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 occupant restraint 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) and by preventing the wearer from being thrown from the vehicle.

Types of seat belts

 * Lap: Adjustable strap that goes over the waist. Used frequently in older cars, now uncommon except in some rear middle seats. Passenger aircraft seats also use lap seat belts.
 * 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.
 * Lap and Sash: Combination of the two above (two separate belts). Mainly used in the 1960s and 1970s. Generally superseded by three-point design.
 * Three-point: Similar to the lap and sash, 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. Until the 1980s three-point belts were commonly available only in the front seats of cars, the back seats having only lap 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 of the developed world 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.
 * Criss-cross: Experimental safety belt presented in the Volvo SCC. It forms a cross-brace across the chest.
 * Five-point harnesses: 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 legs 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: Similar to a five-point harness but includes 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. They were introduced in aircraft for the first time in, by Adolphe Pegoud, who became the first man to fly a plane upside-down. However, seat belts did not become common on aircraft until the 1930s.

Edward J. Claghorn was granted US patent 312085 on February 10, 1885 for a safety belt

American physicians in the 1920s advocated the use of seat belts in cars. Some of them even outfitted their cars with seatbelts. Plastic surgeon Claire L. Straith and physician C. J. Strickland were at the forefront of that demand. Strickland founded the Automobile Safety League of America. The American public showed little interest.

Engineer Hugh De Haven invented the inertia reel and created the concept of "wearing" the car and "packaging" passengers.

Safety belts were tested by Col John P. Stapp, using a rocket sled and himself as the guinea pig, among others. His studies explained the phenomenon that most people injured or killed in plane crashes didn't die when the plane hit the ground, but when the person hit the inside of the plane.

Edward J. Hock invented the safety belt first used by the Ford Motor Company as standard equipment, while he was on active duty with the military as a flight instructor. In 1955 his idea was accepted by the naval authorities, and Hock was awarded $20.50 for his invention. The original schematic and blueprints shows that he utilized scrap parachute strapping to implement his idea. He was never awarded anything other than the $20.50 award, a letter of recognition, a picture with military "brass", and a newspaper article to his credit.

The three point seat belt (the so-called CIR-Griswold restraint) was patented in 1951 by the Americans Roger W. Griswold and Hugh De Haven. There's no information about this design being used in cars

Saab was the first car manufacturer to introduce 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.

Nils Bohlin of Sweden invented a particular kind of three point seat belt for Volvo, who introduced it in 1959 as standard equipment. Bohlin was granted US patent 3,043,625 for the device.

Most U.S. automobiles were sold with front seat belts standard in the 1964 model year. Rear seat belts were made standard in 1968.

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

Technologies
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.

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.


 * 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.

There is some research and tort liability case findings which suggest that some seatbelts are subject to inertial release (explanation needed). Although the NHTSA has been urged to deal with this defect it has yet not been properly addressed. "In 1992, the Institute for Injury Reduction (IIR) petitioned NHTSA to investigate such defects, but after several months, NHTSA denied the petition. Its own laboratory results indicated that inertial forces could cause a buckle to open, but that it didn’t happen in real world accidents." Source: Seat Belt Defects

Automatic seat belts

 * Main article: Automatic seat belt

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 characteristic "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 and/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.

Use of seat belts by expectant mothers
For pregnant mothers, the fetus is protected by a sac full of amniotic fluid. This sac is quite strong and the fluid inside acts like a cushion to protect the fetus. The sac is pliable so it can change shape to a certain degree. The proper use of a seat belt will divert the pressure points off the sac, and thus the fetus would only be minimally affected. The lap belt should be worn low over the pelvic bones and not against the soft stomach area. The shoulder belt should be worn across the chest. Both should be worn snugly.

Reminder chime and light
In North America and some other parts of the world, cars sold since the early 1970s have included a seat belt light on the dashboard, reminding the driver and passengers to buckle up. These systems also included a warning buzzer which sounded for several seconds before turning off (with the warning light), regardless of whether the car was started. 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.

By the early 1980s, many automakers selling in the U.S. market had replaced the buzzer (along with all other buzzers for functions such as headlights-on) with a seatbelt warning chime, though for some models, this change was not implemented until the 1990s or even the early 2000s. Today, many of these carmakers use a red figure with its seatbelt on to serve as its seatbelt warning light, and it may stay on for several minutes after the car is started and the driver's seat belt is not fastened.

In Europe 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. In some systems (i.e. older Volvos), the seatbelt is connected to the turn signal relay, making clicking sounds constantly until the front passengers are buckled up.

Some newer cars from Audi, BMW, Ford, Honda, Hyundai, and Toyota will intermittently flash the reminder light and sound the chime until the driver (and sometimes the front passenger, if present) fasten their seatbelt.

Legislation

 * Main article: Seat belt 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 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 some, stemming from observed finding that following the passage of seat belt laws, road fatalities often did not decrease.

Risk compensation
Some have proposed that the number of deaths was influenced by the development of risk compensation, which says that drivers adjust their behaviour 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 seatbelt legislation.