Maybe I'm missing something, but I'm never able to make an elevated highway become a ground highway or vice versa. When I try to drag one and overlap it to the existing part it just turns red.

I like to have my highways end by turning into an avenue and monorails can't pass over elevated lines at all, with elevated rail requiring a subway transition.

None of my cities are generally well-planned and I usually make the terrain so hilly just for the challenge of placing infrastructure. I like creating landscapes and giving myself a hard time just trying to set up a city on it.

Well, make sure you have about three sqares each way to merge the ground with the elevaded highway. Also, make sure there is nothing that changes the highway before that (a road under it, the end of an exit ramp). Also, if you built something over the ground highway, delete it and rebuild it again when you are done (if you are working in a small area).

For avenues, just have enough room to merge it. You can merge it with elevaded and ground in Rush Hour (not sure about the first version).

Thanks.

Got it to work, found out that it didn't work in my other cities because the terrain wasn't too suitable for highways (too hilly) and sometimes due to the fact that I would get confused when you transition them because it doesn't show the transition when you drag it over in yellow, if you know what I mean.

Just for fun I built a racetrack city, complete with a fire station so that you can drive a fire engine along it plus a monorail that weaves in and out of the track and landmark buildings along the track. Would have posted a screenshot but the city wasn't really worth saving. :D

I like building weird things in my city.

That race track sounds cool! :D Too bad we can't see it. That would have been awsome! ;)