A Traffic Homicide Investigator (THI) is a term used primarily in the United States of America for a police employee, generally a sworn law enforcement officer, who is assigned to investigate fatalities resulting from motor vehicle collisions.
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Overview
Traffic Homicide Investigators have special training that goes beyond standard accident investigation and equips them to conduct an investigation of the site of a serious or fatal collision and to treat it as a crime scene. They are usually a traffic officer who has investigated a great number of nonfatal crashes and have advanced towards more in-depth crash investigation. Although they may not be the first responder to a traffic-related fatality, they will be called upon to take over the investigation at an early stage and see it through to its conclusion.
Method
Traffic Homicide Investigators are "part physicist, part policeman and part victim's advocate". They carefully document the scene of a crash, ideally in a manner thorough enough to allow specially trained officers to reconstruct the scene at a later time, after the roadway has been reopened. This includes documenting the precise details of the roadway on which the fatality occurred; the state of any vehicles involved, including the use of safety features; the condition, legal status and attitude of the operator or operators of any vehicles, and any other local factors such as the weather and the presence or absence of appropriate road signage.
In addition, investigators will conduct interviews with drivers and witnesses to assist in building up an overall picture of what led to the crash.
United Kingdom
In the United Kingdom, specialised collision investigators working as part of the local police force's traffic unit will usually handle collisions that result in death or serious injury. They receive training equivalent to this in the USA and conduct their investigations with similar methodology.