The photos below show the front bumper height of some cars at its highest position, which means empty and parked cars. In the case of an emergency braking, this height is substantially reduced, and if the truck rear underride guard is above it, the car from bumper underrides the truck's guard, thus promoting the "WEDGE EFFECT".
When the truck's underride guard hits the car above its front bumper, the front seat car passengers are severely wounded in the legs and in the chest, by compression with the panel and steering wheel.

CONCLUSION: THE TRUCK UNDERRIDE GUARD AND THE CAR FRONT BUMPER SHOULD BE PLACED AT THE SAME HEIGHT. Since there are cars with different front bumper heights, rear underride guards must be placed at a maximum height of 400 mm from the ground on an unloaded truck in order to be effective in avoiding underride of light passenger vehicles.

PRESENT CRASHWORTHINESS TESTS, WHICH ARE QUITE IMPORTANT FOR OTHER TYPES OF COLLISIONS, ARE USELESS FOR COLLISIONS AGAINST WRONG PLACED OR WEAK TRUCK'S REAR UNDERRIDE GUARDS! THESE CRASH TESTS, WHICH ARE ALSO USED TO ADVERTISE THE CAR ENERGY ABSORBING PERFORMANCE, ARE NEVER CARRIED OUT WITH OBSTACLES ABOVE THE CAR FRONT BUMPER. THE REASON IS OBVIOUS: THE CAR FRONT STRENGTH IS LOCATED AT THE LEVEL OF ITS FRONT BUMPER.