Here some samples /blogs/michael/2015/05/01/useful-cityengine-rules
And here how to create RPKs Help
There are some simple street rules in ESRI.lib (comes with CityEngine, see Navigator). There is the Example Complete Streets downloadable through CityEngine (Help -> Download Tutorials and Examples). David Wasserman also has a complete street rule which has many advanced features.
It is worth mentioning that the street rules available (mine included) do not work in ArcGIS Pro, even if you export them as RPKs. I am not sure what Esri's plans to do with procedural streets in Pro, but I have been told a few times it might be a few years out.
Thank you so much for your answer. I haven't heard about it before.
Okay, so it seems that there is no way to create streets in Arcgis Pro with rules.
Is there a way to create them in another interface and then import them to Arcgis Pro? (with the aim to create a scene layer package to upload to Arcgis online)
In case you are interested, the updated Complete Streets Rule has more information about it here: Complete Street Rule Update
The answer I think is yes, you can create streets in CityEngine and export them as a scene layer package directly from CityEngine. You should either also create your buildings in CityEngine, or at least import their footprints/parcels so that you can have a good placement of streets relative to other assets/buildings.
Others might have better advice.