Road death investigation and the importance of forensic evaluation.
As a road policing officer based in the United Kingdom, I have seen advances in the last few years in automobile technology that has and continues to aid the investigating officer with a serious injury or fatal road traffic collision. The internal combustion engine has not really changed since the 1930’s. What has changed is the various black boxes that have been added to make that engine work more efficiently. Add to this, advances in chassis design and crumple zones making the modern automobile a very safe place to be; within limits! Advances in science, in particular the human gene and DNA have enabled investigators to solve cases many years down the line where when none existed. Interestingly, the science of investigating a collision has not really fundamentally changed since Sir Isaac Newton proffered the laws of motion 300 years ago!
So here we are today. With the advent of 4G, many car manufacturers are introducing that means of telecommunication within their vehicles. The prospect of broadband speed traffic over a mobile phone network means that the possibilities are endless. Real time engine management with the manufacturer; route tracking and stolen vehicle recovery. In essence a black box for the vehicle. This will aid greatly the investigating officer. But, these are only extra tools in the kit bag of an investigating officer.
In the UK, we treat a fatal road traffic collision as an unlawful killing until we know otherwise. That means we treat the scene as if it were a murder. Road closed and taped off; no one leaves the scene until spoken to and the collision site is kept sterile to preserve the evidence. More of this in a moment. Most importantly in any major investigation is continuity of evidence. I appreciate this web site is devoted to cold case investigation. Rarely do we in the road policing world have such a scenario but it does occur; in the way of a hit and run. Importantly though, if the collection of evidence at the scene is flawed then this can have very serious implications 4 or 5 years down the line when a civil claim is pursued.
So as a road-policing officer when I am first at the scene, the next twenty minutes or so are madness! Survey, assess, disseminate to my comms room. The over riding principle is the preservation of life. Only after that has been addressed does the question of evidence rear its ugly head. At any scene you will usually find the paramedics and fire service. They each have their own duties to perform; often in conflict with your own. Do the fire services cut out a deceased driver from their vehicle, which will inevitably destroy impact evidence? Do you allow the other emergency services onto the road, possibly over the collision scene, creating the very real danger of destroying marks left on the road? Clearly, the role of a road-policing officer is a difficult one in those first twenty minutes!
The scene of a collision is a gold mine of forensic evidence but not always. With the advent of ABS and increased stopping distances, vehicles do not always leave marks on the carriageway. Certainty to show who was driving, significant DNA evidence will be left on a deployed airbag. But to reconstruct the collision is more problematic. Witnesses will truly tell you what they perceived happened. It’s not quite the same as what actually happened and which is why it is so important that the scene of a collision is protected. For a scene never lies.
Using the principles of math’s and physics, a road collision scene can tell you a great deal. A vehicle goes through the four stages of braking. Prior to leaving tire marks on the road surface the wheels are travelling slower than the vehicle under braking. Any stones picked up in the tires will leave “grindings” or scratch marks on the road surface. These can be as little as 2 to 3mm in length. Clearly, to make the scene sterile is very important, as these marks are very easy lost.
Gravity, coefficient of friction and length of braked distance allows us to calculate the velocity of the vehicle at point of application of brakes. Transposing to get u (initial velocity) = the square root of the co efficient of friction x gravity x the length of the tire mark will give you the speed on braking. So, as you can see the investigation of a road collision is forensically based and far from simple. The equation above is just one of many that a collision investigator will use.