Invisible Road spline, 1-Way / 2-Lane, Low Traffic (1 vehicle every 60 secs), 35 mph. Visible in Surveyor.
DO NOT mix with 2-Way and/or 1-Lane/3-Lane traffic-generating assets within the same spline row.
Make sure all of your spline segments are facing the same direction. If used with Traffic Stoppers you may have to rotate the stopper by 180 degrees to get the attached spline segments facing the right direction.
Note that only the end piece of a spline row will generate traffic at regular intervals when in Driver, but there may be some additional traffic generated once at intermediate spline points when loading the route.
Known bug with current Trainz versions: Entering Driver via Surveyor (Ctrl+F2) may still show some surveyor-only segments of the spline until zooming in or out a little bit. Unfortunately there's no better solution to have a surveyor-only asset generating traffic when invisible resp. when in Driver.