Welcome to OpenGeofiction. Be creative and have fun!
Here are some important notes regarding roadway design.
Motorways, Freeways, Auto-routes, and the like are dual carriage-ways and both directions of the motorway should be drawn.
streets should be "connected" at intersections. many new cities show them simply overlapped. if it crosses and is not a grade separation (aka bridge tunnel) it should be connected at that point with a node.
If you desire you can add traffic signals to your intersections.
Bridges should not be unnecessarily long. and make sure to place it on the appropriate layer (such as Layer = 1) or where two bridges cross. Layer 1 and Layer 2 for the second bridge that is higher. 3, 4, 5, etc. same thing with tunnels and subways (layer = -1)
I am sure there are more pointers but these are a few to start.