I cannot see how he is immune from prosecution. HE is not the head of state. The family are not immune - I recall Princess Anne (as she was) being done for speeding in her Scimitar.
Anyway - not that many road accidents result in a prosecution. Accidents happen and unless there is clear evidence of evidence of a crime, then an accident it will remain and the insurance companies will sort out the mess.
Was there no close protection officer in the car with him? - I would have thought there always would be, but perhaps they sometimes follow in another vehicle.
Think if you read that wiki link then he and members of the royal household are immune from prosecution when they are on her land/property and they can't be arrested in front of the monarch and both the monarch and the state can't be prosecuted ergo she/the monarch is the state.