The first fatal Road Traffic Accident (RTA) occurred over a hundred years ago. Formal police records of Road Traffic Accidents didn't begin until 1926 and systematic investigation didn't come about until 1948. The Metropolitan Police Motor Driving School (MPMDS) at Hendon, North London, established traffic law and vehicle examination courses in 1959 for 'older, more experienced officers ... operating specially equipped Traffic Accident Cars'.

Based on American experience in this area, the first specific courses on accident reconstruction and investigation began in the early 1970s in Kennington, South London.

Today's Accident Investigators (AIs) are the most specialist officers in the service, being traffic patrol constables qualified to drive cars and motorcycles to police advanced standard, and also hold LGV and PCV licences. Support is provided Home Office forensic scientists.

Training the Traffic Accident Investigators
Road safety can be an emotive subject and 'every driver is an expert'. One of the first high profile Transport Ministers, Ernest Marples, is on record as saying `I've got twenty million road safety advisers !' People working for any of the emergency services who have to deal with road traffic accidents, understand that it takes more than passing a driving test to make an expert.

In most accidents at least two people are involved, each of whom will blame each other. To determine the truth and find the real cause of an accident, AIs are trained to stand back and look at the facts calmly and dispassionately. Any decision to prosecute is based on what would be reasonable to a competent driver. The level of driving which is considered competent is that defined in the Highway Code.

The main full time AI courses run at the Technical Training Wing, Hendon are the:

	accident investigation course (five weeks)
	vehicle examiners' course (four weeks)
	tachograph calibration course (eight days)
	traffic law updating course (one day)

The City & Guilds of London Institute who are a recognised technical skills examining body, oversee the training. The 'City' refers to the Corporation of London and the 'Guilds' are the ancient Craft Guilds of London established in 1878.

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Traffic Accident Detective work
At the accident scene, the AI makes a study of the inter-relation between the three main influences:

To do this the AI will collect RTA data on:

	Who was involved
	Where it occurred
	When it occurred
	In what circumstances it occurred

The data is analysed and an assessment of accident causation and attribution factors are reported to the Crown Prosecution Service and in the event of a fatality, the local Coroner's Court.

Firms of solicitors specialising in traffic law, hire private AIs to assist defence clients. Such AIs are often retired police service officers who have set up their own self employed businesses as Accident Investigation Consultants.

Although the Law Court system has an inherently adversarial nature, AIs maintain professional integrity and a detached attitude towards accident reconstruction. In Court, AIs are not permitted to draw conclusions. These are a matter for the trial judge. The AI's role is to assist the judge in making primary findings of fact, for instance, scientific data from which deductions, such as the speed of a vehicle can be made.

AI's make increasing use of computer software packages to process and analyse specific accident data and trajectories accurately. The data input is derived from:

	mathematical models, formulas and calculations derived from the laws of physics
	statistical correlations collected from previous RTAs and field test results
	vehicle occupant crash models
	pedestrian impact models
	manufacturers' vehicle handling and tyre performance data

Forensic science, based on Locard's Principle that states 'every contact leaves a trace' is as important to road traffic accidents as it is to murder scenes. The difference is that the AI is dealing with tyre marks and vehicle damage rather than fingerprints and cloth fibres.

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Accident Reconstruction
Putting the theory and police training into practice involves collecting all the relevant evidence possible from the accident scene. This involves recording, in every case, the date, time and place; class of road and speed limit; vehicle type(s) and registration number(s); state of light and class of street lighting (if any). The AI surveys the accident site, takes photographs and draws a scaled plan to show the sight lines of view available to the drivers concerned. Information is drawn from:

	Length and position of tyre marks caused by emergency braking
	Environmental constraints such as the road geometry, street furniture, central reservations, verges, kerbs, type of road surface and adverse cambers
	Critical speed (eg. the maximum speed any vehicle can negotiate a bend)
	Impact damage to vehicle(s)
	Debris on ground such as light cluster remains
	Injuries sustained by victims
	Amount of distance a pedestrian or rider is thrown during the accident
	Local weather reports
	Witness statements
	Gatso and traffic monitoring cameras
	Tachograph records from heavy goods and public service vehicles (in the UK & Europe, but not the US)
	Police vehicle examiner's report
	In car accident data recorder

In some cases, computer simulations of the accident have been produced as evidence for Court purposes.

Tyre marks and road surfaces. A driver's emergency braking is governed by Newton's Laws of Motion. When the driver presses the brake pedal, the brakes act, the car decelerates, one set of wheels lock, then the other set of wheels lock. Where the wheels have locked, it is the friction between the tyres and the road surface that stop the car. With the wheels locked, a car will lose about 15 mph very second, there being next to no difference in tyre capability, irrespective of make, width or vehicle speed.

