Adjustments
Adding new points
The easiest way to add new points to a Path is by holding Ctrl + Q and clicking LMB over the landscape.
You can also add points in the middle of a Path, by selecting a PathSpline point and then using Ctrl + Q + LMB combination.
Another way to add new points is by selecting the first or last PathSpline point and dragging it while holding Alt.
You can also right click on the PathSpline and select Add Spline Point Here.
Unreal also allows creating some spline shapes automatically (e.g. circle, arc, straight line). To do that, right-click on the spline point and open the "Spline Generation Panel". After adding the points, just close the panel, so the Path update can proceed.
Projecting onto geometry
By default, Paths follow the height of the PathSpline. This is useful when creating Paths that need to maintain a steady height, such as a highway, tunnel or bridge. For smaller roads it is often useful to project the Path onto the landscape using ProjectOntoGeometry property in the PathAdjustmentProperties.
There is also a property that controls whether the first/last points should be projected as well. This matters when the ends of the Path are connected to intersections.
The best method of projecting a Path only onto specific objects (e.g. landscape) in a level is to create a trace channel with the default response set to "Ignore" and setting the response to the "Block" for the selected objects only. The Path can be configured to only project onto objects with that trace channel set to "Block".
The video below demonstrates results of this method. Path projection ignores the tunnel above the landscape if the "Collision Channel" is set to the "PathProjection". Otherwise, if it's set to the "WorldStatic", Path gets projected onto all the objects, including the tunnel.
When projecting a Path onto the landscape, it's usually crucial to apply vertical smoothing to the Path.
Vertical offset
You can adjust the vertical offset of a Path relative to the PathSpline by changing the Vertical Offset property.
Smoothing
Path can be smoothed out using Smoothen Over Distance Horizontally and Smoothen Over Distance Vertically properties. The larger the value, the more smoothing is applied.