Inspection report last year indicated cracks in the structure.
Well now, I didn’t know that. Can I retract my post?
Just a guess. But it is possible that when the bridge was built in the ‘60s the shoulder was not designed to carry the full load of 80,000 pound trucks bumper-to-bumper.
Now we are here 40 years later. The steel has developed some cracks and the contactor working on the repairs shoves all of the traffic onto the one lane that was never designed to carry it. The steel begins to bend under the heavy load of the evening rush hour, and the failure cascades through the rest of the bridge.
As I say, just a guess.