Professional Road Accident Investigation


In Germany during 1885 Karl Benz, and apparently working separately, Gottlieb Daimler produced a type of rear mounted, self propelled, petrol fuelled, single cylinder four stroke engine vehicle. Whilst there had been other self propelling vehicles Daimler and Benz are accredited with producing the first motor car. Initially the motor car was for the wealthy. However, mass production of the Model T Ford, starting in about 1913, made the motor car available to the developing world.

By 1899 the first fatal road accident in Great Britain had occurred. A driver and passenger were killed in a motor car which rolled over whilst, 'cornering at speed.' Apparently earlier, in 1896, a Bridget Driscoll was run over and killed in the grounds of Crystal Palace. It is unknown how many injuries have been caused over the intervening period, however during 1997 there were in the order of 3.86 million insurance claims arising from our use of vehicles.
It is not surprising to learn that, in the years since its inception, the motor car has become the single greatest contributor of accidents.


The Concise Oxford Dictionary defines 'accident' as
(i) An event which is without apparent cause or is unexpected.
(ii) An unfortunate event causing physical harm or damage brought about unintentionally or
(iii) The occurrence of things by chance.

Over the past few years there has been a move away from using the word accident to describe an incident involving damage, injury or death on the road. Instead the police, media and some insurance companies have taken to using more descriptive words. The change has been driven, by those, who have suffered damage, or relatives of those injured or killed, who do not accept the actions, or lack of action, on the part of the responsible driver, falling in to the normal understanding and meaning of the word accident. Consequently the words crash and collision are being more widely used.

It is seldom found that a single factor was responsible for a collision, but for many there is a desire to isolate a sole cause. More often than not a series of events existed simultaneously to produce the unfortunate result. There are considered to be three key areas which may feature in every accident.

A vehicle defect will be contributory in a small number of collisions with a road layout or surface defect being the next greatest contributor but the overwhelming element is the road user and his inability to cope with his surroundings. Consequently the following definition of a crash may be more representative:

A rare random multi factor event
in which one or more road users
failed to cope with their environment.


The most obvious reason must be establishing whether there is any wrong doing by any involved party. Analysis of trends and common features of crashes, from which it is hoped reductions in the number and severity of such incidents may be possible, is undertaken by the government, who publish comprehensive yearly statistics. 
Insurance companies, solicitors, transport managers and individuals will want to establish what can be proven from the available evidence. Forensic examination of the scene and vehicles together with the actions of those involved may then form the basis on which a successful claim, or prosecution for that matter, may be made or refuted.

It is therefore imperative that all crashes are investigated thoroughly and professionally.


After a collision occurs physical evidence needs to be recorded. Details such as road and weather conditions, position of vehicles involved, road markings and signing together with any defects or obstructions. The presence and type of street lighting and any defects if appropriate. The exact positions of any marks made in or on the road or verges, pre collision or, as a consequence of the collision including dimensions are vitally important. Additionally detailed records, with photographs, of damage sustained by any vehicle together with details of injuries to pedestrians must be obtained. Consequently it is imperative to visit the scene as soon as possible after the incident to observe and record before evidence is destroyed, disappears or remedial works are conducted.

It may well be that an involved party or the police have some details, however, it is unlikely that sufficient detailed information has been recorded and is available to support a comprehensive investigation. It is easy to misinterpret or fail to appreciate the effect of a scene feature without actually visiting a scene personally.
In summary we consider it essential to visit, examine, record and photograph each crash scene and vehicle as soon as possible after the incident.


A fundamental difference between gathering evidence at a crime scene and investigating a road traffic collision is that the automotive crime scene investigator has but one opportunity to gather all the available evidence before the road is re-opened to the public. Furthermore road collision 'scenes' often cover a far greater area than those normally associated with collection of forensic matter. It is seldom possible, or indeed desirable, to keep such extended sections of the road sealed off for long period's time. The collision investigator needs to collect all the available evidence from the scene sufficient to allow rigours examination and interpretation of any forensic evidence found.
There are three elements to be considered in any collision:

At a collision scene the collision investigator would typically gather information about the following:

A typical set of collision investigation scene tools will include: Tape measure, theodolite, camera and flash gun, sight boards and a 'Skidman' meter. Field notes upon which to record the various details of the vehicles and environment. Although generally they cover the same topics it is common practice that investigators tailor their own field note forms.

