Regulators demand that automakers spend boatloads to save lives, so why not make trucks do the same?
There are a few reasons that semi trucks can be such dangerous things for a passenger car to crash into, and not all of them include the sheer difference in mass between the two bodies or the fact that many truck drivers on the road travel with boredom and fatigue as their main companions. It has mainly to do with the fact that modern cars and trucks are not built to play well with each other during episodes of physical contact.
Organizations like the IIHS and NHTSA have done an excellent job of forcing automakers to build cars that don't kill or maim occupants when they come in contact with each other by crashing test cars against each other. Thing is, when two cars collide, they usually strike impact points that engineers have designed with fortitude for real-life crash events.
One area that gets passed over are the A-pillars. When a car strikes a semi truck, the high edge of the box trailer is out of the way of these crash structures and exerts all of the force on the A-pillars, which inevitably collapse and decapitate occupants. Rear trailer guards are one way to reduce the severity of these accidents by bringing the impact spot lower where the car's safety systems are, but the sides of the trailer usually remain untouched. It was a lack of safety systems like the underride guard (along with Autopilot) that contributed to the death of Joshua Brown, but it's entirely possible for truckers to install these.
These crash tests show us how big a difference an underride guard makes using America's favorite fleet vehicle, the 2010 Chevy Malibu. Truckers pay attention, because this can both save lives and fuel thanks to the magic of aerodynamics.