Why would you want run-offs? Are you Hermann Tilke?
Anyway, for my Lippo Village circuit I did it using terrain.
For lines on the track, download the road XPacks by ennisfargis. To create custom lines. create a wall. Remove the two side panels, adjust the width of the top one and lower it to about 0.02Y (2cm off the track). Uncheck "collidable". Click the materials tab, and replace it with any of the ennis textures. Then, set the Scale X to 0.01 and the Scale Y to 1.00. Set the X Offset to -0.02 and the Y Offset to 0.00. Set the panel length to 0.5. Then manipulate the walls nodes in the normal way.
And if the road has considerable elevation or camber, large bumbs etc. the lines may behave oddly. Decrease panel length to make it hug the ground smoother. If that doesn't work, you can create new control points to problem areas or make a new material or shape crossection (ctrl-click&drag along the wall) You can also make the shape control points "Grounded" to make them follow the terrain/track shape (you may have to raise them a bit, couple of millimeters to centimeters, sometimes the points drop just a little with Grounded selected, not a big problem..). If the road is relatively straight and flat or has continuos angle, Kyle's post is spot on. I usually don't bother about scaling textures, i just use small 32x32px grainy white texture with 80-90% alpha transparency.
If you want the full track length white lines or similar, it is best to put them straight to road texture in photoshop/gimp.
Terrain is the easiest tool to create runoffs, the problem comes when the runoff area has a major elevation changes, the terrain form sharp angles between faces. Then a Wall or open ended track is pretty much only options.
There's away to make smoother terrain using an open-ended track that forms the outer edge of runoff. Adding a single panel terrain (no reduction to anchor points) to that new short track. Do the same on the drivable track, create the same kind of terrain. Then by carefully merging the terrain points between the helper track and the course, the terrain becomes much smoother as it has wide but short panels that are formed using those two tracks spline. You have to lasso the points and merge them with "M" key! That averages the points height so the merged point should form close to the center of runoff area and it's height should be also halfway between the original unmerged points.
You can finetune the far edge of runoff by adjusting the helper track camber, elevation and curvature. You can't make drastic moves to track with terrainattached to it. You have to create manually the final runoff far edge by forming new terrain points in top view ; put the helper track about 2m farther than the actual edge will be. That is because if you delete the short help track, the closest panels are deleted too.
You can create the terrain by using 2 or more panels but then the track cambers etc affect only the closest (first "line" of terrain) terrain panels. I usually do it in two stages, first the road shapes with simple terrain (one panel). Then delete and create new terrains with more panels. The both side should have the same or similar terrain generationsettings! Specially the reduction should be kept to minimum to get the wide and short panels.
Test this first in a simple test project to get the hang of the merging process. This is good to do before the project get's too big, as you will have to press undo at least once...