After talking with numerous cities and states not pre-treating dry pavement appears to be standard procedure.
Weather records show there was no precipitation that evening, so plows would not be going back over dry bridges.
It's likely moisture got onto the bridge from car tires, and car exhaust. The tires carry slush from city streets and exhaust includes water vapor. The air temperature that morning hovered around seven below and any water would freeze very quickly.
Other agencies say once a road is dry or cleared, it's not gone over again unless more work is needed.
Typically when someone calls in a complaint about a slick spot. Trucks can't be everywhere and they also can't be held back to cover very small areas.
They're using our tax dollars to clear the roads, so when the job is done, the agencies want to get the trucks off the roads.