters and how they came together in a fateful manner.
Do you want to have character 3 in a wheelchair - facing his life without hope of walking again? Perhaps using a monologue to tell the audience how he feels?
Do you want the families of each of the characters trying to come to terms with what has happened?
The final scene must have impact, and make clear the whole point that you intend for your drama.