THE PLT HAD DEPARTED THE ARPT ON THE SECOND LEG OF A X-COUNTRY FLT, WHEN AN ACCIDENT OCCURRED. HE REPORTED THAT HE WAS UNABLE TO OBTAIN A WX BRIEFING PRIOR TO DEPARTURE AND INTENDED TO CALL FLT WATCH ONCE AIRBORNE. HE REPORTED THAT AFTER DEPARTURE, A THUNDERSTORM WAS ENCOUNTERED AND HE INITIATED A 180 DEG TURN BACK TO THE ARPT. THE PLT STATED THAT DUE TO SEVERE TURBULENCE, HE ELECTED TO MAKE AN EMERGENCY LANDING ON A ROAD. WHEN THE ACFT WAS ON FINAL APCH TO LAND, A TRUCK AND TRAILER PULLED OUT ON THE ROAD AND THE PLT ATTEMPTED A GO-AROUND. HE REPORTED THAT DURING THE GO-AROUND, ANOTHER DOWNDRAFT WAS EXPERIENCED AND HE VEERED THE ACFT TO THE LEFT TO AVOID THE TRUCK. THE ACFT THEN HIT AN OVERPASS AND DESCENDED TO THE ROAD BELOW. OTHER LOCAL PLTS STATED THERE WAS NO TURBULENCE IN THE AREA THAT DAY, BUT THE WEATHER WAS DETERIORATING RAPIDLY.