Hi André,
Thank you for this qestion.
In default graph drawing mode, one wants the terrain be smothed to align to the network graph.
But you are using a premodeled bridge as a base for the graph. This means the feature has to be deactivated.
Settings for new streets can be controlled by using the Street Creation Settings dialog.
To open the dialog, choose Graph → Street Creation Settings... from the main menu.
There you have to uncheck the "Align terrain" option.
For more details please refer the CityEngine Help on that topic:
Creating and Editing Street Networks Manually
Best regards