I am a retired Metropolitan Police Traffic constable. I was for some time attached to the investigation unit. Whilst I agree that 100 per cent of fatal accidents cannot be accurately reconstructed, nearly all can. Evidence is always left at the scene in some form or another, allowing one to reconstruct the speed and direction of vehicles, pedestrians etc. It would be a serious neglect of duty if police did not investigate every sudden or violent death, which must be presumed criminal until proved otherwise. I will however agree with you that it not always necessary to completely close roads, providing the scene and personal are protected from passing traffic. In my day, we were in charge of the streets, but this did not mean that we demonised drivers.