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A three-point seat belt.
A seat belt, sometimes called a safety belt, is a safety harness designed to secure the occupant of a vehicle against harmful movement that may result from a collision or a sudden stop. As part of an overall automobile passive safety system, seat belts are intended to reduce injuries by stopping the wearer from hitting hard interior elements of the vehicle, or other passengers (the so-called second impact), are in the correct position for the airbag to deploy and prevent the passenger from being thrown from the vehicle. Seat belts also absorb energy by being designed to stretch during an impact, so that there is less speed differential between the passenger's body and their vehicle interior, and also to spread the loading of impact on the passengers body.

The final, so-called 'third impact' after a passenger's body hits the car interior, airbag or seat belts, is that of the internal organs hitting the ribcage or skull. The force of this impact is the mechanism through which car crashes cause disabling or life threatening injury. The sequence of energy dissipating and speed reducing technologies - crumple zone - seat belt - airbags - padded interior, are designed to work together as a system, to reduce the force of this final impact.

Types of seat belts

Lap seat belt (or "two points") in an aircraft
Three points seatbelt in a car.
The three points belt is the standard seat belt for road cars.
Five points harness in a racing car.


Lap

Adjustable strap that goes over the waist. Used frequently in older cars, now uncommon except in some rear middle seats. Passengers aircraft seats also use lap seat belts to prevent injuries.

Sash

Adjustable strap that goes over the shoulder. Used mainly in the 1960s, but of limited benefit because it is very easy to slip out of in a collision.

Three-point

Similar to the lap and shoulder, but one single continuous length of webbing. Both three-point and lap-and-sash belts help spread out the energy of the moving body in a collision over the chest, pelvis, and shoulders. Volvo introduced the first production three-point belt in 1959. The first car with three point belt was a Volvo PV 544 that was delivered to a dealer in Kristianstad on August 13, 1959. The three point belt was developed by Nils Bohlin who earlier had worked on ejection seats at Saab.

Until the 1980s, three-point belts were commonly available only in the front seats of cars; the back seats had only lap belts or diagonal belts. Evidence of the potential for lap belts to cause separation of the lumbar vertebrae and the sometimes associated paralysis, or "seat belt syndrome", has led to a revision of passenger safety regulations in nearly all developed countries requiring that all seats in a vehicle be equipped with three-point belts. Since September 1, 2007, all new cars sold in the U.S. require a lap and shoulder belt in the center rear.

Besides regulatory changes, "seat belt syndrome" has led to tremendous liability for vehicle manufacturers. One Los Angeles case resulted in a $45 million jury verdict against Ford Motor Company; the resulting $30 million judgment (after deductions for another defendant who settled prior to trial) was affirmed on appeal in 2006.
Harness


Belt-in-Seat (BIS)

The BIS is a three-point where the shoulder belt attachment is to the backrest, not to the b pillar. The first car using this system in the United States was the 1990 Mercedes-Benz SL. Some cars like the Renault Vel Satis use this system for the front seats. This system allegedly is safer in case of rollover, especially with 4–8 years old children, though other sources dispute this claim. Bob Lange, executive director of safety integration for General Motors Corp...concurred that such seats cost more to build but, interestingly, don't boost safety ratings. "We were hoping to record a [safety] performance improvement, but it didn't happen," Lange said. "We found we could provide the same level of protection with a body-mounted belt."

Experimental production car safety belts

  • Criss-cross Experimental safety belt presented in the Volvo SCC. It forms a cross-brace across the chest.
  • 3+2 Point Seatbelt: Experimental safety belt from Autoliv similar to the criss-cross. The 3+2 improves protection against rollovers and side impacts.
  • Four point "belt and suspenders": An experimental design from Ford where the "suspenders" are attached to the backrest, not to the frame of the car.
  • Inflatable Safety Belts: An airbag is included within the belt for the rear seat belts.


Five-point harnesses

These restraints are safer but more restrictive than most other seat belt types. They are typically found in child safety seats and in racing cars. The lap portion is connected to a belt between the leg and there are two shoulder belts, making a total of five points of attachment to the seat. (Strictly speaking, harnesses are never to be fastened to the seat—they should be fastened to the frame/sub-frame of the automobile.)

Six-point harnesses

These harnesses are similar to a five-point harness but include an extra belt between the legs, which is seen by some to be a weaker point than the other parts. These belts are used mainly in racing. In NASCAR, the six-point harness became popular after the death of Dale Earnhardt. Earnhardt was wearing a five-point harness when he suffered his fatal crash. As it was first thought that his belt had broken, some teams ordered a six-point harness in response.

