What are passive restraints? How do they keep you safe during collisions?

December 14, 2020

Learn how passive restraints, such as airbags and seat belts, work to keep you safe in the event of a collision


A man putting on a seat belt passive restraint system

In our previous blog, we discussed collision avoidance systems: Important auto safety features that help you avoid getting into a crash in the first place. But while that technology has undoubtedly made driving safer, accidents do still happen. Fortunately, when designing the safest possible vehicles, manufacturers didn't stop there; they created passive restraints to protect you and your passengers during a collision. 

So, what are passive restraints? What are the mechanisms behind these life-saving features and how do they work to keep you and your passengers safe? Today, we're taking a look at two of the most essential life-saving features your vehicle could possibly have: airbags and seat belts! Read on!

What is a passive restraint system, exactly? 


The term "passive restraint" refers to a vehicle safety device or feature that is activated by the force of a collision or sudden stop with the intention of preventing injury to the occupant. By definition, they don't require any input from the driver to function. 

While other safety systems, such as anti-lock brakes and electronic stability control, work to prevent accidents, passive restraints, or passive safety systems, work to protect you during an impact. The most common passive restraint systems in vehicles today are airbags and seat belts, or safety belts. Below, we'll take a look at each of these systems and discuss how they came to be and how they work to save lives during a collision. 

Airbags

How were airbags developed?


Although some form of airbag technology has been around since 1919, versions made specifically for automobiles weren't invented until the 1950s. After several decades of improvements, the first commercially available airbags hit the market in the 70s, but weren't truly widespread until the 90s. 

Some of the first automotive airbags used compressed air to inflate the cushion, which were either activated by bumper contact or by hand. Neither of these methods proved to be very effective as they simply couldn't deploy in time to properly cushion the driver from an impact. Shortly thereafter, manufacturers switched over to chemical and electrical methods of inflation, which we still use some version of today.

For the first several decades, airbags came with their own share of safety concerns. They often inflated too quickly and forcefully, which could cause significant injuries even during low-speed accidents. Fortunately, advancements have been made and regulations put in place to ensure that airbags are as safe and effective as possible. After decades of crash tests and simulations, the deployment process has been fine-tuned to ensure that they are fast enough to be effective without causing harm in the process. 

As a result, modern front airbag systems "reduce driver fatalities by 29 percent and fatalities of front-seat passengers age 13 and older by 32 percent," according to the National Highway Traffic Safety Administration. In combination with lap and shoulder seat belts, "the risk of death in frontal crashes has been reduced by 61 percent." 

How do airbags work? 


Airbags are meant to prevent head and neck injuries in the event of a collision by creating a cushioned barrier in between you and the dashboard or steering wheel. But crashes happen fast; one minute you're speeding down the road and the next, you're not. That's why the method of deployment is just as important as the bag itself when it comes to saving lives. 

To ensure the timing is right, airbags are outfitted with an accelerometer which detects sudden changes in speed. When they sense that a vehicle has been in a collision, it sends an electrical impulse to a container filled with a mixture of chemicals. The impulse detonates an ignition compound, creating a small explosion. The heat from this explosion interacts with the other chemicals, causing them to degrade into a gas which inflates the airbag.

At this point, you might be thinking, "Doesn't that whole process take a lot of time?"  From start to finish, it certainly is a rather involved process and sounds as though it would take time - too much time, perhaps. In reality, all of this takes place in 30 milliseconds! In that short flash of of an instant, the bag is inflated and 20 milliseconds later, the passenger safely hits the bag. 

Seat belts


According to the NHTSA, seat belts save almost 15,000 lives each year. Nearly half of vehicle-related fatalities involve individuals who were not wearing a seatbelt. Seat belts are commonplace these days, and are one of the most important safety features in any car - but that wasn't always the case. 

When were seatbelts invented?


The concept of a seat belt (sometimes called a safety belt) is far from a new invention. Although they didn't find their way into automobiles until much later, horse-drawn carriages often had some form of safety belt or harness as early as the 1800s. When vehicle manufacturers began to add seat belts to some of their vehicles, they were typically lap belts, or two-point belt systems. It wasn't until 1959 that an engineer working with Volvo adapted an aircraft harness into the three-point, Y-shaped seat belt system that we use to this day. (Interesting fact: The first vehicle to come standard with the three-point belt was the 1959 Volvo 122!)

Even after the invention of the three-point automobile harness, it would still take a number of years for them to become truly ubiquitous in the industry. They were often sold as upgrades or modifications at gas stations and auto parts stores, but weren't required by law. However, with The National Traffic and Motor Vehicle Safety Act of 1966, laws began to emerge requiring manufacturers to place safety belts in the vehicles they built.

How do seatbelts work?


On the surface, it may seem as though seat belts are fairly simple mechanism; it's a belt that prevents you from flying forward. How complicated could it be? Although the belt itself (also known as the webbing) is certainly a simple idea, there's a lot more going on behind the scenes.

Inertia: Why seat belts are so important. In order to understand how a seat belt works (and why it's so important that it works), let's take a trip back to physics class for a moment. Newton's first law says that an object at rest or in motion will stay at rest or in motion unless acted upon by another force. "Inertia is the resistance of any physical object to any change in its velocity," and it's the reason your coffee goes flying out of the cup holder when you slam on your brakes. 

If you're driving at 60 mph, everything inside your car is traveling at that same speed (including you and your coffee). When your vehicle speeds up or slows down gradually, you and the car appear to move as one, almost as though you had the same inertia. That is deceptive, however, because everything inside the car has its own inertia, independent from the vehicle. If you slam on your brakes, or crash into an object, the vehicle's speed drops instantly and you, the driver, do not. That is to say, you keep going until something stops you. Fortunately, your seat belt can be the something that slows you down (but your coffee is on its own). 

