What seat belts leave behind

In civil actions, since Froom v Butcher [1976] QB 286 (CA), failure to use the available restraint is considered contributory negligence, and any award to the injured party may be reduced by up to 25 per cent. It is rare that those unrestrained occupants who survived the crash admit to not using the available restraint, which is not surprising considering that they a) broke the law, and b) face a 25 per cent reduction on their claim.

There is a necessity, therefore, to establish independently whether or not the seat belt was used. That is not the full story, though. While it may be established that the seat belt probably wasn't being worn, when considering the issue of contributory negligence, the court must also decide what effect the lack of restraint had upon the injuries received. A seat belt is not a panacea, and the benefits it affords in a crash range from avoidance of all injuries, to nil, and all levels in between.

There are several features of the accident that must be considered when establishing the use and effectiveness of the seat belt system:

• The forensic evidence on the seat belt hardware;
• The injuries received;
• The source of the injuries;
• The impact type;
• Defects in the seat belt system.

The function of the seat belt

In order to appreciate what forensic evidence one might expect to find on a seat belt, and what injuries one might expect to see on a restrained occupant, it is important to understand the function of a seat belt, and how that function may be affected.

There are three distinct phases to any given crash:

• The impact or deceleration of the vehicle with the struck object;
• The impact or deceleration of the occupant with the vehicle interior;
• The impact or deceleration of the occupant's internal organs;
• The classic, and most easily visualised, being the brain against the skull casing.

How these phases manifest themselves is dependent upon whether the occupant is restrained, or unrestrained, at the time of the crash. For example, in a 30mph frontal impact with a rigid object, the vehicle will decelerate in a controlled manner, deforming the front of the vehicle by approximately two feet. The average deceleration of the vehicle is 15g.

The occupant begins to move forwards in response to the decelerating vehicle, and will themselves only decelerate when contacts are made with the vehicle's interior. In the unrestrained case, head and chest contacts are usually made with the vehicle's interior when the vehicle has almost stopped. The occupant is still moving at 30 miles per hour, and will decelerate at a rate that is dependent on the compliance of the particular internal feature that contact is made with. If the steering wheel deforms one foot from a chest contact, then the chest will suffer an average deceleration approaching 30g. If the windscreen deforms six inches from a head strike, then the head will suffer an average deceleration approaching 60g. (This is somewhat simplified as the features struck may still be moving forwards at the point of contact, and a chest deceleration will affect the head contact speed). Internal organs will decelerate in response to the contacts made, and that deceleration will be from a combination of visceral, shear, and linear impact forces.

In the restrained case, the occupant moves forwards making initial contact with the seat belt. The occupant is now tied to the vehicle, and restraining forces are applied to the occupant's pelvis, rib cage and shoulder. The occupant therefore benefits from deceleration that remains in the still-crushing vehicle. The webbing of the seat belt stretches to some degree, allowing further space for deceleration. The overall effect is that contacts with the vehicle interior may be avoided altogether, or that the effect of those contacts are mitigated. Naturally, this has a direct bearing on the third impact of the occupant's internal organs.

Space and slack

Of significant importance in the seat belt function is the space available in which to decelerate the occupant, and the slack in the seat belt system. Forensic evidence on the seat belt hardware is also important '“ a testing of the sea belt functions and the extent to which the plastic of the seat belt may have moved, or melted, as a result of the build-up of heat on impact.

Other forensic evidence may be drawn from the appearance of the seat belt webbing itself. The webbing is sewn back on itself at its ends. Under very high loads the stitching may begin to break. Indeed this feature has been used occasionally as a load limiter to help reduce occupant injury from seat belt loads. A loop is sewn into the webbing, the stitches of which are designed to progressively fail at a prescribed loading.

Webbing is woven from several thousand filaments. During sustained use, the edges of the webbing often becomes 'fuzzy'. Low level magnification of the broken filaments that constitute the 'fuzz' have a near-uniform length, and no particular end characteristic. In contrast, broken filaments sustained during crash loads have done so very quickly which generates heat. As the filament snaps, the filament springs back, and the heated end 'mushrooms'.

