Five witnesses observed the single-engine tail wheel airplane approach and attempt to land on a highway with vehicle traffic. The airplane struck a set of electrical wires that crossed the highway and impacted the surface of the highway. Examination of the engine and airframe revealed no evidence of a mechanical problem. Toxicological testing revealed .029ug/ml diphenhydramine in the pilot's blood. Diphenhydramine is an over-the-counter antihistamine with sedative effects.