Seven-point harnesses (5+2)

Aerobatic aircraft frequently use a combination harness consisting of a five-point harness with a redundant lap-belt attached to a different part of the airframe. While providing redundancy for negative-g maneuvers (which lift the pilot out of the seat), they also require the pilot to un-latch two harnesses if it is necessary to parachute from a failed aircraft.

History

Seat belts were invented by George Cayley in the late 1800s, though Edward J. Claghorn was granted the first patent ( , on February 10, 1885 for a safety belt).

American car manufacturers Nash (in 1949) and Ford (in 1955) offered seatbelts as options, while Swedish Saab first introduced seat belts as standard in 1958.

After the Saab GT 750 was introduced at the New York motor show in 1958 with safety belts fitted as standard, the practice became commonplace.

The first three point seat belt (the so-called CIR-Griswold restraint) was patented in 1951 by the Americans Roger W. Griswold and Hugh De Haven, and developed to its modern form by Nils Bohlin for Swedish manufacturer Volvo - who introduced it in 1959 as standard equipment. Bohlin was granted for the device.

In 1970, the state of Victoria, Australiamarker, passed the first law worldwide making seat belt wearing compulsory for drivers and front-seat passengers.

Technologies

Seat Belt uncovered Inertial Reel


Most seat belts are equipped with locking mechanisms (or inertia reels) that tighten the belt when pulled fast (e.g. by the quick force of a passenger's body during a crash) but do not tighten when pulled slowly. This is implemented with a centrifugal clutch, which engages as the reel spins quickly. Alternatively, this function may be secured by a weighted pendulum or ball bearing: when these are deflected by deceleration or roll-over they lock into pawls on the reel.

Types of inertia reel type seatbelts:

NLR (no locking retractor): Commonly used in recoiling lap belts

ELR V (emergency locking retractor - vehicle sensitive): Single sensitive mechanism, composed of a locking mechanism activated in an emergency by deceleration or rollover of the vehicle. Thus, the seatbelt is sensitive to the vehicle's motion.

ELR VW (emergency locking retractor - vehicle and webbing sensitive): Dual sensitive means a seatbelt retractor that, during normal driving conditions, allows freedom of movement by the wearer of the seatbelt by means of length-adjusting components that automatically adjust the strap to the wearer, with a locking mechanism that is activated by two or more of the following:
  • deceleration or rollover of the vehicle,
  • acceleration of the strap (webbing) from the retractor, or
  • other means of activation.


A recent study by McCoy & Chou (2007) from the Ford Motor Company (Safety Test Methodology, SP-2123) demonstrated that the standard inertia reel seatbelt does not stop the head from making contact with the interior of the roof on a standard rollover test in their dynamic Rollover Component test System (ROCS). Even with modern pre-tensioning devices the head contacts the interior of the roof and the neck suffers 'visible' compression.

Pretensioners and webclamps

Seatbelts in many newer vehicles are also equipped with "pretensioners" and/or "Webclamps".
  • Pretensioners preemptively tighten the belt to prevent the occupant from jerking forward in a crash. Mercedes-Benz first introduced pretensioners on the 1981 S-Class. In the event of a crash, a pretensioner will tighten the belt almost instantaneously. This reduces the motion of the occupant in a violent crash. Like airbags, pretensioners are triggered by sensors in the car's body, and most pretensioners use explosively expanding gas to drive a piston that retracts the belt. Pretensioners also lower the risk of "submarining", which is when a passenger slides forward under a loosely worn seat belt. An alternative approach being looked at by major car companies is the CG-Lock technology whereby the occupant is held in position via the lap belt in order to prevent the passenger from coming out of position in the event of a crash.
  • Webclamps clamp the webbing in the event of an accident and limit the distance the webbing can spool out (caused by the unused webbing tightening on the central drum of the mechanism) these belts also often incorporate an energy management loop ("rip stitching") which is when the lower part of the webbing is looped and stitched with a special stitching. The function of this is to "rip" at a predetermined load, which reduces the load transmitted through the belt to the occupant, reducing injuries to the occupant.


Inflatable Seatbelts

The Inflatabelt Seatbelt utilities tubular inflatable bladders that are contained within a outer cover. When a crash occurs gas is introduced into the bladder causing the inflation of the bladder to increase the area of the restraint contacting the occupant and also shortening the length of the restraint to tighten the belt around the occupant improving the protection. The inflatable sections may b shoulder only or Lap and Shoulder. The system supports the head during the crash better than a web only belt. It also provides side impact protection. The inflatable Seatbelt was first invented by Donald Lewis and tested at the Automotive Products Division of Allied Chemical Corp.

