Sam Kodsi is a Consulting Professional Engineer and the President of Kodsi Forensic Engineering. Sam worked in seatbelt component manufacturing before shifting gears and becoming an expert in forensic engineering, crash reconstruction, injury biomechanics, and driver behaviour. In light of the seatbelt’s 60th anniversary, Sam answered some questions about buckling up.
Q: How does the seatbelt fit in amongst newer safety technologies like airbags, blind spot monitoring, and backup cameras?
A: The cost of manufacturing a single modern seatbelt is significantly less than $100 and even replacement seatbelts typically sell for under $200. So, the seatbelt is not only the #1 most effective public health measure, it is also the most cost-effective one. Seatbelts are the simplest form of restraint, and an integral part of the passive safety systems of a vehicle. A seatbelt alone is way more effective than an airbag would be on its own. There was an unfortunate drop in seatbelt usage in the 1980s and 90s when vehicles began to include airbags, but it is a complete fallacy that airbags replace a seatbelt. Airbags supplement seatbelts, which is why airbags are called the SRS (supplemental restraint system). With that said, seatbelts and airbags work together to be more effective than the seatbelt on its own, and the pairing is most effective in low- to moderate-speed crashes involving a frontal component. Seatbelts are also effective in keeping occupants contained in the vehicle during a rollover. Even during a rear end impact, the seatbelt reduces the forward rebound motion.
Q: Can you tell us anything interesting about how a seatbelt works that still makes it so effective at preventing injuries and fatalities?
A: If one crashes at some speed, especially if it’s a frontal impact, their body will want to continue moving at that speed. The seatbelt applies forces in the opposite direction to reduce the occupant’s movement and prevent the person from contacting the interior of the vehicle. Seatbelts restrain the torso, and hence, the body, from a secondary impact with the steering wheel, dashboard, windshield, and even other people in the vehicle.
Q: Some people express concern over wearing a seatbelt for fear of it causing injury while restraining them during a collision. Is this a legitimate concern?
A: That is a myth. There have been very rare occasions where someone has been hurt by their seatbelt or could have benefited from being ejected. But the prevalence of that is so incredibly low that it can essentially be dismissed; the probability that a seatbelt will benefit you in a crash is much, much higher than somehow getting injured by it. The seatbelt will almost always be helpful if worn properly, which means that it has to fit securely around shoulder, chest and across the upper thighs or pelvis.
Q: Has the seatbelt changed much since 1955 when it became standard safety equipment in all new vehicles?
A: Although it has not changed nearly as much as other heavily-electronic vehicle modules, there have been some advancements. Early seatbelts were made from nylon webbing, while newer seatbelts use polyester webbing, which is even stronger. There has also been an evolution of the number of attachment points, from lap belts which had two points, to the now commonplace lap and shoulder belts which have 3 attachments points. In the 1980s, seatbelt pretensioners were introduced into vehicles and made use of the same sensors that trigger vehicle airbags. When the pretensioner is activated by the sensors, an explosive gas is fired to drive a mechanism that very quickly retracts the belt. This action increases the seatbelt’s effectiveness by further reducing occupant motion during a crash, even before airbags deploy. Pretensioners are especially helpful with pulling someone back into position if they are leaning forward or in an otherwise unfavourable position just prior to airbag deployment.
Q: Statistics show that seatbelt use has increased. Has that also resulted in a reduction of unrestrained occupants in the collisions you have investigated over the last 15 years?
A: The 1950s saw the seatbelt become a mandatory piece of safety equipment in some countries and jurisdictions, while in other countries it was only an option during that same period. For a time after seatbelts were introduced, less than 1% of drivers used them. Even when seatbelts became regulated automotive safety equipment in the 1960s and 70s, only about 15% of people were using them. Things have come a long way; currently in Canada, usage is at an average of 95%, with usage a bit lower in rural areas of the country. The more prevalent use of seatbelts has definitely been noticeable in my work over the last 15+ years during which I have been investigating collisions, particularly for front seat occupants. In the mid-90s, occupants were unrestrained in about 1 in 4 or 5 of the cases that I investigated. Nowadays, I would approximate that less than 1 in 10 of the motor vehicle collision cases I assess have an unbelted occupant.
An important consideration is that while Canadians’ seatbelt usage in Canada is pretty impressive, that is not the case abroad. Seatbelt laws and enforcement are not as stringent in many countries. However, just because a country doesn’t have seatbelt laws, doesn’t mean that Newton’s law doesn’t apply there. For residents of developing countries, seatbelt use is markedly lower, resulting in notably higher collision-related fatalities. While safety regulations in those countries are still evolving, public education is in demand. It took nearly 40 years for seatbelt usage in North America to exceed 90%. It will take years of awareness in developing countries for citizens to buckle up for their own good.
Q: How can you tell if an occupant was wearing their seatbelt during a collision?
A: There are several ways that we can get to this answer.
The force of a belted occupant’s body moving very quickly during a collision stretches or “loads” the belt in a way that leaves physical evidence we can assess, keeping in mind that the force exerted on the belt is proportional to the weight of the occupant, as well the decceleration(rate of speed change) experienced during the crash. During that decceleration, the very quick movement and its force can cause the seatbelt webbing to rub against the D-ring and latch plate. We look at evidence of local abrasions, scuffing, or melting on the D-ring or latch plate. We also look for loading or plastic transfer marks on the polyester webbing of the belt. In vehicles with a pretensioner, seatbelt use can be indicated by a “stuck” seatbelt where the pretensioner has fired. In our seatbelt examinations, we carefully document this sensitive evidence, which should always be preserved as soon as possible before the markings degrade, or the seatbelts or vehicle become unavailable.
The black box is increasingly becoming another source of information on seatbelt use for front passengers. Some black boxes can even give us information on the front occupants’ seat track positions (for example, forward or back). So in more investigations, we can obtain valuable information not only about seatbelt usage, but how the seatbelts were used in the context of the seat and surrounding environment, which can be extremely pertinent to injury biomechanics assessments.