Though traditional airbags were a significant development, they were most effective in straightforward crashes. In a front-end collision, for example, the engine compartment is designed to collapse and absorb most of the impact. A car that is hit in the side, however, has only the door and a few inches of space to protect the occupant. Drivers and front seat passengers were left vulnerable to side impacts; rear passengers were an afterthought.
Rather than reinvent the wheel, automakers studied crash test data to figure out where additional airbags would be most beneficial. Side collisions are most dangerous to a vehicle's occupants because the crumple zone is so small, so airbags were added to doors and front seats to protect the chest and thorax area of front passengers.
An early device was the head airbag, or Inflatable Tubular Structure, commonly found in many vehicles after 1998. When the car senses a side impact, the tube emerges and inflates, forming a firm 5 inch (12 centimeter) barrier across the front and rear windows. This tube prevents occupants' heads from being smacked into hard surfaces; however, it offers no protection from injury caused by limb movement or flying debris.
So, along came the advent of side curtain airbags, which "unroll" in a collision to provide protection to passengers in a side impact collision. Curtain airbags are most often mounted within the headliner above the doors and windows. As with traditional airbags, when the sensor detects an impact, some deploy through the material, and others are stored in a compartment beneath a panel that blows out.
Two convertibles introduced a variation on the technology, demonstrating the side curtain airbag's versatility. Porsche and Volvo both claim to be the first manufacturers to incorporate door-mounted side curtain airbags [sources: Mello, Wardlaw]. The airbags incorporate vertical air chambers (somewhat resembling an air mattress) that have a stiff structure, allowing them to deploy upward to cover the window area. Because these airbags are not based in the roof, they offer protection from side collisions even when the convertible top is down.
Though the technology is similar to other types of airbags, side curtains offer extra protection that traditional airbags might not, which we'll discuss in the next section.