Traffic system means pedestrian, people walking around and cars driving around in a loop. Terrain includes visibility hidden transformgroup (TG) named splines. TG splines has sub TG's trafficSystem and pedestrianSystem. Below examples are from mapUS.i3d file.
trafficSystem has trafficLoop01 through 04 splines (assuming we can place any number of splines here?). Sub TG parkedCars which has TG's called parkedCars placed into parking lot spots.
trafficLoop01 spline E control vertex needs to be next to S so the spline will form a loop. When vehicle gets to E it can proceed to S right away. You could also hit O key to close the loop. Check the log file for errors if your spline is not accepted.
trafficSystem attributes onCreate: TrafficSystem.onCreate and xmlFile: $data/maps/mapUS_trafficSystem.xml
pedestrianSystem has pedestrianLoop01 and 02.
pedestrianLoop01 is the same type as trafficLoop01.
pedestrianSystem attributes onCreate: PedestrianSystem.onCreate and xmlFile: $data/maps/mapUS_pedestrianSystem.xml
mapUS traffic and pedestrian splines are not closed, but appears you can close them if you wish (please confirm! 2019-03-27).