Automatic seat belts

Some vehicles have shoulder belts that automatically move forward to secure the passenger when the vehicle is started. A separate lap belt is usually included, and the lap belt must be fastened manually. Automatic seat belts have fallen out of favor recently, since the airbag became mandatory in many countries.

Use of seat belts by child occupants

As with adult drivers and passengers, the advent of seat belts was accompanied by calls for their use by child occupants, including legislation requiring such use. It has been claimed that children in adult restraints suffer lower injury risk than unrestrained children.

The UK extended compulsory seatbelt wearing to child passengers under the age of 14 in 1989. It was observed that this measure was accompanied by a 10% increase in fatalities and a 12% increase in injuries among the target population. In crashes, small children who wear adult seatbelts can suffer "seat-belt syndrome" injuries including severed intestines, ruptured diaphragms and spinal damage. There is also research suggesting that children in inappropriate restraints are at significantly increased risk of head injury, one of the authors of this research has been quoted as claiming that "The early graduation of kids into adult lap and shoulder belts is a leading cause of child-occupant injuries and deaths." As a result of such findings, many jurisdictions now advocate or require child passengers to use specially designed child restraints. Such systems include separate child-sized seats with their own restraints and booster cushions for children using adult restraints. In some jurisdictions children below a certain size are forbidden to travel in front car seats.

In rear seats

In 1955 (as a 1956 package) Ford offered lap only seat belts in the rear seats as an option within the Lifeguard safety package. In 1967 Volvo started to install lap belts in the rear seats. In 1972 Volvo upgraded the rear seat belts to a three point belt.

In crashes, unbelted rear passengers increase the risk of belted front seat occupants' death by nearly five times.

Reminder chime and light



In North America, cars sold since the early 1970s have included an audiovisual reminder system consisting of a light on the dashboard and a buzzer or chime reminding the driver and passengers to fasten their belts. Originally, these lights were accompanied by a warning buzzer whenever the transmission was in any position except park if either the driver was not buckled up or, as determined by a pressure sensor in the passenger's seat, if there was a passenger there not buckled up. However, this was considered by many to be a major annoyance, as the light would be on and the buzzer would sound continuously if front-seat passengers were not buckled up. Therefore, people who did not wish to buckle up would defeat this system by fastening the seatbelts with the seat empty and leaving them that way.

By the mid-1970s, auto manufacturers modified the system so that a warning buzzer would sound for several seconds before turning off (with the warning light), regardless of whether the car was started. However, if the driver was buckled up, the light would appear, but with no buzzer. New cars sold in the United States in 1974 and the first part of the 1975 model year were sold with a special "ignition interlock", whereby the driver could not start the car until the seat belt was fastened; however, this system was short-lived.

Today, the belt warning light may stay on for several minutes after the car is started if the driver's seat belt is not fastened.

In Europe and some other parts of the world, most modern cars include a seat-belt reminder light for the driver and some also include a reminder for the passenger, when present, activated by a pressure sensor under the passenger seat. A Swedish study showed that of the cars with seat belt reminder 8 of 10 drivers not using the seat belts were driving an Audi.

Some cars will intermittently flash the reminder light and sound the chime until the driver (and sometimes the front passenger, if present) fasten their seatbelts, and they sometimes even lock the speed to 10 km/h or less.

Legislation

Observational studies of car crash morbidity and mortality, experiments using both crash test dummies and human cadavers indicate that wearing seat belts greatly reduces the risk of death and injury in the majority of car crashes.

This has led many countries to adopt mandatory seat belt wearing laws. It is generally accepted that, in comparing like-for-like accidents, a vehicle occupant not wearing a properly fitted seat belt has a significantly and substantially higher chance of death and serious injury. One large observation studying using US data showed that the odds ratio of crash death is 0.46 with a three-point belt, when compared with no belt. In another study, that examined injuries presenting to the ER pre- and post-seat belt law introduction, it was found that 40% more escaped injury and 35% more escaped mild and moderate injuries.

