The Army Engineer answer is that plank roads are put down in areas where bogs and such needed to be passed by wagons. They float, are usually roped together and hold up fairly well. Post , Railroad ties,etc. will also work. If put in with a base course of light agggregate they are even stronger. In WWII they used pierced steel planking for roads over sand, mud, for airfields and revetments. I think they were also used some in WWI and I know they were in Vietnam.
Bus as your satement implies, someone may have named a road Plank.