On dry road surfaces, hard continuous braking achieves the shortest stopping distance. The rate of stopping in the dry at speeds between 20 mph and 80 mph is constant, though it is affected slightly by the kind of vehicle and the type, condition and temperature of the road surface.

The drag factor, or friction co-efficient of the road surface is the value of resistance between the tyre and the road surface. On a dry surface this is taken between 0.69 and 0.75 for most cars. With buses and lorries this can decrease with an increased load and is usually lower by virtue of components and tyre design.

Emergency braking on dry surfaces leaves a deposit of black rubber, beginning at the point where the driver hits the brake hard. The length of tyre mark is proportionate to the square of the speed. The car's speed is determined by measuring the length of these tyre marks, the result is set against the shortest stopping distances shown in the Highway Code.

As with many things in life, there can be exceptions and qualifications, for instance vehicles fitted with ABS tend to leave only scuff marks on the road surface.

On wet road surfaces, cadence braking can achieve a shorter stopping distances. The rate of stopping varies depending on the thickness of the layer of water, depth of tyre tread, the texture of the road surface, and the speed at which the tyres at skidding. The friction co-efficient is usually less than 0.5 and can be as low as 0.2 on snow and ice. The skid resistance of wet surfaces decreases with the increase of a vehicle's speed, in the case of lorries and buses this can cause a considerable lengthening of the stopping distance.

Emergency braking in the wet may still lay rubber, but the AI will have to wait for the road to dry before being able to look for it. Where no tyre marks have been left, determination of the vehicle's speed becomes more speculative.

Choice of the most suitable type of road surface necessary to cope with different traffic conditions is made by the Highways Authority. Different textures include coarse concrete, coarse granite, fine textured asphalt, quartzite, mixed aggregate and 'Shell-Grip'.

With use, surfaces become worn. Tests on motorway surfaces on the M5 near Worcester using coarse granite showed the following braking distances at 95 km per hour (60 mph):

This table shows that tyre grip on a newly laid tarmac road surface will be poor until the surface becomes worn and the stones become exposed offering more grip. This grip is however lost at below zero temperatures where water freezes between the stones, becoming black ice. This often occurs on bridges over water.

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Critical limits. A drivers' reaction time and misuse of road speed are major factors in most RTAs and when determining whether a charge of 'dangerous driving' or 'driving without due care and attention' is to be laid. Although the Highway Code assumes an average reaction time of 0.7 of a second, allowances have to be made for the driver being distracted, so a reaction time of around 1 and 2 seconds is normally used.

To determine the circumstances, an AI can have the road closed in order to conduct stopping distance tests at the accident site. Where possible the vehicle involved in the RTA is used, otherwise a comparable similar police vehicle is an acceptable substitute.

To conduct the test a pneumatic chalk gun fitted to the vehicle. Immediately the AI begins braking, the gun fires leaving a chalk mark on the road. At that point the velocity of the vehicle is ascertained, either using a radar gun, the calibrated speedometer of the car or any other suitable device. The measurement from the chalk mark to where the vehicle comes to rest provides the precise braking distance, from which the AI will be able to calculate the friction co-efficient of any skidding tyres upon that road surface. By knowing the coefficient of friction, the initial speed can be calculated of any vehicle leaving tyre marks on the road surface at the accident scene.

To ensure accuracy of the results and minimise any risk of error, the test is repeated several times. Once the original speed of the car is calculated, it's skidding time, any impact speed along the tyre marks, time to impact, etc can also be calculated.

Road surface tests reveal that friction values in dry conditions are not speed dependant, however in the wet the friction value reduces with speed. Consequently in wet conditions braking distances increase dramatically with small increases in speed.

Measurement of the co-efficient of friction is increasing being made by a portable box of electronics attached to the car involved in the accident in place of the pneumatic chalk gun equipment.

The maximum cornering speed <http://www1.tpgi.com.au/users/mpaine/rollover.html> a vehicle is capable of can be also be determined by the friction value of the road surface. Attempting to negotiate too tight a corner or bend at a speed beyond the critical limit is one of the main causes of vehicle occupant fatalities. The front tyres remain on their steered course, but the unsteered rear wheels loose adhesion, putting the car into a spin which presents one side of the car, (the least protected part) to the front. Should the side of the car hit a lamp post, tree or another approaching vehicle the chances of escaping injury are very low. Impact depth is calculated at one inch for each mph of impact speed.

Hitting something solid, side on at 30 mph gives you a less than 15% chance of getting out of the car without injury. Increase the impact speed to 40 mph and your chance of survival is reduced to 1 in a 1,000.

Heavy braking and trying to steer at the same time is a problem. Any braking increases the weight on the front wheels, making steering harder, increasing the slip angels on the front tyres which can cause sudden under-steer and reduced stability. Habitual heavy braking and steering increases wear on the tyres and steering joints, making steering arduous and increases the possibility of control loss.

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Impact damage. Frontal impacts with other road users and rigid objects account for the greater proportion of serious and fatal injuries.

