On May 11, 1995, about 1715 Alaska daylight time, a wheel equipped Cessna 170B airplane, N4366B, sustained substantial damage during a forced landing onto Alaska State Highway 1, near Chickaloon, Alaska. The solo private pilot was not injured. The 14 CFR Part 91 cross country flight operated in visual meteorological conditions. The flight departed Northway, Alaska, about 1500. The pilot reported that while in cruise flight at 6,500' msl, the engine began to vibrate abruptly, followed almost immediately by a loud noise and the separation of the propeller from the engine. He said the only suitable forced landing area was the highway, and he was forced to land downwind and uphill due to surrounding terrain. The airplane bounced, and went into a ditch adjacent to the highway. The pilot could give no reason why the propeller separated from the engine. In his written statement to the NTSB, he noted that the propeller, the propeller mounting flange, and the forward portion of the crankshaft had separated from the engine and were not recovered.