The highways got redone a lot in the Rush Hour expansion, making them generally more useful and worthwhile.
The ground highway is a new type of highway. It has a lower construction cost, but is harder to plan around in your city. Roads and avenues can cross ground highways by forming bridges (as well as via overpass), but railways cannot.
The ground highway can be connected directly to an elevated highway, as well as to an avenue.
In Rush Hour, highways can not just form cloverleaves but also T-intersections, which take up less space. Another new thing is the addition of overpasses/underpasses, which makes it easier to connect highways to the road network. One problem is that overpasses/underpasses do not work well with avenues, because of the inability to switch sides.
Because of the new additions, you can now cycle through the various available intersections using thekey.