The effects of seat belt laws are disputed by those who observe that their passage did not reduce road fatalities. There was also concern that instead of legislating for a general protection standard for vehicle occupants, laws that required a particular technical approach would rapidly become dated as motor manufacturers would tool up for a particular standard which could not easily be changed. For example, in 1969 there were competing designs for lap and 3-point seat belts, rapidly-tilting seats, and air bags being developed. But as countries started to mandate seat belt restraints the global auto industry invested in the tooling and standardized exclusively on seat belts, and ignored other restraint designs such as air bags for several decades

Risk compensation

Some have proposed that the number of deaths was influenced by the development of risk compensation, which says that drivers adjust their behavior in response to the increased sense of personal safety wearing a seat belt provides.

In one trial subjects were asked to drive go-karts around a track under various conditions. It was found that subjects who started driving unbelted drove consistently faster when subsequently belted. Similarly, a study of habitual non-seatbelt wearers driving in freeway conditions found evidence that they had adapted to seatbelt use by adopting higher driving speeds and closer following distances. Similar responses have been shown in respect of anti-lock braking system and, more recently, airbags and electronic stability control.

A 2001 analysis of US crash data aimed to establish the effects of seatbelt legislation on driving fatalities and found that previous estimates of seatbelts effectiveness had been significantly overstated. According to the analysis used, seatbelts were claimed to have decreased fatalities by 1.35% for each 10% increase in seatbelt use. The study controlled for endogenous motivations of seat belt use, which it is claimed creates an artificial correlation between seat belt use and fatalities, leading to the conclusion that seatbelts cause fatalities. For example, drivers in high risk areas are more likely to use seat belts, and are more likely to be in accidents, creating a non-causal correlation between seatbelt use and mortality. After accounting for the endogeneity of seatbelt usage, Cohen and Einav found no evidence that the risk compensation effect makes seatbelt wearing drivers more dangerous, a finding at variance with other research.

Increased traffic

Other statistical analyses have included adjustments for factors such as increased traffic, and other factors such as age, and based on these adjustments, a reduction of morbidity and mortality due to seat belt use has been claimed. However, Smeed's law predicts a fall in accident rate with increasing car ownership and has been demonstrated independently of seat belt legislation.

Use in vehicles other than cars

Buses

Pros and cons had been alleged about the use of seatbelts in school buses. See also Seat belts in school buses.

Trains

Use of seatbelts in trains has been investigated. Concerns about survival space intrusion in train crashes and increase of injuries to unrestrained or incorrectly restrained passengers led the researches to discourage the use of seat belts in trains.

:It has been shown that there is no net safety benefit for passengers who choose to wear 3-point restraints on passenger carrying rail vehicles. Generally passengers who choose not to wear restraints in a vehicle modified to accept 3-point restraints receive marginally more severe injuries.


Aeroplanes

The "Father of Crash Survivability", Hugh De Haven, was a plane pilot. His interest in crash survivability was sparked by him surviving a plane crash during the First World War.

See also



References

  1. Allt om Motor: Trepunktsbältet 50 år
  2. Federal Motor Vehicle Safety Standards; Occupant Crash Protection. Final Rule
  3. Karlsson v. Ford Motor Co., 140 Cal. App. 4th 1202 (2006).
  4. Autoliv Inc - What we do - Seatbelts
  5. The first seat offered in the U.S. that incorporated a three-point belt—on the 1990 Mercedes-Benz SL—
  6. It's a seat belt praised in several internal GM memos obtained by the 11 News Defenders that say this different belt system "improves safety performance", "occupant retention", "rollover protection" and works "for all size occupants in all seat positions". This lap and shoulder belt system looks just like the old one, but for one obvious difference -- this lap and shoulder belt attaches not to the frame of the vehicle but to the seat.
  7. default - Concept Cars - Volvo Models - www.volvocars.com/intl
  8. 3+2 Point Seatbelt and Side-Support Airbag, from Autoliv
  9. Ford researches possible next-generation safety belt technology
  10. TIME magazine article on issue about Earnhardt
  11. [1]
  12. Trollhattan Saab — Saab 9-1, 9-3, 9-4x, 9-5, 9-7x News
  13. "Kids at Risk: When Seatbelts are NOT Enough", by Karp H, Reader's Digest (US Edition), November 1999.
  14. Volvocars corporate website
  15. "Unbelted rear passengers 'biggest danger in crash"
  16. Allt om motor: Åtta av tio utan bälte kör Audi
  17. 'The Effects of Mandatory Seat Belt Laws on Driving Behavior and Traffic Fatalities' by Alma Cohen and Liran Einav at Harvard Law School
  18. Seat-belts in School Buses?
  19. Pros, Cons of School Bus Seatbelts
  20. Assessment of three-point passenger restraints (seatbelts) fitted to seats on rail vehicles PDF, page 33


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