https://www.roadandtrack.com/car-cultur ... belt-1959/
In the 1950s, Vattenfall, the Swedish national power company, had a problem—its workforce was dying in car crashes. Two engineers with the power company, Bengt Odelgard and Per-Olof Weman, were given the task of making employees' cars safer. Their research led to the invention of the three-point seat belt by Volvo, Sweden's largest car company.
Based on research by the US Air Force, Odelgard and Weman came to the conclusion that a diagonal restraint, rather than a simple lap-belt, would vastly improve safety. In 1956, Vattenfall started fitting two-point across-the-chest safety belts to its cars, and soon after that, Volvo boss Gunnar Engellau became interested in the tech.