After four more hours (in addition to the last three days), I have to pause this exercise completely! The reason is simple: Because of my intention to put in as many details as possible, I apparently did mistakes which led to some kind of a vicious circle. The main problem are the shadows. To create a gentle 3D-illusion, this characteristic was crucial to me, so I primarily focused on this topic. At the beginning the process went fine. It was not a big deal to create a shadow for the vehicles or the street lamps for example, but then the street barriers became a little more difficult. The difference here was the fact, that I had to combine individual shapes to one big shape to prevent gaps and irregularities. Afterwards the surface looked acceptable, but the contour was torn apart somehow. - Unfortunately the most important property of a smooth shadow.
I was able to compensate it a little bit, but when I had to deal with the shadows of the trees (which have to lay over everything underneath them), I came to my current limits. As you might see in the latest image (see above), these shadows have sharp edges unlike the other ones. They are also not adjusted to other objects, because it would have been necessary to cut the shadows at several spots. This led to more disrupted contours, and the mistake became bigger than before.
Another problem increased due to performance issues of my computer. - Something I just can not understand since I also use this system for video editing and post production purposes, what might need more of its internal memory (16 GB of RAM and a good graphic card as support) than Illustrator. More and more bugs appeared! After selecting a shape or a group of shapes, it was no longer highlighted. A little later, I could no longer select objects directly. It was like the layers have been locked down (which is an useful option normally).
Original sketch of the highway and the bridge before working on Illustrator. In the beginning, the process was not supposed to last longer than two evenings / 6-8 hours. Due to the mentioned problems, I finally paused the project after almost 13 hours.
During editing the paths, they also stopped to lock on other paths, what made it impossible to work accurately. So my conclusion is, that I probably made a few essential mistakes, which I now have to figure out. In the meantime, I will pause the "highway project" and get started with the next steps of my helpful book by Monika Gause.