When compiling the reconstruction report a collision investigator will use a number of other tools that will include calculators together with certain formulas to analyse such things as speed from skid marks. Computers and software will be used to produce scale plans from scene measurements, asses crush damage, pedestrian or vehicle occupant kinematics, position of the sun relative to a driver's view and simulation software to animate a collision from any forensic evidence found.

Reference may also be made to statistical data, vehicle manufacturer data and where appropriate technical reports and papers on related subjects.

It is estimated that there are over 3 million CCTV and other surveillance cameras operating within the UK. Consideration will have to be given whether whole or part of the incident may have been recorded by one of these devices. Many employ time lapse images and these will need to be careful analysed.

There will frequently, but not always, be eye witnesses to collisions. Witness statements should be taken from all those people who have seen the incident or the moments or minutes leading up to it. Witness accounts can be 'tested' against the physical evidence found.

Using the final position of vehicles together with any tyre skid, scuff and scratch marks it may be possible, using formulas, to assess the pre impact speeds of those vehicles involved in the collision. However before these calculations can reliably be applied to tyre and scuff marks the coefficient of friction between the two surfaces, tyre and road surface for example, needs to be established. With tyre skid marks this is commonly investigated with a 'Skidman' accelerometer device. Under best evidence practice it should be the vehicle that made the marks which is used during the skid testing. It will be appreciated that this will not always be possible due to damage sustained by that vehicle or the fact it is no longer at the scene! The laws of friction govern the results of skidding vehicles.

With most cars it makes little difference how wide a tyre is in relation to the degree of grip available. Consequently there is little difference in the ability to stop from one car tyre to another - in the dry. Some specialist motorcycle tyres have an improved capability and commercial tyres, those on goods and passenger vehicles, can return lower frictional values than cars under some circumstances.

Where the road surface is wet the overriding factor, in the ability to stop, is the depth of tread and the ability of the tread pattern to clear water away from the road tyre contact area. To achieve grip, in the wet, there must not be so much as a drop of water between the tyre and road surface. In circumstances where there is a greater depth of water than the depth of tread the possibility exists that a phenomenon sometimes referred to as 'aquaplaning' may occur.

It is a misunderstanding that the black marks associated with skidded tyres are created solely from tyre rubber. When a tyre is 'locked' against the road surface and sliding the heat build up between the tyre and bitumen surface draws the bitumen to the road surface leaving the skid mark. On concrete surfaces tyre skid marks are less well defined and are of course entirely from tyre rubber and a form a type of 'cleaning' of the concrete surface.

The length of a tyre skid mark is proportional to the square of the vehicle's speed. Careful examination of the marks may reveal small scratches prior to the start of any black skid marks. These scratches will have been created by tiny debris on the road, or loosened chipping's and will have been created before the tyre has started to skid, but is none the less under heavy braking. Additionally examination of the end of the skid mark needs careful consideration. Definite square ended skid marks tend to indicate a tyre has skidded to a stop. Whereas a fading out of the skid mark may indicate the driver has released the brakes before braking to a stop further away. The Laws of Friction govern skidding vehicles . It is not possible to steer a skidding vehicle. A deviation in a skidded mark maybe attributable to:

Alternatively sudden deviations in marks indicate where another force has interacted with that vehicle and changed the direction of the skid thus potentially indicating a point of impact.

The introduction on most modern cars of ABS is beginning to make skid marks less common. However careful scene examination can reveal scuff marks from vehicles that have braked heavily albeit the ABS has activated. Where no skid or ABS scuff marks have been identified the collision investigator will have to resort to other methods to assess pre-impact speeds of involved vehicles.

Dynamic safety systems which are being employed by Mercedes Benz, and other manufacturers, include ESP (Electronic Stability Program), to improve a vehicle's handling, braking and thus stability and ADS (Adaptive Damping System). Some of these facilities are often selectable by the driver to give a different 'feel' to the driving experience. These features may eliminate some of the sensations a driver is accustomed to receiving and lead him in to a false sense of security or ability with that particular vehicle. As the many manufactures hand-book's will say; these safety systems do not allow you to exceed physical limits!

Where a vehicle collides with another vehicle or roadside furniture and sustains damage it is sometimes possible to measure that crush damage and then with other information use computer programs to assess the change of velocity suffered at impact.

