
I couldn't find a highway labor project to add your grandpa to. The WPA ended in 1943, because the men were either needed for the war, or all of the other jobs that happened because of the war. Thanks for adding him to the WPA. A ditch grader is someone who uses heavy equipment, similar to a bulldozer, but with a V shaped blade, that cuts a trench to lay the pipe in. Pipes are often laid near roads, but not always, so I don't know that being a ditch grader (which is a great job) goes with highway labor.
Hope that helps.