Invisible Road spline, 45 mph. Visible in Surveyor.
Make sure to have a traffic-generating asset (Traffic Generator / Vehicle Spawner or just a piece of visible road) facing the correct way at the end of your spline row to spawn vehicles at regular intervals when in Driver.
As this asset doesn't generate any traffic on its own, you may mix it with either 1-lane or multi-lane - and either 1-way or 2-way - traffic-generating assets within the same spline row.