Hey there,
Your question is complex and you can go about it in many different ways. I’ve been thinking about creating a traffic system as a future project and thought about 2 main approaches - Node Based (AI goes from node to node) and spline-based (AI follows a spline). I decided to go with splines for more control since the AI will be driving a car.
As far as creating roads - feel free to use my free community tool the Modular Road Tool to create your roads quickly :).
You can get it HERE
Hope this helps!