This VERY easy technique will help you to produce an attractive & realistic highway Junction, similar to ones seen all over the UK and the rest of Europe which use a one way roundabout to control traffic flow around the Junction.
Although I presume you already know how to produce a sunken highway, I will explain to those new to the game. Using smoncrie's "sunken" hole digger lots, dig a small strip about 10 tiles long and then expand using single road tiles. Once you have completed this, lay a ground highway in the channel.
Decide on the location of your Junction and demolish a strip of the ground highway large enough to construct the Junction (somewhere around 15-20 tile length). Once demolished, "fill in" the empty channel you now have, in order to bring it back up to ground level. I find the easiest way to do this is with single road tiles.
Now place a raised highway section in the area you have just levelled.
Connect up the ground highway to the raised highway. In this tutorial, the transition is a little bumpy (although not too bad) but with a bit of practice you can make this transition as smooth as if your ground highway were at ground level rather than sunken. The results look very pleasing indeed.
Now place two single road tiles either side of the raised highway where you want the first on/off ramp to be. This will prevent a double on/off ramp being formed when we run a one way road under the highway.
Repeat the process for the other set of on/off ramps and connect up the one way road into a rough expanded version of a roundabout.
All that remains is to connect up your road network, add a few grass filler tiles and trees and you're done.