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Also called safety belts. Seat belts are found in cars and other vehicles that travel on roads and have a passenger compartment with seats. So, you will find them in large trucks, but not motorcycles. Seat belts are also part of commercial airplane seats and the seats of some boats.

Today, most states in the United States require the driver and any front seat passenger of road vehicles to be wearing their seat belts while the vehicle is in motion. The fine for a violation can be surprisingly high! This hasn't always been the case. I remember riding in the car with my mom in the 60s. She developed an impressive "right arm out" reflex -- anytime she had to brake drastically, she'd fling her right arm out to keep me, or my little brother, or the groceries, from pitching forward.

Older cars had only lap belts -- just one belt that goes from left to right and holds your butt in place. These were better than nothing, but people wearing them still got injured, because their face tended to slam into the steering wheel or dashboard. At some point during the 70s (? anyone remember for sure when?) cars began to be equipped with shoulder belts too, in addition to the lap belt. This additional belt goes over the outside shoulder (left for the driver, right for the passenger) and down to fasten along with the lap belt next to the inside hip. These belts are fairly effective at keeping someone's face from slamming into part of the car in case of a sudden stop, but a new type of injury began to be seen: a sort of brain trauma caused when the head is violently slung to the side. Today (2000), most new cars in the US have airbags, plus lap belts, plus shoulder belts. This can protect you against a pretty serious crash, but airbags are basically explosive and have been shown to seriously injure short people and infants.

It's still a Very Bad Idea to get into a car crash.

The complete evolution of the seat belt is the five point seat belt, or harness. They feature a strap over each shoulder, a strap around each side of the body, and a crotch strap. All of the straps come together over your belly and latch there; One either turns a knob or slaps a button to release it. There are also four-point harness-type seat belts, and six-point, but the five point types are the most common.

Five point belts are required for most classes of racing. The shoulder belts usually strap to a roll cage or roll bar behind the seats, the side belts to mounting bolts beside the seat, and the crotch strap to a bar running between the front bolts holding down the seat itself. Five point belts are generally sufficient to keep you in your seat even if you roll the vehicle. Five and six point belts both serve to keep you from submarining, or sliding down towards the toe panel in a head-on collision, but six point belts have the added advantage (especially to male drivers) that they do not tend to crush the crotch region if such a thing should happen, as the fifth belt in five-points runs straight down from the clasp, bisecting the crotch region, potentially more literally than one might appreciate.

The US Code, or United States Code of Federal Regulations, mandates certain requirements for seat belts in Title 49, Volume 5, Part 571. Every racing organization, including the FIA, NHRA, SCCA, and so on, have more stringent requirements than the US government. For example, modern cars are required to have both lap and shoulder belt for each front seat passenger, and a lap belt for the rear passenger. Seat belt straps are required to be made out of webbing, or a woven material, to enhance their flexibility and lifetime. The webbing must be at least 46mm wide, support 26,689 N for a lap belt alone, or 22,241 N for the lap belt, and 17,793 N for the shoulder belt. They are tested for their resistance to abrasion, light, and corrosion. The buckles have to meet certain characteristics for how hard it may be to press the button, how hard it can be to slide the buckle closed, et cetera.

The FIA, by contrast, requires a four, five, or six-strap belt. The FIA specifications for a five or six point belt include crotch and pelvic straps at least 44mm wide where they cross the thigh, and at least 25mm elsewhere; Other straps have to be at least 70mm wide, considerably wider than the US Code requirements. Also, the buckle is required to have between 20 and 40cm^2 of contact with the wearer, to spread out any impact there. Other requirements are substantially higher.

The downside to five point belts is that they take more time to put on, and they will do you very little good if you do not connect all of the belts, as they are designed to work with all of them securely fastened. Proper adjustment is even more important with five point belts than with normal lap or shoulder and lap belts. They also reduce your freedom of motion. However, this reduction also makes you considerably safer, and you shouldn't be doing anything while you're driving that you can't do while strapped in with five points, either. In the US, as long as they are listed with the [U.S. Department of Transportation (USDOT), there is no case for them not being street legal as long as you follow the statutes of 49CFR571.209, Seat belt assembly anchorages.


References:

49CFR571.209 Code of Federal Regulations Title 49, Volume 5. October 1, 2001
From the U.S. Government Printing Office via GPO Access

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