Traffic on this bridge is estimated 71,000 vehicles a day,
Read more here: http://www.tri-cityherald.com/2013/05/23/2406557/i-5-bridge-collapses-in-nw-wash.html#storylink=cpy
Currently the thinking is an oversized load impacted on the spire of the bridge bringing a section down. Two cars and three people went into the river, all are well but banged up.
I've been over this bridge a lot - driving from Canada to see my grandparents in the states. The traffic on I-5 is huge. They have a detour set up that goes over a local bridge. Hopefully it holds up, the delays are going to be epic.