MOTORISTS OBSD THE HELICOPTER FLYING LOW OVR & ALONG AN INTERSTATE HIWAY. IN THE VICINITY OF A HIWAY REST AREA, THE HELICOPTER MANEUVERED TO THE SIDE OF THE HIWAY, THEN FLEW BACK OVR THE MEDIAN. TWO WITNESSES SAID IT CAME TO A HOVER, THEN STARTED TO CONTINUE ALONG THE ROADWAY. SUBSEQUENTLY, IT HIT A PWR LINE THAT CROSSED OVR THE HIWAY & CRASHED ONTO THE MEDIAN AFTER TRAVELING ANOTHER 426 FT. AFTER IMPACTING THE MEDIAN, THE HELICOPTER TRAVELED ANOTHER 57 FT BEFORE COMING TO REST. THE ACDNT OCCURRED IN THE VICINITY OF WHERE THE PLT'S WIFE & A FRIEND OF THE PASSENGER WERE TRAVELING BY AUTO. THE PLT & HIS WIFE DENIED KNOWING EACH OTHER WERE IN THE VICINITY; ALTHO, BOTH WERE RETURNING FROM THE SAME RECREATIONAL ACTIVITY. THE PLT STATED THAT HE WAS ATTEMPTING A PRECAUTIONARY LANDING AT THE REST AREA DUE TO BAD WX. AN INVESTIGATION REVEALED THAT DRG THE ACDNT, THE ROTOR OF THE HELICOPTER HIT THE PWR LINE ABOUT 28 FT AGL. A WITNESS SAID THAT LIGHT RAIN WAS FALLING WHEN THE ACDNT OCCURRED.