The State Patrol is investigating a fatal traffic crash on the Interstate in Janesville about 8:00 a.m. Wednesday.  Officials say a 42-year-old man from Belvidere was killed when his motorcycle crashed into a barrier at the 171 northbound exit.  Witnesses say the motorcyclist started to exit, but changed direction at the last moment to resume traveling on the Interstate and made contact with the crash barrier.  Troopers say he was dead when they arrived.