This is a quick and simple technique for "merging" a single lane split highway to a regular ground highway.
First create a stretch of ground highway and demolish a small section where you want the merge to occur.
Connect a single tile length of avenue to one end of your highway like so:
Next, run a small one way road section to join at the convergance of the highway and the avenue. Be sure that the direction of the one way road matches the direction that the split highway lane will have once connected.
Once you have this completed, run another ground highway up to the one way road so that the lanes connect.
Now run a section of rail along the unwanted lane of the new highway to demolish it. Once completed, demolish the rail line as well.
You will now have your split highway connecting to the ground highway and as long as you have taken care to ensure traffic flow is in the correct direction, the junction will work like any other in the game. The next step is to re-connect the original highway using a new section of ground highway.
You will now have the finished product. Although the merge is noticable, it is not too obvious (particularly at higher zooms) and a few trees will help to disguise it all the more. The important thing is, it works.