Looking at the MDOT website, the traffic incident closing the Key Bridge was posted at 1:27 AM.
The Washington Post has police audio at the time of the closure (https://www.washingtonpost.com/dc-md-va/2024/03/26/baltimore...). A quick summary of the timeline from that audio:
* There was a request to close the bridge when the ship lost power, which went over police dispatch about a minute before the bridge collapse (the bridge collapse is reported at timestamp 1:09 in the audio).
* Someone was able to hold the outer loop traffic at ~0:20 in the audio, as they reported they were already driving along at the time.
* Inner loop traffic is reported stopped at ~0:56 in the audio. I suspect there may already have been a police car there because of the construction on the bridge.
* Between 0:20 and 0:56, the conversation is about pulling the workcrew off the bridge. The police officer blocking inner loop traffic, after reporting stopping traffic, is indicating that he's waiting for a second unit to arrive before going onto the bridge to collect them.
* At 1:09, the bridge is reported collapsed, and multiple officers confirm. There is a question as to which traffic is stopped--the people blocking inner loop traffic are unable to confirm outer loop stoppage, but the person holding outer loop informs them of the stoppage at the end of the recording.
So traffic seems to have been stopped for about 10-50 seconds before the bridge collapse, depending on the exact length of time between someone stopping traffic and radioing in that they did so. From what I can tell, it sounds like outer loop traffic was stopped in time solely by sheer coincidence, while the inner loop traffic may have been existing police presence (for the construction zone) changing posture to a full closure.