What do you get when you cross a seatbelt and an airbag?

The best ideas are often the simplest, and that certainly looks true of the latest car safety innovation.

What are the two best-known safety features experienced directly by passengers? The seatbelt and the airbag. But back-seat passengers only have a seatbelt, and installing an airbag for those seats is a tricky process. So why not combine the two?

That’s just what Ford’s research and advance engineering department has done. It’s created an inflatable model designed to solve two of the main problems with the seatbelt.

Firstly, while it does a great job of preventing impact with the seat in front (or worse, the dashboard or windscreen), it does mean that what impact does take place – between the chest and the seatbelt – is across a small area, thus increasing the force. That’s thought to be even more risky for rear seat passengers as they are more likely to be children or elderly people and have weaker bones in the chest.

Secondly, the traditional seatbelt design means that a crash can leave the neck snapping quickly to one side and may even mean the head slamming into the door.

The new system, which connects to the same triggering process as front-seat airbags, takes just 40 milliseconds to inflate, increasing the seatbelt’s surface area by around 400%. Consumer tests showed only one in ten people found the new design to be less comfortable than a traditional seatbelt.

The system has been researched and tested for several years, including tests to make sure it wouldn’t pose a danger upon deployment to a child who had fallen asleep with their heads tucked over the belt, or had wriggled about and managed to get the shoulder strap behind their back.

The system will debut next fall as an optional extra on the Explorer SUV, with an expected cost of below $400.