In 2002 there were in excess of 38,000 pedestrian casualties as a consequence of them coming in to conflict with motor vehicles. Unfortunately about 8500 of them need the attendance of a collision investigator. It is currently being reported that just 10% of pedestrians struck at speeds in the order of 40 mph are likely to survive. In the cases of a pedestrian collision the investigator will need to try and establish:

The location and nature of bodily injuries will help to establish the pedestrian's orientation at impact. This may help confirm where they emerged in to the road from. This may enable the investigator to establish whether the driver would have been able to stop in the distance available had he been travelling at an appropriate speed or show the collision was unavoidable.

When considering pedestrian collisions investigators may make use of research papers covering such topics as walking and running speeds of pedestrians, damage patterns on vehicles and how conspicuous the pedestrian was to the driver, particularly if the collision occurred after dark. It may also be possible to use the displacement a pedestrian has been 'thrown' to assess the impact speed of the striking vehicle.

Needless to say the perspective for the pedestrian must also be considered: Did they look before moving in to the road and had they looked would the vehicle and hence the perilous nature of their movement have been obvious? Suffice to say the high rate of pedestrian collisions indicates humans are notoriously poor at judging the approaching speeds of vehicles.

In all cases an examination of the involved vehicles needs to be undertaken in order to establish whether there could have been any mechanical defect or component failure which may be connected to causation. In many cases the collision investigator will undertake this task. However it is becoming common to find suitably qualified automotive engineers, working for the police, undertaking this part of the investigation and reporting their findings to the collision investigator and senior investigating officers.

Typically the condition and operation of brakes, steering, tyres, suspension and electrical systems will be investigated. Where component failures are found it will be necessary to ascertain whether these are as a consequence of the collision or had failed or existed previously. It is sometimes necessary to send components, tyres and bulbs for example, away for specialist forensic analysis.

Large goods vehicles and certain passenger carrying vehicles are fitted with tachographs. The tachograph is a device that record's distance and speed against time on to wax discs referred to as charts. The tachograph charts cover periods of 24 hours and can be microscopically analysed to provide a detailed record of a vehicles movements leading up to an incident. Correctly used tachograph charts show how much and when rest periods have been taken throughout the period in question.

It is not uncommon for unscrupulous drivers and operators of goods and passenger carrying vehicles to interfere with the information recorded by the tachograph, mainly to extend the hours that they drive. Modern tachographs are electronic and have features to prevent interference. However as with many other devices there are often ways to overcome the electronics and cheat on the instrument. The latest generation of modular tachographs uses encrypted data. In the near future the tachograph will be digital and the data no longer recorded on wax discs but saved on to credit sized cards.

Certain goods and passenger carrying vehicles may be required to be fitted with speed limiter devices which are frequently 'driven' from the tachograph. Consequently these are also open to tampering and abuse.

The existence of 'black box' data recorders in cars and commercial vehicles is on the increase in Europe. These devices continually monitor and record inputs from sensors around the vehicle and in the event of a collision retain that information. Typical operational data inputs include: Seat belt use, use of indicators, horn or lights. Down loaded data from the module can be analysed and accurately establish impact dynamics of the vehicle. Ultimately the information indicates driver responses prior to an impact.

On completion of an investigation a technical report is sent to the Crown Prosecution Service and/or in the case of a fatality Her Majesty's Coroner. In Civil proceedings reports are either confidential to the instructing party or prepared for the court.

Generally reports are of a technical nature and include photographs of the vehicles and the scene. Scale plans of the immediate scene may be accompanied by area maps.

Suffice to say it is not the role of the collision investigator to apportion blame, that is a task for the Crown Prosecution Service and the courts to decide. The collision investigator is there to record all the scene evidence and interpret those findings where they would be outside the normal understanding of the court or to help the court where appropriate. As such collision investigators are frequently called upon to give expert evidence during a trial, at an inquest or in civil proceedings.

Finally there are many people who may offer services to review data from collision scenes, examine photographs, interpret results from computer programs, pontificate and even 'speculate' about likely causes in a report. However it is unlikely they will have even been to the scene of a single collision. There is little better training and experience for reconstructing collisions than attending the scenes of 'real' collisions and studying the scene and vehicles in situ over a number of years. The moral of the story is, no matter how good your ability to calculate and manipulate data, you cannot learn the skills and gain the experience on a two day or week course. Choose your investigator with care!

An illustrated version of 'What investigators do,' is available upon request