He said he and his girlfriend were about 400 yards away on the Burlington Bridge when they heard "just a loud bang."
"Then we looked over and saw the bridge was down in the water," he said.
He pulled over and saw three vehicles in the water, including the camping trailer that landed upside-down, he said.
The bridge was not classified as structurally deficient, but a Federal Highway Administration database listed it as being "functionally obsolete" — a category meaning that the design is outdated, such as having narrow shoulders and low clearance underneath.
The bridge was built in in 1955 and had a sufficiency rating of 47 out of 100 at its November 2012 inspection, Transportation Department spokesman Noel Brady said Friday. The state average is 80, according to an Associated Press analysis.
The bridge was inspected two times last year, in August and November.
Washington state was given a C in the American Society of Civil Engineers' 2013 infrastructure report card and a C- when it came to the state's bridges. The group said more than a quarter of Washington's 7,840 bridges are considered structurally deficient or functionally obsolete.
The bridge was 1,112 feet long and 180 feet wide, with two lanes in each direction, Brady said. There are four spans, or sections, over the water supported by piers. The span on the north side is the one that collapsed.
It's a steel truss bridge, meaning it has a boxy steel frame. The vertical clearance from the roadway to the overhead supports is 14.6 feet.
The mishap was reminiscent of the August 2007 collapse of an I-35W bridge in Minneapolis that killed 13 people and injured another 145 when it buckled and fell into the Mississippi River during rush-hour.
Sligh was thankful.
His wife was "doing OK" and he had "lots of cuts," he said. "You're kind of pinching yourself and realize you're lucky to be alive."
Baker reported from Olympia, Wash. Associated Press writers Chris Grygiel in Seattle and Terry Tang in Phoenix also contributed to this report.