I had just gotton on route 5 north from the Agawam rotary. After I merged onto route 5 I drove a short distance then I heard a loud noise from the rear of my vehicle. Immediately after the loud noise my rear end kicked out and I lost complete control of my truck. I hit the brakes and tried to slightly counter steer and I still had absolutely no control over the truck. I then proceeded to go over a straight edge bridge curbing and then a bridge guardrail. I did not hit any potholes,their were none and I was driving in a straight line at approximately 40 mph. My truck was hanging over the bridge.