Hi - I'm a nubie but this is what I found:
1. In the editor, open the Toolbox Tab and check the LINEAR OBJECTS TOOL icon (looks like RR track).
2. Then go to the Browser Tab and check ROADS icon.
3. From the list, select the type of road surface you want and lay it out the same way you would as track.
4. In the same list you will see SR-US-TRAFFIC-something. This is actual randomly generated AI cars and trucks. L=low flow, M=medium flow, H=high flow.
5. You then superimpose the traffic pattern on top on the road surface.
Super easy to do. I was going crazy trying to figure out how to do it too - so I hopes this helps.
