lease PM Cjah if you have any questions about this article.
Set street tiles above the place where your sunken highway is just before the start of the tunnel. Make sure to do that on both sides of the tunnel.
Go into god mode and raise the land up to form the land bridge. If your river or sea is deep it may take longer to get the land raised. Then build the tunnel with the highway or avenue.
Now lower the land bridge carefully near the site of both tunnels. Make everything level and add trees.
Now add Jeronij's walls or any other walls.
Do the same to the other side and you will have a complete underground tunnel. It can be used in all settings - rural/urban and even industrail.
Special thanks to: SkiGeek, ThatMonkeySim, and Voar Tok for their assistance on tutorial.