An investigator looking at an accident collision involving vehicles has to determine what caused the incident. This is done by working backwards from the end result of the collision in order to find the sequence events that took place leading up to and during the accident. The analysis of skid marks whilst being very basic, plays an important role in determining the cause of a traffic collision. Skid marks are produced when the driver of a vehicle suddenly applies the brakes at speed causing the wheels to lock and resulting in the vehicle sliding before coming to a halt. Skid marks are typically straight due to the fact that steering is not possible when the front wheels are locked.
Skid marks can provide an accurate indication of speed, however they are not necessarily definitive and therefore, any calculation of speed derived from them is not going to be entirely accurate. The speed the vehicle was travelling when skid mark was produced is known as the skid speed and is a ‘conservative value’, due to the fact that the wheels do not lock-up instantly under heavy breaking. This is mostly due to automatic braking systems (ABS) in modern cars which prevent the front wheels from locking before the back wheels do. With this in mind, it is safe to assume that the driver of a vehicle may have been travelling at a speed in excess of the calculated skid and that the speed calculated will be a minimum speed, as the vehicle will have had to have been travelling at least that fast in order to produce it.
Assuming that the vehicle is travelling in a straight line and both sets of wheels are aligned during a skid, there will be two distinct marks along the surface of the road. At the beginning of a set of skid marks, there is likely to be what is known as a "shadow skid". This is a light mark produced as the wheels began to slow. If this is measured and added into the total length of the skid the more accurate the skid speed becomes. If there are multiple skid marks present, the thinner marks will have been caused by the rear tires, whilst the front marks will be wider as whilst the car is coming to a halt, the weight is forced forward on to the front wheels due to inertia. If there are two distinct dark marks then it is likely that the vehicle stopped in a straight line and the rear tyres skids overrode those of the front tyres, which typically produces a darker skid. In the case of four distinct marks being present, it is likely that the vehicle was not travelling in a straight line. In each case, the skid marks should be measured individually and an average length of each mark taken. It should also be taken into consideration that not all marks are continuous, as shadow skid marks can occur early into the skid. Another problem is that skid marks quickly fade due, particularly due to weather and other vehicles, so it is essential to measure them as quickly as possible.
Using the measurement of the skid marks and other considerations in order to calculate the speed at which the vehicle was travelling before producing the skid mark, the following calculation can be used is S = √30.D.f.n
Where: S = Speed, 30 = A constant value, D = Distance of skid, f = Drag factor on the particular road surface and n = Braking efficiency (%)
Drag factor is the term used for the interface between the tyre and road surfaces. It is important to know the drag factors of different road surfaces since the friction between tyre and surface will be different in each case. The drag factor can be found through experiment using specially tuned equipment and vehicles, however the drag factor for most road surfaces can be found listed in books or tables. For example, the drag factor values for asphalt are between 0.50 to 0.90 and for show, 0.10 to 0.55. The lower the drag factor, the less grip is offered by the road surface, which directly affects the length, colour and appearance of the skid, therefore requiring the drag factor of the particular skid mark to be taken into consideration in each case.
The other important factor in calculating the speed which the vehicle was travelling is the braking efficiency of the vehicle in question. For instance, if each wheel provides an equal amount of braking force, then the braking efficiency is 100%. Braking efficiency is dependent upon several factors such as the type and weight of the vehicle, tyre and brake wear and so on. Other types of tyre mark, such yaw, scuff and scrub marks can be used to determine things such as the direction of travel of a vehicle spinning out of control or if a tyre had come in contact with a surface (respectively). The presence of tyre marks or impressions in a surface can also be used to determine the type of tyre which caused it, which can be very useful in identifying whether or not a particular vehicle was present at the scene.
In conclusion, skid marks are a vital piece of physical evidence in the investigation of a vehicle collision due to the fact that they can be used to determine several factors leading up to a collision, such as direction of travel and minimum speed
- Alex Wood
