Francis Scott Key Bridge collapse in Baltimore...

according to the post above, after the ships mayday, they were able to halt traffic and that the only persons on bridge was road crew fixing roadway.

I think the fact that it was close to 4am kept this from being a major catastrophe in terms of lives. It could have been so much worse.
I believe it was at 1:40am EST. But your right that had it been during the day possibly 100's would have been killed. From every video I have seen I can't find a shot that shows a car or truck on the bridge at the moment it collapsed.