Determining the events of a crash with another vehicle or object, the AI will use calculations that take into account the vehicle's mass, weight, length, width, position of its centre of gravity, angle of impact, its moment of inertia or yaw and velocity change.

Where a pedestrian has moved out into a vehicle's path, research data correlating average child running speed and adult walking pace to vehicle impact damage and pedestrian injury can be used to determine the vehicle's speed.

Type of injury to pedestrians. Injuries inflicted where a pedestrian is hit by a vehicle vary according to a person's size and age.

Frontal impact with an adult pedestrian at 30 mph and the chances of survival are 1 in 1,000. The minimum injuries will be to the knees and thigh. In the case of a vehicle's speed being 40 mph, the chances are that the pedestrian will be thrown up in the air, caused to rotate and as the body comes down head first, the head will experience a severe impact with the bonnet or windscreen resulting normally in death (Drunken pedestrians have however been known to survive !)

A child will receive chest injuries and life threatening damage to the vital organs even where impact speed is only 20mph. Injuries to elderly persons result in an increased likelihood of shock occurring, again usually resulting in bereavement.

Injuries sustained by victims. Physical descriptions of the injuries, abrasions and skin wounds sustained by both fatalities and survivors; photographs of the victims in their original pre-treatment state; X-rays; scans; doctors, ambulance and nursing notes are necessary to an AI, though it is not possible to determine a vehicle's speed from the extent of injuries alone.
Medical evidence of this nature is also sought after by hospital consultants, so it isn't unusual for it to `transfer' into personal collections for subsequent use in lectures !

Car occupant injury. Anthropomorphic 'crash test dummies' are commonly used by vehicle manufacturer's and research organisations to determine the exact effects that crashes have on vehicle occupants. Variable's include a person's:

	height and build
	age
	physical characteristics
	tolerance to injury
	seating position in front or back on impact
	decision to use a seat belt or not

Depending on the type of impact, injuries are caused to the driver by the:

	protrusion of the steering assembly into the chest
	intrusion of external objects including parts of other vehicles
	forward movement of unrestrained rear seat passengers or loads
	vehicle's sudden change of velocity
	seating position in front or back on impact
	decision to use a seat belt or not

Injuries to the vehicle occupant's head, chest or abdomen are the most life threatening. The risk of serious injury is about six times greater to unbelted front seat occupants in the striking car. Where a vehicle overturns its occupants will normally escape death or serious injury if they are belted in.

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The amount of distance a pedestrian or rider is thrown during an accident. It is possible to calculate a car's speed reasonably accurately by using a 'throw distance formula'.

This technique is particularly useful where there's an absence of sufficient tyre marks, as in the case with cars fitted with ABS. The AI will need to determine the type of collision, the points of impact and landing, taking into account any amount of bouncing and rolling.

The theory of `pedestrian throw distance' assumes that slippage occurs constantly at any period while the pedestrian and ground are in contact. Any slope in the road and other angles can be accommodated for in the calculation.

Numerous staged crash victim simulations have been carried out to validate this projectile formula, including the use of corpses in Germany and life size dummies in the UK !

The higher the impact speed with a pedestrian, the greater the 'throw distance'. A throw of 60 feet suggests a vehicle speed of between 28 and 36 mph depending on the type of vehicle and whether the pedestrian was a child or an adult.

Where a car's impact speed with a pedestrian is in excess of 35 to 40 mph, the likelihood is that the pedestrian will travel over the car's roof and will land in front of any following traffic, however, where the pedestrian gets caught on a broken windscreen they may get carried along by the impact vehicle.

Witness statements. To avoid memory problems, bystanders are interviewed immediately at the accident scene. Unfortunately, their unreliable perception, sometimes to the point of exaggeration, where vehicle's speeds especially motorcycles are over-estimated, hinders the investigation.

Similar discrepancies occur with the statements made by drivers to the police and their insurance companies, which inevitably suggest that somebody or something else was at fault.

Tachograph records. This mechanical device is like a clock which, using three needles calibrates data onto a white wax disc. When viewed under a powerful microscope provides an accurate chart of an entire journey measuring the vehicle's speed and distance travelled in kilometres, and the points where the driver loaded, unloaded and rested.

Second generation tachographs providing a fully electronic alternative, or a smart card are coming into use in the Europe.

Police vehicle examiner's report. The examiner checks first the components where most wear occurs, that is, the vehicle's brakes, tyres and steering.

The inspection looks also for other clues, such as stretched seat belt webbing which indicates that the belt experienced high forces. Should the RTA have occurred during darkness on an unlit road, the intensity and aim of the headlights will be worthy of note.

In car accident data recorder. Comparable to an aircraft's 'black box', their purpose is to electronically record and store data for the period leading up to, during and just after the crash. The captured information can be made available to AIs and insurance companies so as to establish the objective facts of the incident.

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