With the current emphasis on vegetated ditches and bioswales, what is the rule of thumb for roadside ditch depth?
If the ditch is designed to provide positive drainage of the highway it needs to extend below the subgrade. Generally 6 inches is enough, but 12 inches is used in many cases as we know ditches tend to silt in between cleaning. All ditches should have erosion protection and seeding with grass or other vegetation is very effective unless the ditch is too steep (greater than 5 percent slope).
If the road has underdrains to provide the positive drainage (such as in a curbed section), any ditch/swale only needs to be deep enough to carry surface water from the road and side area. It may be much shallower. In either case, the fore slopes next to the highway should be as flat as possible for highway safety. 3 to 1 or flatter slopes are recommended, but are not always possible do to right-of-way and other constraints along the side of the highway.