The CHP said as many as 20 cars ended up with flat tires around 6 a.m. Friday when they ran over the debris on Highway 24 in Orinda, the San Francisco Chronicle reported.
One vehicle sustained a puncture to its fuel tank and fire crews were dispatched to clean up leaked gasoline from the shoulder of the road.
"It looked like a long piece of a bumper," motorist Wayne Siwik said. "I pulled over to check my tires and, thankfully, they were fine."
The highway patrol did not describe the debris or comment on where it came from.