Looks like he had last second doubts and slowed down. If he was going the speed limit, either the load, the truck or the bridge (or any combination of the three) would have given out.
It might have been at speed and just barely caught the beam. It looks like the damage is to the upper corner of the item on the passenger side while the driver's side looks untouched. This would match the slope of the bridge where the right lane is marked 15' 11" and the shoulder is marked 15' 7".
The bridge is a concrete box beam which would take some of the impact but still allow an "almost" to pass through. A look at Street View in Google Maps (mile marker 265) shows previous hits on the bridge.