See that horizontal bar mounted behind the rear wheels of the trailer? That’s the Under-run Protection Bar:
It is also installed on the front and the sides

Accidents are designed to be bumper-to-bumper contact. This is because modern cars are constructed to keep the passenger compartment intact, whilst allowing the front and rear crumple zones to absorb the majority of the impact.
Crash test research has shown that this can reduce injury to the passenger.

Trucks have large overhangs and no bumper
A – Pillars
The A-pillars of a car are not designed to absorb or resist impacts. As a result, the passenger compartment takes the full brunt of the impact. This causes severe head and upper body injuries to the passengers. These type of accidents have an extremely high fatality rate.
As a result, in the case of a collision, the smaller and lower vehicle tends to “ride under” the truck’s long overhang
You can see how the rear end of a truck has deformed the A-pillars (ie. the vertical pillars on either side of the windshield):
That’s where the Under-run protection bar comes in. Think of it as a bumper for trucks:
The Under-run protection bar (front, rear, sides) enables small and large vehicles to make bumper-to-bumper contact. This greatly reduces fatalities & serious injuries to the occupants in the car.

PROBLEM: A lot of under-ride bars are not constructed well enough. This causes them to collapse when they are hit hard, or just on one side. This results in the car traveling under the truck.