One year after a container ship ran into the Francis Scott Key Bridge in Baltimore, Maryland, causing the structure’s collapse, a study has identified other major US bridges that are surprisingly vulnerable to similar catastrophic ship strikes – and their collective risk is so high that such incidents may occur every few years.
Modern bridges can reduce the chances of ship collisions with measures like increasing the spacing between support piers and adding protective barriers. Such design standards are supposed to reduce the odds of major ship strikes to less than 1 in 10,000 per year, but researchers have calculated that face significantly higher risk.
The Key Bridge itself would have a 1 in 48 annual risk of collision – if it hadn’t already collapsed on 26 March 2024, killing six road maintenance workers and impeding shipping at the port of Baltimore for months. “While it was shocking for the world to see this happen on such a large scale, it wasn’t unprecedented and it wasn’t an aberration,” says at Johns Hopkins University in Maryland.
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Shields and his colleagues have calculated the collision risks for 240 major US bridges so far, and they plan to cover 400 bridges by the end of their study, which is funded by the US National Science Foundation. To find the odds, they consider factors such as the ship traffic passing beneath each bridge, the risk of a nearby ship going off course and the chances that a wayward ship hits the bridge.
The most vulnerable site identified so far is the Huey P. Long Bridge in Louisiana, which is crossed by 40,000 vehicles per day. It has a 1-in-17 chance of a potentially catastrophic collision each year. Next is the San Francisco-Oakland Bay Bridge in California, used by more than 115,000 toll-paying cars per day, which has a 1-in-22 chance of a collision each year.
Some high-risk bridges are already getting better protection. For instance, new collision barriers packed with stones and sand are being installed around the Delaware Memorial Bridge, with a scheduled completion date of September 2025. And the San Francisco-Oakland Bay Bridge is undergoing a risk assessment to pave the way for upgraded protective measures.
Bridges worldwide still face increased risks as ship sizes have grown. The Key Bridge’s pier protection system previously stopped a 119-metre-long container ship called Blue Nagoya from smashing into the structure during a minor collision in 1980. But the drifting container ship Dali, which caused the catastrophic collapse in 2024, was 300 metres long and weighed 10 times as much as Blue Nagoya.
“As the ships get larger, the risk assessment has to account for that fact,” says Shields.
