Couple of key points:
Ship appears to have lost power twice within 30 minutes of departing the port of Baltimore.
Pilot on ship immediately called coast guard, declared may day, and advised bridge be shut down. You can see this on the video where traffic appears to stop suddenly. This occurred less than 7 minutes before impact.
Police shut down both directions of the bridge as soon as they could. For context, about 32,000 cars traverse the structure each day. Had this been even 4:30 or 5:00 am it would have been a totally different scenario