Most collisions are caused by driver error. Less than two
percent of collisions are the result of a mechanical problem with a
The Collision Investigation Unit is on hand to investigate any
fatal road traffic collisions or ones in which a person has
When the unit is called to a collision, they will seal off the
scene and start to investigate.
“We are trained to look for specific evidence such as tyre and
scratch marks on the road surface, but there is an abundance of
information that can be collected at the scene,” explains PC Mike
Baddeley from the Collision Investigation Unit. Officers rely on
maths and physics and the laws of motion to make their calculations
and must undergo special training in these areas to join the
“The evidence is interpreted and used to reconstruct the
collision. We can often show how a vehicle was moving – was it
swerving, had the driver control of the vehicle? In addition, we
can often calculate the speeds of vehicles at the time of the
collision quite accurately”
All marks are photographed and the scene surveyed using the
latest techniques and equipment. A scale plan is produced using a
CAD software package. The plan and photographs are referred to in
the report so others can visualise the scene.
Some stored information can be extracted from the car’s Engine
Control Unit which controls all sorts of systems on a modern
vehicle. This information can show what certain systems, such as
the traction control or ABS, were doing at the time of the
collision giving the Investigators more evidence for their
“Physical evidence at a scene can be a lot more helpful than eye
witnesses,” Mike explained. “A collision takes only a few seconds.
It is difficult for a witness to a very traumatic event to take in
and process a lot of information in such a short period of
All this evidence is used to help prepare an unbiased report on
the circumstances and cause of the collision.
Sometimes these reports prompt the Highways Agency and local
councils to alter the layout of a road or a junction to help
prevent further collisions.
The Collision Investigation Unit attends about 200 collisions in
a year, mostly on the main roads through the county, despite
Cheshire having a very extensive motorway network the Unit only
spends about 5% of its time investigating motorway collisions.
“We compile a report for every fatal collision and a few very
serious ones. Our reports are passed to the Crown Prosecution
Service (CPS) and the Coroner. The CPS then decide whether any of
the drivers involved have committed an offence and whether they
should be prosecuted. Our evidence is pivotal in many cases.” Mike
said. “Personally I hope our reports help the families of the
collision victims understand what happened and help them come to
terms with their loss.”