In Florida at a local PD here is how it worked: Patrol initially responds to the crash and upon arrival they will notify dispatch if there is a fatality or if anyone involved *looks* like they will be fatal shortly. A team of traffic homicide investigators will respond in this case. The entire scene including vehicles, people involved and witnesses is roped off, roads closed, traffic diverted, evidence photographed and collected, witnesses and other drivers/pedestrians interviewed. All nearby video from buildings, stores, dashcam, traffic cameras, etc. is collected.
If there is a body bodies etc. on scene EMS will not remove them after they are pronounced. The medical examiner will respond then detectives will start next of kin notifications. Same if they are transported to the hospital, next of kin still notified. This has to be done in person, not over the phone...Ever.
Blood draw for involved driver if there are signs of impairment (there usually are) is done on scene, all blood collected is entered into refrigerated evidence for a toxicology screen later.
The scene is then marked with paint then everything is cleaned up and collected. Some agencies have laser equipment that will measure the scene and convert it to a 'to scale diagram' right there. Other agencies have a laser on a tripod with an encoder and will need to 'shoot' the scene later using the paint markers as a reference point for shooting the reflector. Each point is shot individually and takes forever to complete. Think of a survey crew on the side of the road with one person shooting the laser and the other person holding a reflector (prism) on a stick. This makes a paint the dots looking template from which the scene is reconstructed to scale on a computer for court and insurance purposes. The roadway is usually closed again for this to occur.
The vehicles are seized and towed to an inside secured storage for evaluation. A download of the vehicle EDR is done with owner permission, if no permission a subpoena will be completed. At no time is anyone allowed access to the vehicles until the inspection and EDR reading is done. Yes, I know you are making payments on the car and this is your daily driver. Sorry, it stays in secured storage until we get what we need from it.
This is the first half of the investigation. The second half involved waiting for tox to return, evaluating all of the evidence, doing all of the math involved for vehicle speed and direction calculations based on skid marks, EDR data, and video evidence. Then writing an entire report and bringing charges, if any, against persons involved. The comes depositions, court time, trial, civil judgement cases, etc.
It takes months and months for even the seemingly simplest investigations to be completed.
Pssst.. I may have been a traffic homicide investigator in a former life....
