MIAMI — Sgt. Jenna Mendez was heading to work at the Sweetwater Police Department on Thursday when she caught the long red light at Southwest 112th Avenue. She was not happy: She had a 2 p.m. meeting to make.

With the windows rolled up and music on, she waited. Then she saw the brand new bridge about 300 feet in front of her, just installed by Florida International University, suddenly come down. It fell all at once.

Her first thought: Why would construction workers do such a thing?

“Obviously it was a collapse, but I couldn’t comprehend it,” she said on Friday. “I was thinking: ‘Why did they just block all those lanes of traffic?’”

Ahead of her, Alexa Duran and her friend, Richard Humble, both F.I.U. students, had also been waiting out a red light in a Toyota 4Runner.