Why people died in accident are called Shaheed?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The term Shaheed (martyr) is only used for a person on whom the Book of Allaah or the Sunnah of the Prophet stated that he is as such, either he is mentioned by name or his death is described as being a martyr [because of dying in a specific way]. We have already issued Fatwa: 88945 on the categories of martyrs, so please refer to it. So the evidence is what is mentioned in the Book of Allaah or the Sunnah of the Prophet and not what people say. However, some scholars are of the view that whoever is killed by a car, or that his car rolled over and killed him without any negligence on his part, is considered as a person who died under falling debris [meaning that he is considered as a martyr], as mentioned in Fatwa: 89399. The same thing applies to a person who dies when his car catches on fire, or is drowned in a river or the like, but he has to be washed, enshrouded and people should pray on him because the ruling of a martyr who dies in a battle in this world does not apply to him. 

Allaah knows best.