Question: what do you do?

Mark Hill answered on 17 Mar 2011:

Hi einstein54,

I am a police officer, who is a collision investigator. I go to fatal road crashes and collect all the physical evidence, to allow me to find the cause of the crash. This is for the courts, especially if someone is at fault and may be prosecuted, but it is also to see if things need to be altered to prevent a similar crash in the future, to save a life, or lives.

I collect the evidence but starting at the end – where the vehicles and bodies have come to rest, and then try and work back along the skid marks, scratch marks and debris, toward where the vehicles came from. I photograph, electronically survey, measure and collect evidence from the scene. Sometimes I use scenes of crime procedures for casting tyre prints, lifting body marks and finger.palm prints, remove airbags for DNA analysis and collect bulbs for later lab analysis. I may put all the vehicles back onto thier tyre marks at the point of the collision, to show how they were positioned in relation to each other when they collided. That one photograph can explain a lot. Finally I do a skid test, by skidding my vehicle from a known speed. That is important.

Later, I will fully mechanically examine the vehicles, for faults that were present before the collision, that may have caused or contributed to it. I will examine evidence in our lab, using different tests and microscopes.

Then I will sit down and get my calculator out. The skid test that I did at the scene will allow me to calculate the co-efficient of friction, the grippiness between the tyre and the road surface. From that, I can use Newton’s laws of mechanics and maths to try and calculate the speed of vehicles and pedestrians involved. Post-mortem evidence, from looking at injuries, can also help in working out how a collision happened. The way that a bone may break, and any fragments that are produced, can tell me which way that bone, therefore body, was hit from. Was the pedestrian facing toward, away from or walking left or right in front of the vehicle. This is very important.

After all of this, and having downloaded my survey data, I produce a full report, with a scale plan. This is critically peer reviewed in my office, the maths verified and then printed and bound. The report is then presented in my evidence in courts and inquests, to explain how the crash happened. This assists with helping to prosecute the guilty drivers, but equally important, may also prevent innocent drivers from being prosecuted. I go to the court for everyone there, not just for the prosecution, or the defence. I am there for the judge and jury as well.

As you have gathered, I love my job and wouldn’t want to do anything else instead. I am really lucky in being able to use science in real life (and death) events. It is this practical application of the sometimes ‘boring’ science and maths that makes my job so interesting.

Thank you for your question.

Mark.