WHILE EN ROUTE AT 5000' MSL, THE PLT TRANSMITTED THAT HE HAD AN ENG PROBLEM & REQUESTED CLEARANCE TO LAND AT THE PROVIDENCE/THEODORE FRANCIS GREEN STATE ARPT, WHICH WAS APRX 12 MI AWAY. THE PLT THEN ASKED THE CONTROLLER IF THAT WAS THE CLOSEST ARPT; HOWEVER, DUE TO THE RADIO CONGESTION, THE CONTROLLER HAD DIFFICULTY UNDERSTANDING THE PLT. VECTORS WERE PROVIDED TO THE PROVIDENCE ARPT. SUBSEQUENTLY, THE ACFT CRASHED ACROSS AN INTERSTATE HIGHWAY APRX 2 MI FROM THE ARPT WHILE ON A HEADING IN THAT DIRECTION. DURING THE CRASH LANDING, THE ACFT COLLIDED WITH A CONCRETE DIVIDER/BARRIER. AN INVESTIGATION REVEALED THAT THE ENG CRANKSHAFT, PN 639243, HAD FAILED FROM FATIGUE AT TWO LOCATIONS. THE FORWARD FAILURE WAS THRU THE CRANKCHEEK BETWEEN THE #2 MAIN BEARING JOURNAL & THE #3 CONNECTING ROD CRANKPIN. THE AFT FAILURE WAS THRU THE CRANKCHEEK BETWEEN THE #4 CONNECTING ROD CRANKPIN & THE #3 MAIN BEARING JOURNAL. BOTH FATIGUE CRACKS HAD SUBSURFACE ORIGINS.