The webbing may exhibit glass particles or powder from air bags. Comparison of the length of webbing covered with such evidence shows how much webbing was reeled out when the evidence was deposited, and hence whether it has been worn or not.

Webbing failure

Occasionally webbing will be seen to be separated. The nature of the break must be closely examined. Webbing is often cut by rescue services to release an occupant. The webbing, though, is also occasionally cut through if the roof needs to be removed from the vehicle. This can lead to confusion when an occupant was not wearing the seat belt, whereas the cut through the webbing suggests otherwise. When webbing is cut intentionally, the fibres at the end of the cut show a uniform length, and do not exhibit any mushrooming of the filaments.

Webbing may become cut by contact with sharp surfaces during the impact. Matching the cut to the sharp surface shows how much webbing was reeled out, and will therefore establish if the seat belt was being worn or not.

The webbing may fail at the D ring under high loads if the webbing becomes 'roped' or bunched up in one corner of the D ring. The failed ends of the webbing show variable length, with mushrooming of the filaments. The distinction between webbing that has failed in this manner and webbing that has been intentionally cut is apparent by the lack of uniformity of the failed webbing. In addition, the webbing leading to the failed end exhibits considerable longitudinal crimping.

When webbing fails purely as a result of overload, there is massive disruption of the webbing material, such that the webbing ends resemble horse tails.

Webbing is only one of the factors of seat belt forensics that can come under scrutiny. The mounting points may also become deformed, although this generally only occurs under very high loading. Seat belts also occasionally leave marks on the occupant's clothing, or on the seat back, which can also serve as an indication of usage or non-usage.

Defects in the seat belt system

It is rare for a seat belt system to contain manufacturing or design defects. In litigation, it is occasionally specified that an occupant was wearing their seat belt, but the seat belt released itself because of some defect with the buckle. The occupant now exhibits typical unrestrained injuries that they now wish to be compensated for, either from the negligent driver of the other vehicle, or from the seat belt manufacturer.

Unintentional release of the buckle can be achieved where there is mis-latching of the buckle and tongue, foreign bodies inside the buckle, inadvertent operation of the buckle release button, or crushing of the buckle. This last scenario is very rare, but occasionally happens in side impacts where the seats are forced together.

The injuries received

Injuries that an occupant receives are another useful way of helping establish use or non-use, in certain circumstances.

Restrained occupants in moderate to heavy impacts almost always suffer contact with the vehicle interior. In a moderate to severe frontal impact the restrained occupant will strike the facia with their knees, and their head will usually arc down to strike the steering wheel. If the steering wheel intrudes and/or there is sufficient slack in the seat belt, the restrained occupant will sustain a chest strike on the steering wheel. There is relatively little difference in the contact source for the unrestrained occupant, except that the unrestrained occupant's head may strike the windscreen. The difference is in the force with which the contacts are made, and, in like-for-like impacts, one would expect the injury level to be lowest for the restrained occupant.

Occasionally a restrained occupant exhibits bruising and abrasions from the shoulder diagonally across the chest, and at the iliac crests of the pelvis, the pattern of the injuries matching the geometry of the seat belt to the occupant's seating position. If it was that easy, then there would be no necessity to look any further in establishing seat belt use. Restrained occupants rarely exhibit this pattern of injury.

Apart from the above injury type, it is not possible to establish seat belt use purely on injuries sustained. Information about the vehicle is required, and particularly the impact direction and the amount of intrusion to the occupant in question.

Important factors such as biomechanical tolerance also need to be considered. A healthy young male will be able to sustain crash loads far above the level that will injure an elderly female. A restrained elderly female involved in a moderate frontal impact may exhibit severe chest injuries, particularly to the ribs and sternum, that would normally only be associated with a heavy, unrestrained chest impact with the steering wheel.

Conclusion

In-depth knowledge of injuries expected versus impact type, intrusion, age, sex, height and weight are necessary to differentiate use from non-use. There is a vast amount of information available in the literature. This area is the subject of a great deal of continuing research around the world '“ too large to be considered within the confines of this paper.