Well, we do use over/underpasses here, when & where there's enough space available.
So they're not abundant, but we got some.
The Brits often use
2 bridges on Motorway entrance/exit complexes, with a big roundabout laid out over the Motorway.
That's also quite costly, but it has the added advantage of physically preventing people from going the wrong way on the motorway - whereas in Belgium the exit and entrance are often right next to each other, and relatively many people take the wrong one ...