The force a seat belt must exert upon you in order to stop your forward movement is considerable. For this reason, seat belts are specifically designed to spread that force across the largest possible area on the most durable parts of your body (like your shoulder, chest, and pelvis). In the case of a three-point harness (which is combination of both a lap and shoulder belt), the belt stretches across your pelvis and chest before fastening themselves to three points on the vehicle's frame. 

The larger the area you can spread the stopping force, the better. That's why harnesses in race cars and fighter jets have six or even seven-point connections. On the other hand, lap belts only have one strap, and thus the stopping force is spread across a much smaller section of your body. That is why you should use a three-point system whenever possible. 

The mechanics of seat belts. As you're probably aware, seat belts are designed to become tense when a sudden stopping force is applied - you may have noticed this if you try to pull the belt out too quickly, or even if you brake suddenly. The most common type of seat belt has a fairly basic mechanism that it uses to control the slack in normal circumstances, but also keep it taught in the event of collision. 

In normal conditions, the slack of a seat belt is controlled by a spool and spring, which allows you to pull as much of the belt out as needed before reeling the rest in. When your vehicle comes to a sudden stop, or when the belt is jerked too quickly, a locking mechanism prevents it from extending any further. 

Around this spool is a gear with a series of teeth, which is key to the locking process. In some vehicles, there is a small, weighted pendulum that swings forward when the vehicle comes to a sudden stop. As it swings forward, it pushes a lever into the gear's teeth, which prevents the spool from rotating any further. Yet another locking mechanism relies on something called a centrifugal clutch, which turns as you pull the belt outward. Pull slowly, and nothing happens, but pull quickly, and the weight in the clutch spins outward, causing a lever to lock into the gear. 

Over the years, these simple mechanisms remain relatively unchanged. Newer vehicles may make use of electronic accelerometers and sensors to engage the locks, and they may feature a pretensioner. 
 
Pretensioners. Ideally, there won't be slack in the belt when the locking mechanisms kick in; too much slack means you're not properly being held in place. However, these locks don't remove excess belt length - they simply prevent it from extending further. That is where pretensioners come into play. 

Pretensioners are an additional safety measure designed to actually tug the belt during a crash, which pulls the passenger towards the seat and holds them firmly in place. Usually, they rely on the same sensors that trigger the airbags, causing them to activate at the same moment. 

Like the airbag deployment system, many pretensioners rely on pyrotechnics to pull the belt back into place. After the electronic sensor detects a sudden deceleration, it detonates a small explosive in a chamber with a piston. This explosion sends the piston upward, which turns a gear attached to your belt's spool, quickly winding it up.

What passive restraints are required by law?


Having passive restraint systems in your vehicle is clearly important for your safety and the safety of your passengers. We have laws for all sorts of things when it comes to driving, so how has the law adapted to these safety measures over the years?

Are airbags required by law?


When airbags were being developed, it was thought that they may replace the need for a seat belt entirely. Of course, this notion was abandoned as the data began to show that the combination of a safety belt and an airbag was the safest method. 

Although many vehicle manufacturers included airbags as a standard safety feature, they weren't required by law until the Intermodal Surface Transportation Efficiency Act of 1991. This piece of legislation put regulations in place that required all automobiles and light trucks made after 1998 to have airbags installed for the driver and the right front passenger. 

Unlike seat belts, an occupant doesn't typically have a choice as to whether or not they use an airbag. The laws require them to be placed in a vehicle and as long as you're driving such a vehicle, you are technically using one. That having been said, it is possible in rare cases to have a deactivation switch installed. However, this can only be done by an authorized dealer or repair shop, and requires authorization from the NHTSA.

Are seat belts required by law?


When seat belts were new, people were hesitant to use them. They seemed like a hassle, and it wasn't clear that they were truly necessary or even helpful. But as time went on, numerous studies began to paint a clear picture: Seat belts are an amazing, life-saving invention.

In the United States, seat belt use laws are left to individual states to make, but the legislation requiring manufacturers to place them in vehicles was implemented at the national level in the late 60s. At first, the law simply required belts to be placed in all seating positions, but later specified that they needed to be three-point belts.

Although manufacturers have been required to place seat belts in vehicles for years, states didn't implement laws requiring occupants to use them until the 80s - using a seat belt was completely voluntary. In 1984, New York was the first state to put laws into place requiring occupants to use them. 

Since then, nearly every single state has put seat belt usage laws into place (with the exception of New Hampshire). Of course, the specific requirements and types of laws vary from state to state. 

Texas has what are known as primary enforcement laws, which means an officer has the ability to pull a driver over and issue a ticket (up to $200) for not wearing a seat belt. These laws require everyone to wear a safety belt, regardless of where in the vehicle they are seated. Children below the age of 8 must be in a child car seat unless they are taller than 4 feet 9 inches, in which case they must wear a seat belt. 

An airbag, which is a passive restraint system

Passive restraints don't just save lives, they can save you money, too! If your vehicle is equipped with factory passive restraint systems, you may qualify for a discount on your auto insurance

For more information about our insurance products, request a free quote online, or reach out to one of our trusted agents today!


Read more: Airbags and seat belts help protect you in the event of a crash. But what safety features help you avoid the collision in the first place? Read part 1 of our blog series exploring vehicle safety features to learn more!

by Geoff Ullrich

About the Author

Geoff Ullrich is a writer and Content Marketing Specialist at Germania Insurance.