The underpass is available only to vehicles which fit, which is to say ordinary height passenger cars or even just ordinary height half-width vehicles.
Why is this such a good idea? It’s vastly cheaper to make such an underpass. Because it’s so shallow, it is cheap to dig and shore up the walls. You can start the downramp much closer to the intersection because you don’t need to go so far down. It’s a tiny fraction of the cost of a regular overpass or underpass which requires lots of space to go up and down, and must be high enough for big trucks to pass underneath.