The Minnesota Department of Transportation(MnDOT) has thousands of sensors in the highways all over that state that regularly update traffic conditions. Below is a graph of the traffic data for the sensors on either side of the 35W bridge on August 1st. By looking at this graph you can see the exact moments when traffic was immediately halted by the bridge collapse.
The different colors correspond to the different in-road sensors displayed on the map to the left.
Data and map from MnDOT's real-time traffic data.